{"id":852,"date":"2026-10-23T06:46:20","date_gmt":"2026-10-23T06:46:20","guid":{"rendered":"https:\/\/brainfogremedy.com\/articles\/?p=852"},"modified":"2026-10-24T07:07:52","modified_gmt":"2026-10-24T07:07:52","slug":"metformin-brain-fog","status":"publish","type":"post","link":"https:\/\/brainfogremedy.com\/articles\/metformin-brain-fog\/","title":{"rendered":"Metformin Brain Fog: What Is Known About B12 and Thinking"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\"><strong>Metformin brain fog is not a phrase on the label. The label does say B12 fell below normal in about 7 percent of trial patients over 29 weeks, and since June 2022 the UK regulator calls low B12 a common effect that can impair thinking. Whether that explains one person&#8217;s fog is a blood test question for the prescriber.<\/strong><\/p>\n\n\n\n<div class=\"msq-audio\" data-article=\"metformin-brain-fog\" data-minutes=\"14\">\n<style>.msq-audio{margin:1.25em 0 1.75em;padding:16px 18px;background:#EEF3F8;border:1px solid #CBD8E6;border-radius:12px}\n.msq-audio-lead{margin:0 0 8px;font-size:.95em}.msq-audio audio{width:100%;margin:0 0 12px;display:block}\n.msq-toc-title{margin:0 0 6px;font-weight:700}.msq-toc ol{margin:0;padding:0;list-style:none}\n.msq-toc li{display:flex;gap:10px;align-items:center;padding:6px 6px;border-bottom:1px solid #CBD8E6;border-radius:8px}\n.msq-toc li:last-child{border-bottom:0}.msq-toc li.is-active{background:#fff}\n.msq-toc-play{flex:0 0 auto;width:30px;height:30px;border-radius:50%;border:0;background:#1F4E79;color:#fff;font:inherit;font-size:.72em;line-height:30px;padding:0;cursor:pointer}\n.msq-toc-play.is-playing{background:#B4531A}\n.msq-toc-time{flex:0 0 auto;min-width:3.1em;font-variant-numeric:tabular-nums;color:#4A5560;font-size:.9em}\n.msq-toc a{color:#1B1F24;text-decoration:none;line-height:1.35}.msq-toc li.is-active a{font-weight:700;color:#1F4E79}\n.msq-audio-note{margin:10px 0 0;font-size:.82em;color:#4A5560}\nh2.wp-block-heading[id]{scroll-margin-top:24px}\n.msq-hplay{display:inline-block;vertical-align:middle;width:26px;height:26px;margin:0 0 0 10px;border-radius:50%;border:0;background:#1F4E79;cursor:pointer;padding:0;position:relative;top:-2px}\n.msq-hplay::before{content:'\u25b6\ufe0e';display:block;color:#fff;font-size:11px;line-height:26px;text-align:center}\n.msq-hplay.is-playing{background:#B4531A}.msq-hplay.is-playing::before{content:'\u2016';font-size:13px;font-weight:700}\n.msq-bar{position:fixed;left:0;right:0;bottom:0;z-index:9000;display:flex;align-items:center;gap:10px;padding:8px 12px;padding-bottom:calc(8px + env(safe-area-inset-bottom));background:#FFFFFF;border-top:1px solid #CBD8E6;box-shadow:0 -4px 16px rgba(0,0,0,.08);font-size:.92em;color:#1B1F24;transition:transform .2s ease}\n.msq-bar.is-hidden{transform:translateY(130%)}\n.msq-bar-play{display:inline-flex;align-items:center;gap:8px;border:0;border-radius:999px;background:#1F4E79;color:#fff;font:inherit;font-weight:700;padding:8px 14px 8px 12px;cursor:pointer;white-space:nowrap}\n.msq-bar-play.is-playing{background:#B4531A}\n.msq-bar-icon::before{content:'\u25b6\ufe0e';font-size:12px}.msq-bar-play.is-playing .msq-bar-icon::before{content:'\u2016';font-size:14px;font-weight:700}\n.msq-bar-chapter{flex:1 1 auto;min-width:0;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}\n.msq-bar-time{flex:0 0 auto;font-variant-numeric:tabular-nums;color:#4A5560;font-size:.9em}\n.msq-bar-rate,.msq-bar-close{flex:0 0 auto;border:1px solid #CBD8E6;background:#fff;color:#1B1F24;font:inherit;font-size:.85em;line-height:1;border-radius:8px;padding:7px 9px;cursor:pointer}\n.msq-bar-close::before{content:'\u2715'}\n.msq-bar.msq-bar-pill{left:auto;right:14px;bottom:14px;padding:6px;padding-bottom:6px;border:1px solid #CBD8E6;border-radius:999px;box-shadow:0 6px 20px rgba(0,0,0,.15)}\n.msq-bar.msq-bar-pill .msq-bar-chapter,.msq-bar.msq-bar-pill .msq-bar-time,.msq-bar.msq-bar-pill .msq-bar-rate{display:none}\nbody.msq-has-bar{padding-bottom:76px}\n@media (max-width:600px){.msq-bar-time{display:none}.msq-bar{gap:8px;padding-left:10px;padding-right:10px}}\n.msq-s.msq-now{background:#EEF3F8;box-shadow:0 2px 0 #1F4E79;border-radius:2px}\ntr.msq-now td,tr.msq-now th{background:#EEF3F8;box-shadow:inset 0 -2px 0 #1F4E79}\n@media (hover:hover){.msq-s,tr[data-i]{cursor:pointer}.msq-s:hover{background:#EEF3F8}}<\/style>\n<script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"AudioObject\",\"@id\":\"https:\/\/brainfogremedy.com\/articles\/metformin-brain-fog\/#audio\",\"name\":\"Metformin Brain Fog: What Is Known About B12 and Thinking, read aloud\",\"description\":\"Synthetic voice reading of the article, recorded 23 October 2026.\",\"contentUrl\":\"https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/metformin-brain-fog-audio-202610231221.mp3\",\"encodingFormat\":\"audio\/mpeg\",\"duration\":\"PT13M32S\",\"uploadDate\":\"2026-10-23\",\"inLanguage\":\"en-GB\",\"isAccessibleForFree\":true,\"mainEntityOfPage\":\"https:\/\/brainfogremedy.com\/articles\/metformin-brain-fog\/\",\"hasPart\":[{\"@type\":\"Clip\",\"name\":\"Metformin Brain Fog: What Is Known About B12 and Thinking\",\"startOffset\":0,\"endOffset\":67},{\"@type\":\"Clip\",\"name\":\"Does metformin cause brain fog?\",\"startOffset\":67,\"endOffset\":134,\"url\":\"https:\/\/brainfogremedy.com\/articles\/metformin-brain-fog\/#does-metformin-cause-brain-fog\"},{\"@type\":\"Clip\",\"name\":\"What does the metformin label say about B12, and how has it changed?\",\"startOffset\":134,\"endOffset\":333,\"url\":\"https:\/\/brainfogremedy.com\/articles\/metformin-brain-fog\/#what-does-the-metformin-label-say-about-b12-and-how-has-it-c\"},{\"@type\":\"Clip\",\"name\":\"How often does B12 fall on metformin, and does it matter for thinking?\",\"startOffset\":333,\"endOffset\":448,\"url\":\"https:\/\/brainfogremedy.com\/articles\/metformin-brain-fog\/#how-often-does-b12-fall-on-metformin-and-does-it-matter-for\"},{\"@type\":\"Clip\",\"name\":\"Does metformin make thinking worse or better over the years?\",\"startOffset\":448,\"endOffset\":566,\"url\":\"https:\/\/brainfogremedy.com\/articles\/metformin-brain-fog\/#does-metformin-make-thinking-worse-or-better-over-the-years\"},{\"@type\":\"Clip\",\"name\":\"What do people taking metformin report about brain fog?\",\"startOffset\":566,\"endOffset\":703,\"url\":\"https:\/\/brainfogremedy.com\/articles\/metformin-brain-fog\/#what-do-people-taking-metformin-report-about-brain-fog\"},{\"@type\":\"Clip\",\"name\":\"What should you ask the prescriber about B12 monitoring?\",\"startOffset\":703,\"endOffset\":812,\"url\":\"https:\/\/brainfogremedy.com\/articles\/metformin-brain-fog\/#what-should-you-ask-the-prescriber-about-b12-monitoring\"}],\"contentSize\":\"5.7 MB\"}<\/script>\n<p class=\"msq-audio-lead\"><strong>Listen to this article<\/strong>, about 14 minutes. A synthetic voice reads the article and announces each section title as it begins; tables are read row by row and the questions at the end are left out.<\/p>\n<audio class=\"msq-audio-player\" controls preload=\"metadata\" src=\"https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/metformin-brain-fog-audio-202610231221.mp3\"><\/audio>\n<nav class=\"msq-toc\" aria-label=\"In this article\"><p class=\"msq-toc-title\">In this article<\/p>\n<ol>\n<li data-start=\"0.0\"><button type=\"button\" class=\"msq-toc-play\" data-start=\"0.0\" aria-label=\"Play from: Metformin Brain Fog: What Is Known About B12 and Thinking\">&#x25B6;&#xFE0E;<\/button><span class=\"msq-toc-time\">0:00<\/span><a href=\"#top\">Metformin Brain Fog: What Is Known About B12 and Thinking<\/a><\/li>\n<li data-start=\"66.66\"><button type=\"button\" class=\"msq-toc-play\" data-start=\"66.66\" aria-label=\"Play from: Does metformin cause brain fog?\">&#x25B6;&#xFE0E;<\/button><span class=\"msq-toc-time\">1:07<\/span><a href=\"#does-metformin-cause-brain-fog\">Does metformin cause brain fog?<\/a><\/li>\n<li data-start=\"134.44\"><button type=\"button\" class=\"msq-toc-play\" data-start=\"134.44\" aria-label=\"Play from: What does the metformin label say about B12, and how has it changed?\">&#x25B6;&#xFE0E;<\/button><span class=\"msq-toc-time\">2:14<\/span><a href=\"#what-does-the-metformin-label-say-about-b12-and-how-has-it-c\">What does the metformin label say about B12, and how has it changed?<\/a><\/li>\n<li data-start=\"332.7\"><button type=\"button\" class=\"msq-toc-play\" data-start=\"332.7\" aria-label=\"Play from: How often does B12 fall on metformin, and does it matter for thinking?\">&#x25B6;&#xFE0E;<\/button><span class=\"msq-toc-time\">5:33<\/span><a href=\"#how-often-does-b12-fall-on-metformin-and-does-it-matter-for\">How often does B12 fall on metformin, and does it matter for thinking?<\/a><\/li>\n<li data-start=\"447.52\"><button type=\"button\" class=\"msq-toc-play\" data-start=\"447.52\" aria-label=\"Play from: Does metformin make thinking worse or better over the years?\">&#x25B6;&#xFE0E;<\/button><span class=\"msq-toc-time\">7:28<\/span><a href=\"#does-metformin-make-thinking-worse-or-better-over-the-years\">Does metformin make thinking worse or better over the years?<\/a><\/li>\n<li data-start=\"566.06\"><button type=\"button\" class=\"msq-toc-play\" data-start=\"566.06\" aria-label=\"Play from: What do people taking metformin report about brain fog?\">&#x25B6;&#xFE0E;<\/button><span class=\"msq-toc-time\">9:26<\/span><a href=\"#what-do-people-taking-metformin-report-about-brain-fog\">What do people taking metformin report about brain fog?<\/a><\/li>\n<li data-start=\"703.04\"><button type=\"button\" class=\"msq-toc-play\" data-start=\"703.04\" aria-label=\"Play from: What should you ask the prescriber about B12 monitoring?\">&#x25B6;&#xFE0E;<\/button><span class=\"msq-toc-time\">11:43<\/span><a href=\"#what-should-you-ask-the-prescriber-about-b12-monitoring\">What should you ask the prescriber about B12 monitoring?<\/a><\/li>\n<\/ol><\/nav>\n<p class=\"msq-audio-note\">Recorded 23 October 2026 from the article as published that day. Press a chapter to jump there; the highlighted line is what is playing. A play button sits beside every section heading, and once you scroll past this player a bar at the bottom of the screen keeps the controls in reach. Tap or click any sentence to hear the reading start from there.<\/p>\n<script>var MSQ_SENT = [[0.0,\"h\",\"metforminbrainfogwhatisk\",47],[4.48,\"p\",\"metforminbrainfogisnotap\",39],[7.24,\"p\",\"thelabeldoessayb12fellbe\",149],[20.08,\"p\",\"whetherthatexplainsonepe\",68],[24.96,\"p\",\"onthispagethemetforminla\",189],[42.12,\"p\",\"peoplesearchthisphrasefr\",39],[45.12,\"p\",\"somehavefeltslowsincesta\",39],[48.16,\"p\",\"othershavebeenonthemfory\",62],[52.08,\"p\",\"thelabeltworegulatorstwo\",98],[59.32,\"p\",\"thispagesetsthemsidebysi\",103],[66.66,\"h\",\"doesmetformincausebrainf\",26],[68.74,\"p\",\"notbythatnameandnotdirec\",56],[73.46,\"p\",\"thecurrentusmetforminlab\",366],[103.58,\"p\",\"theroutethelabeldoesdesc\",49],[107.1,\"p\",\"metformincaninterferewit\",155],[118.54,\"p\",\"sothehonestchainisthemed\",142],[129.86,\"p\",\"eachlinkhasanumberbelow\",23],[132.06,\"p\",\"thechainasawholedoesnot\",23],[134.44,\"h\",\"whatdoesthemetforminlabe\",54],[139.32,\"p\",\"thisistheparttherankingp\",32],[141.48,\"p\",\"thewarningisoldbutitston\",79],[146.8,\"p\",\"thetablereadseachdocumen\",65],[151.04,\"t\",\"glucophagelabelapril2017\",286],[177.28,\"t\",\"metforminlabeldecember20\",279],[205.0,\"t\",\"mhradrugsafetyupdate20ju\",304],[234.72,\"t\",\"niceguidelineng239march2\",256],[260.96,\"t\",\"adastandardsofcare2026\",280],[286.52,\"p\",\"threechangesareworthnoti\",28],[288.96,\"p\",\"the2017brandlabelcalledt\",131],[300.16,\"p\",\"the2025genericlabeldrops\",102],[308.04,\"p\",\"andthemhradrugsafetyupda\",233],[327.4,\"p\",\"thesameadvicesaysdonotst\",79],[332.7,\"h\",\"howoftendoesb12fallonmet\",56],[337.02,\"p\",\"thebestlongrunnumberscom\",155],[349.3,\"p\",\"arodaandcolleagues2016re\",43],[353.38,\"t\",\"lowb12at5years\",44],[361.38,\"t\",\"loworborderlinelowb12at5\",60],[370.98,\"t\",\"lowb12at13years\",45],[379.3,\"t\",\"loworborderlinelowb12at1\",62],[388.9,\"t\",\"anaemiaat5years\",47],[396.38,\"p\",\"twothingsstandout\",17],[398.7,\"p\",\"thegapiswidestwhenborder\",78],[404.86,\"p\",\"andtheoddsofdeficiencyro\",131],[414.54,\"p\",\"thestudydidnottestmemory\",39],[417.42,\"p\",\"theonestudythataskedthat\",53],[421.06,\"p\",\"mooreandcolleagues2013lo\",282],[442.18,\"p\",\"itisquotedeversinceasifi\",41],[445.34,\"p\",\"itdidnot\",8],[447.52,\"h\",\"doesmetforminmakethinkin\",50],[451.12,\"p\",\"heretheevidencepointsthe\",84],[456.6,\"p\",\"zhangandcolleagues2022po\",208],[474.36,\"p\",\"thestudiesdisagreedwidel\",136],[483.84,\"p\",\"theauthorsaskedforrandom\",34],[486.04,\"p\",\"sodidmoore\",10],[486.8,\"p\",\"nobodyhasrunonelargeenou\",40],[489.52,\"p\",\"reportstothefdaareathird\",52],[493.88,\"p\",\"theadverseeventreporting\",126],[502.12,\"t\",\"fatigue\",38],[509.32,\"t\",\"dizziness\",41],[516.8,\"t\",\"confusionalstate\",48],[524.24,\"t\",\"anaemia\",39],[530.88,\"t\",\"somnolence\",42],[537.88,\"t\",\"feelingabnormal\",47],[545.04,\"p\",\"fatiguesitsfourthbehindo\",95],[552.32,\"p\",\"memorycomplaintsthething\",60],[557.4,\"p\",\"thatcouldmeantheyarerare\",90],[563.2,\"p\",\"thereportscannottellthos\",30],[566.06,\"h\",\"whatdopeopletakingmetfor\",46],[569.34,\"p\",\"threethreadsandonecommen\",120],[578.06,\"p\",\"theyareaccountsnotfindin\",141],[587.98,\"p\",\"nothinginthemtellsareade\",46],[590.7,\"p\",\"thesurpriseisthedirectio\",25],[592.46,\"p\",\"sevenofthetwelvedescribe\",59],[596.18,\"p\",\"inathreadfromseptember20\",217],[614.1,\"p\",\"inacommentfromseptember2\",163],[625.86,\"p\",\"inathreadfrommay2026aper\",160],[637.94,\"p\",\"fourdescribedtheopposite\",53],[641.78,\"p\",\"inathreadfromjune2026the\",237],[661.66,\"p\",\"oneaccountcutagainstboth\",137],[672.38,\"p\",\"wheretheaccountsagreewit\",93],[678.94,\"p\",\"wheretheygobeyondittheim\",104],[685.46,\"p\",\"wheretheyaresilentnotone\",89],[692.94,\"p\",\"thatisthegaptheregulator\",129],[703.04,\"h\",\"whatshouldyouaskthepresc\",47],[706.28,\"p\",\"theregulatorsagreetheper\",106],[712.84,\"p\",\"parsonageandcolleagues20\",204],[733.04,\"p\",\"acompanionsurveyof124pri\",180],[749.52,\"p\",\"sotheusefulquestionsaref\",264],[769.0,\"p\",\"noneofthatdecidesanythin\",33],[771.24,\"p\",\"themhrasaysapersoncankee\",182],[783.36,\"p\",\"iffogstartedinthefirstwe\",69],[788.64,\"p\",\"andsincetheusualcausesof\",187],[800.68,\"p\",\"thisarticleisgeneralinfo\",109],[808.24,\"p\",\"speaktoyourdoctoraboutyo\",38]];\n(function(){\n  function init(){\n  var root = document.querySelector('.msq-audio'); if (!root) { return; }\n  var audio = root.querySelector('audio'), items = Array.prototype.slice.call(root.querySelectorAll('.msq-toc li'));\n  var starts = items.map(function(li){ return parseFloat(li.getAttribute('data-start')) || 0; });\n  var names = items.map(function(li){ return li.querySelector('a').textContent; });\n  var anchors = items.map(function(li){ var h = li.querySelector('a').getAttribute('href') || ''; return (h.charAt(0) === '#' && h !== '#top') ? document.getElementById(h.slice(1)) : null; });\n  var article = root.getAttribute('data-article') || location.pathname, minutes = root.getAttribute('data-minutes') || '';\n  var marks = {}, started = false, pausedAt = -1, closed = false, ended = false;\n  var RATES = [1, 1.25, 1.5], rate = 1;\n  try { var saved = parseFloat(localStorage.getItem('msq-audio-rate')); if (RATES.indexOf(saved) >= 0) { rate = saved; } } catch (e) {}\n  audio.playbackRate = rate;\n  function track(name, extra){ try { if (typeof gtag === 'function') { var p = { article: article }; for (var k in extra) { p[k] = extra[k]; } gtag('event', name, p); } } catch (e) {} }\n  function current(){ var t = audio.currentTime, idx = 0; for (var i = 0; starts.length > i; i += 1) { if (t + 0.25 >= starts[i]) { idx = i; } } return idx; }\n  function onScreen(){\n    var line = window.innerHeight \/ 3, idx = 0;\n    for (var i = 0; anchors.length > i; i += 1) { var el = anchors[i]; if (el && line >= el.getBoundingClientRect().top) { idx = i; } }\n    return idx;\n  }\n  function fmt(s){ s = Math.max(0, Math.round(s || 0)); var m = Math.floor(s \/ 60), r = s % 60; return m + ':' + (10 > r ? '0' : '') + r; }\n  function play(from, source){\n    if (from !== null) { audio.currentTime = from; }\n    var p = audio.play(); if (p && p.catch) { p.catch(function(){}); }\n    track(from === null ? 'audio_resume' : 'audio_chapter', { chapter: names[current()], start: Math.round(audio.currentTime), source: source });\n  }\n  var hbtns = anchors.map(function(el, i){\n    if (!el) { return null; }\n    var b = document.createElement('button'); b.type = 'button'; b.className = 'msq-hplay'; b.setAttribute('aria-label', 'Play this section: ' + names[i]);\n    b.addEventListener('click', function(){\n      if (!audio.paused && current() === i) { audio.pause(); track('audio_pause', { source: 'heading' }); return; }\n      play(starts[i], 'heading');\n    });\n    el.appendChild(b); return b;\n  });\n  var bar = document.createElement('div'); bar.className = 'msq-bar msq-bar-pill is-hidden'; bar.setAttribute('role', 'region'); bar.setAttribute('aria-label', 'Audio player');\n  var playBtn = document.createElement('button'); playBtn.type = 'button'; playBtn.className = 'msq-bar-play';\n  var icon = document.createElement('span'); icon.className = 'msq-bar-icon'; icon.setAttribute('aria-hidden', 'true');\n  var label = document.createElement('span'); label.className = 'msq-bar-label';\n  playBtn.appendChild(icon); playBtn.appendChild(label);\n  var chap = document.createElement('span'); chap.className = 'msq-bar-chapter';\n  var clock = document.createElement('span'); clock.className = 'msq-bar-time';\n  var rateBtn = document.createElement('button'); rateBtn.type = 'button'; rateBtn.className = 'msq-bar-rate'; rateBtn.setAttribute('aria-label', 'Playback speed');\n  var closeBtn = document.createElement('button'); closeBtn.type = 'button'; closeBtn.className = 'msq-bar-close'; closeBtn.setAttribute('aria-label', 'Close the audio bar');\n  bar.appendChild(playBtn); bar.appendChild(chap); bar.appendChild(clock); bar.appendChild(rateBtn); bar.appendChild(closeBtn);\n  document.body.appendChild(bar);\n  function showRate(){ rateBtn.textContent = (rate === 1 ? '1' : String(rate)) + 'x'; }\n  showRate();\n  rateBtn.addEventListener('click', function(){\n    rate = RATES[(RATES.indexOf(rate) + 1) % RATES.length]; audio.playbackRate = rate; showRate();\n    try { localStorage.setItem('msq-audio-rate', String(rate)); } catch (e) {}\n    track('audio_rate', { rate: rate });\n  });\n  closeBtn.addEventListener('click', function(){\n    closed = true; if (!audio.paused) { audio.pause(); } paint(); track('audio_bar_close', {});\n  });\n  function mode(){\n    if (!audio.paused) { return 'pause'; }\n    if (ended) { return 'again'; }\n    if (!started) { return 'listen'; }\n    return (pausedAt === onScreen()) ? 'resume' : 'section';\n  }\n  playBtn.addEventListener('click', function(){\n    var m = mode();\n    if (m === 'pause') { audio.pause(); track('audio_pause', { source: 'bar' }); return; }\n    if (m === 'again') { ended = false; play(0, 'bar'); return; }\n    if (m === 'resume') { play(null, 'bar'); return; }\n    play(starts[onScreen()], 'bar');\n  });\n  function paint(){\n    var idx = current(), playing = !audio.paused, m = mode();\n    items.forEach(function(li, i){\n      var on = (i === idx) && playing, b = li.querySelector('.msq-toc-play'), a = li.querySelector('a');\n      li.classList.toggle('is-active', on); b.classList.toggle('is-playing', on);\n      b.textContent = on ? '\u2016' : '\u25b6\ufe0e';\n      b.setAttribute('aria-label', (on ? 'Pause: ' : 'Play from: ') + a.textContent);\n    });\n    hbtns.forEach(function(b, i){ if (b) { var on = (i === idx) && playing; b.classList.toggle('is-playing', on); b.setAttribute('aria-label', (on ? 'Pause this section: ' : 'Play this section: ') + names[i]); } });\n    playBtn.classList.toggle('is-playing', playing);\n    label.textContent = m === 'pause' ? 'Pause' : m === 'again' ? 'Play again' : m === 'resume' ? 'Resume' : m === 'section' ? 'Play this section' : 'Listen, ' + minutes + ' min';\n    playBtn.setAttribute('aria-label', label.textContent);\n    chap.textContent = started ? names[(playing || m === 'resume') ? idx : onScreen()] : '';\n    clock.textContent = started ? fmt(audio.currentTime) + ' \/ ' + fmt(audio.duration) : '';\n    bar.classList.toggle('msq-bar-pill', !started);\n    var show = !closed && (started || 0 > root.getBoundingClientRect().bottom);\n    bar.classList.toggle('is-hidden', !show);\n    document.body.classList.toggle('msq-has-bar', show);\n  }\n  root.addEventListener('click', function(e){\n    var b = e.target.closest ? e.target.closest('.msq-toc-play') : null; if (!b) { return; }\n    var li = b.parentNode, s = parseFloat(b.getAttribute('data-start')) || 0;\n    if (!audio.paused && current() === items.indexOf(li)) { audio.pause(); track('audio_pause', { source: 'index' }); return; }\n    play(s, 'index');\n  });\n  audio.addEventListener('play', function(){ ended = false; closed = false; if (!started) { started = true; track('audio_play', { minutes: Math.round((audio.duration || 0) \/ 60) }); } paint(); });\n  audio.addEventListener('pause', function(){ pausedAt = current(); paint(); });\n  audio.addEventListener('timeupdate', function(){\n    paint();\n    if (audio.duration > 0) {\n      var pct = Math.floor(audio.currentTime \/ audio.duration * 100);\n      [25, 50, 75].forEach(function(m){ if (pct >= m && !marks[m]) { marks[m] = true; track('audio_progress', { percent: m }); } });\n    }\n  });\n  audio.addEventListener('ended', function(){ ended = true; track('audio_complete', {}); paint(); });\n  var ticking = false;\n  function onScroll(){ if (ticking) { return; } ticking = true; setTimeout(function(){ ticking = false; paint(); }, 80); }\n  window.addEventListener('scroll', onScroll, { passive: true }); window.addEventListener('resize', onScroll);\n  paint();\n  }\n  if (document.readyState === 'loading') { document.addEventListener('DOMContentLoaded', init); } else { init(); }\n})();\n\n(function(){\n  function start(){\n    var root = document.querySelector('.msq-audio'), audio = root ? root.querySelector('audio') : null;\n    var map = (typeof MSQ_SENT !== 'undefined') ? MSQ_SENT : [];\n    if (!root || !audio || !map.length) { return; }\n    var scope = root.closest('main') || root.closest('article') || root.closest('.entry-content') || document.body;\n    var SKIP = '.msq-audio,.msq,.msq-tool,.msq-end,.msq-bar,script,style,noscript,figcaption,#rank-math-faq,.rank-math-block,nav,header,footer';\n    function norm(s){ return s.toLowerCase().replace(\/[^a-z0-9]+\/g, ''); }\n    var nodes = [], stream = '', walker = document.createTreeWalker(scope, NodeFilter.SHOW_TEXT, null), n;\n    while ((n = walker.nextNode())) {\n      var el = n.parentElement; if (!el || el.closest(SKIP)) { continue; }\n      var raw = n.nodeValue, keep = [], norms = '';\n      for (var i = 0; raw.length > i; i += 1) { var c = raw.charAt(i).toLowerCase(); if (\/[a-z0-9]\/.test(c)) { keep.push(i); norms += c; } }\n      if (!norms) { continue; }\n      nodes.push({ node: n, a: stream.length, b: stream.length + norms.length, keep: keep });\n      stream += norms;\n    }\n    var segs = [], cursor = 0, lastRow = null;\n    map.forEach(function(m, idx){\n      var kind = m[1], f = m[2], len = m[3];\n      if (kind === 't') {\n        var rows = scope.querySelectorAll('tr'), after = !lastRow;\n        for (var r = 0; rows.length > r; r += 1) {\n          var tr = rows[r];\n          if (!after) { if (tr === lastRow) { after = true; } continue; }\n          var cell = tr.querySelector('td,th');\n          if (cell && norm(cell.textContent).indexOf(f) === 0) { tr.setAttribute('data-i', idx); lastRow = tr; break; }\n        }\n        return;\n      }\n      var p = stream.indexOf(f, cursor); if (0 > p) { return; }\n      segs.push({ i: idx, a: p, b: Math.min(p + len, stream.length) }); cursor = p + len;\n    });\n    var si = 0;\n    nodes.forEach(function(nd){\n      var frag = document.createDocumentFragment(), raw = nd.node.nodeValue, rawPos = 0, any = false;\n      for (var k = si; segs.length > k; k += 1) {\n        var sg = segs[k]; if (sg.a >= nd.b) { break; } if (nd.a >= sg.b) { continue; }\n        var s = Math.max(sg.a, nd.a), e = Math.min(sg.b, nd.b);\n        var rs = nd.keep[s - nd.a], re2 = nd.keep[e - nd.a - 1] + 1;\n        if (rs > rawPos) { frag.appendChild(document.createTextNode(raw.slice(rawPos, rs))); }\n        var span = document.createElement('span'); span.className = 'msq-s'; span.setAttribute('data-i', sg.i); span.textContent = raw.slice(rs, re2);\n        frag.appendChild(span); rawPos = re2; any = true;\n        if (sg.b > nd.b) { break; }\n      }\n      while (segs.length > si && nd.b >= segs[si].b) { si += 1; }\n      if (!any) { return; }\n      if (raw.length > rawPos) { frag.appendChild(document.createTextNode(raw.slice(rawPos))); }\n      nd.node.parentNode.replaceChild(frag, nd.node);\n    });\n    var starts = map.map(function(m){ return m[0]; }), curIdx = -1, lastUser = 0, timer = null, autoUntil = 0, dragging = false;\n    function idxAt(t){ var lo = 0, hi = starts.length; while (hi > lo + 1) { var mid = (lo + hi) >> 1; if (t + 0.15 >= starts[mid]) { lo = mid; } else { hi = mid; } } return (t + 0.15 >= starts[0]) ? lo : -1; }\n    function setNow(i){\n      if (i === curIdx) { return; }\n      Array.prototype.forEach.call(scope.querySelectorAll('.msq-now'), function(el){ el.classList.remove('msq-now'); });\n      curIdx = i; if (0 > i) { return; }\n      var els = scope.querySelectorAll('[data-i=\"' + i + '\"]'); if (!els.length) { return; }\n      Array.prototype.forEach.call(els, function(el){ el.classList.add('msq-now'); });\n      var r = els[0].getBoundingClientRect(), now = Date.now();\n      if (!dragging && now > lastUser + 2500 && (80 > r.top || r.bottom > window.innerHeight - 140)) {\n        var reduce = window.matchMedia && window.matchMedia('(prefers-reduced-motion: reduce)').matches;\n        autoUntil = now + (reduce ? 400 : 1600);\n        els[0].scrollIntoView({ block: 'center', behavior: reduce ? 'auto' : 'smooth' });\n      }\n    }\n    function tick(){ if (!audio.paused) { setNow(idxAt(audio.currentTime)); } }\n    function userScrolled(){ lastUser = Date.now(); }\n    ['wheel', 'touchmove', 'keydown'].forEach(function(ev){ window.addEventListener(ev, userScrolled, { passive: true }); });\n    window.addEventListener('scroll', function(){ if (Date.now() > autoUntil) { userScrolled(); } }, { passive: true });\n    window.addEventListener('mousedown', function(e){ if (e.clientX >= document.documentElement.clientWidth) { dragging = true; userScrolled(); } }, { passive: true });\n    window.addEventListener('mouseup', function(){ if (dragging) { dragging = false; userScrolled(); } }, { passive: true });\n    audio.addEventListener('play', function(){ if (timer) { clearInterval(timer); } timer = setInterval(tick, 150); tick(); });\n    audio.addEventListener('pause', function(){ if (timer) { clearInterval(timer); timer = null; } });\n    scope.addEventListener('click', function(e){\n      if (e.target.closest('a, button, input, select, textarea, label, summary, .msq, .msq-tool, .msq-end, .msq-audio, .msq-bar')) { return; }\n      var sel = window.getSelection ? String(window.getSelection()) : '';\n      if (sel && sel.trim()) { return; }\n      var el = e.target.closest('[data-i]'); if (!el) { return; }\n      var i = parseInt(el.getAttribute('data-i'), 10); if (isNaN(i)) { return; }\n      if (!audio.paused && i === curIdx) { return; }\n      lastUser = Date.now();\n      audio.currentTime = starts[i];\n      var p = audio.play(); if (p && p.catch) { p.catch(function(){}); }\n      try { if (typeof gtag === 'function') { gtag('event', 'audio_chapter', { article: location.pathname, source: 'sentence', start: Math.round(starts[i]) }); } } catch (err) {}\n    });\n    audio.addEventListener('seeked', tick);\n    audio.addEventListener('ended', function(){ setNow(-1); });\n  }\n  if (document.readyState === 'loading') { document.addEventListener('DOMContentLoaded', start); } else { start(); }\n})();<\/script>\n<\/div>\n\n\n\n<p class=\"msq-hook wp-block-paragraph\">On this page: the metformin label as worded in April 2017 and December 2025 set beside the MHRA, NICE and ADA positions with their dates, the 13 year trial&#8217;s B12 numbers, the FDA report counts, and what twelve people on Reddit described.<\/p>\n\n\n\n<div class=\"msq\" id=\"ms-quiz\" data-slug=\"metformin-brain-fog\" data-category=\"what causes it\" data-list=\"5\">\n<style>.msq,.msq-hook,.msq-box,.msq-end,.msq-tool{--msq-accent:#1F4E79;--msq-accent-text:#FFFFFF;--msq-line:#CBD8E6;--msq-warn:#B4531A;--msq-muted:#4A5560;--msq-tint:#EEF3F8;--msq-base:#FFFFFF}\n.msq{margin:1.25em 0 1.75em}\n.msq-hook{border-left:3px solid var(--msq-accent);padding:6px 14px;color:var(--msq-muted);font-size:.97em}\n.msq-box{background:var(--msq-tint);border:1px solid var(--msq-line);border-radius:12px;padding:0}\n.msq-box>summary{cursor:pointer;list-style:none;padding:14px 18px;font-weight:700;color:var(--msq-accent)}\n.msq-box>summary::-webkit-details-marker{display:none}\n.msq-box>summary::before{content:\"+\";display:inline-block;width:1.1em;font-weight:800}\n.msq-box[open]>summary::before{content:\"\\00d7\"}\n.msq-body{padding:2px 18px 18px}\n.msq-intro{margin:0 0 12px;font-size:.95em;color:var(--msq-muted)}\n.msq-list{margin:0;padding:0;list-style:none}\n.msq-q{margin:0 0 18px;padding:0}\n.msq-question{font-weight:700;margin:0 0 8px}\n.msq-options{display:flex;flex-direction:column;gap:8px}\n.msq-opt{text-align:left;background:var(--msq-base);border:1px solid var(--msq-line);border-radius:8px;padding:10px 12px;font:inherit;font-size:.95em;color:inherit;cursor:pointer;line-height:1.35}\n.msq-opt:hover{border-color:var(--msq-accent)}\n.msq-opt[disabled]{cursor:default}\n.msq-opt.is-right{border-color:var(--msq-accent);background:var(--msq-line);font-weight:700}\n.msq-opt.is-wrong{border-color:var(--msq-warn);color:var(--msq-warn)}\n.msq-explain{margin:8px 0 0;font-size:.92em;padding:10px 12px;border-left:3px solid var(--msq-accent);background:var(--msq-base);border-radius:0 8px 8px 0}\n.msq-explain[hidden]{display:none}\n.msq-verdict{font-weight:700}\n.msq-result{margin-top:14px;padding:14px 16px;background:var(--msq-base);border:1px solid var(--msq-line);border-radius:10px}\n.msq-result[hidden]{display:none}\n.msq-result h3{margin:0 0 6px;font-size:1.1em}\n.msq-result p{margin:0 0 8px}\n.msq-jump{margin:0 0 8px;padding-left:1.2em}\n.msq-form{margin-top:10px;display:flex;flex-wrap:wrap;gap:8px;align-items:center}\n.msq-form input[type=email]{flex:1 1 220px;padding:10px 12px;border:1px solid var(--msq-muted);border-radius:8px;font:inherit;background:#fff}\n.msq-btn{background:var(--msq-accent);color:var(--msq-accent-text);border:0;border-radius:8px;padding:11px 18px;font:inherit;font-weight:700;cursor:pointer}\n.msq-btn:hover{filter:brightness(.92)}\n.msq-btn[disabled]{opacity:.7;cursor:default}\n.msq-no{font-size:.9em;color:var(--msq-muted);margin-left:4px}\n.msq-privacy{margin:6px 0 0;font-size:.82em;color:var(--msq-muted);flex-basis:100%}\n.msq-hp{position:absolute;left:-5000px;width:1px;height:1px;overflow:hidden}\n.msq-msg{margin:8px 0 0;font-size:.92em}\n.msq-end{margin:1.5em 0;padding:18px;border:1px solid var(--msq-accent);border-radius:12px;text-align:center;background:var(--msq-tint)}\n.msq-end p{margin:0 0 12px}\n.msq-end .msq-body{text-align:left;padding:16px 0 0}\n.msq-end .msq-start-test[hidden]{display:none}\n.msq-tool{margin:1.5em 0;padding:18px 20px;background:var(--msq-tint);border:1px solid var(--msq-line);border-radius:12px}\n.msq-tool h3{margin:0 0 6px;font-size:1.15em}\n.msq-tool p{margin:0 0 10px}\n.msq-tool .msq-note{font-size:.85em;color:var(--msq-muted);margin:10px 0 0}\n.msq-check label{display:flex;gap:10px;align-items:flex-start;padding:9px 0;border-bottom:1px solid var(--msq-line);cursor:pointer;line-height:1.4}\n.msq-check label:last-of-type{border-bottom:0}\n.msq-check input{margin-top:5px;flex:0 0 auto;width:18px;height:18px;accent-color:var(--msq-accent)}\n.msq-check-count{font-weight:700;margin:10px 0 0}\n.msq-verdict-box{margin:12px 0 0;padding:10px 14px;border-radius:8px;background:var(--msq-base);border-left:3px solid var(--msq-warn)}\n.msq-verdict-box[hidden]{display:none}\n.msq-calc{display:grid;grid-template-columns:1fr 1fr;gap:10px 16px}\n.msq-calc label{display:flex;flex-direction:column;gap:4px;font-size:.92em}\n.msq-calc input,.msq-calc select{padding:8px 10px;border:1px solid var(--msq-muted);border-radius:8px;font:inherit;background:#fff}\n.msq-calc-out{grid-column:1 \/ -1;margin-top:6px;padding:12px 14px;background:var(--msq-base);border-radius:8px;border:1px solid var(--msq-line)}\n.msq-calc-out strong{font-size:1.15em}\n.msq-choose label{display:block;padding:9px 12px;margin:0 0 8px;background:var(--msq-base);border:1px solid var(--msq-line);border-radius:8px;cursor:pointer}\n.msq-choose input{margin-right:8px;accent-color:var(--msq-accent)}\n.msq-choice-result{margin-top:10px;padding:12px 14px;background:var(--msq-base);border-left:3px solid var(--msq-accent);border-radius:0 8px 8px 0}\n.msq-choice-result[hidden]{display:none}\n@media (max-width:600px){.msq-calc{grid-template-columns:1fr}}\nh2.wp-block-heading[id]{scroll-margin-top:24px}<\/style>\n<noscript><style>.msq-explain{display:block !important}<\/style><\/noscript>\n<details class=\"msq-box\">\n<summary>Think you already know this? Five questions, one minute.<\/summary>\n<div class=\"msq-body\">\n<p class=\"msq-intro\">Every answer is on this page. Answer from what you know now; the page fills in the rest.<\/p>\n<ol class=\"msq-list\">\n<li class=\"msq-q\" data-q=\"1\" data-answer=\"b\" data-title=\"Does metformin cause brain fog?\" data-section=\"#does-metformin-cause-brain-fog\">\n<p class=\"msq-question\">1. Which of these does the current US metformin label list as a common reaction from its trial?<\/p>\n<div class=\"msq-options\" role=\"group\" aria-label=\"Question 1 options\"><button type=\"button\" class=\"msq-opt\" data-opt=\"a\">Memory impairment in 7 percent of patients<\/button><button type=\"button\" class=\"msq-opt\" data-opt=\"b\">Asthenia, meaning weakness or tiredness, in 9 percent against 6 on placebo<\/button><button type=\"button\" class=\"msq-opt\" data-opt=\"c\">Brain fog in 1 in 10 patients<\/button><\/div>\n<p class=\"msq-explain\" hidden><span class=\"msq-verdict\"><\/span>The <a href=\"https:\/\/dailymed.nlm.nih.gov\/dailymed\/drugInfo.cfm?setid=0ff1ba75-3c86-4baf-a6b1-1bb883839866\" target=\"_blank\" rel=\"noopener\">metformin label on DailyMed<\/a>, revised December 2025, lists asthenia at 9 percent against 6 on placebo and has no row for memory or concentration. Brain fog is not a label term. <a class=\"msq-section\" href=\"#does-metformin-cause-brain-fog\">Read the section<\/a><\/p>\n<\/li>\n<li class=\"msq-q\" data-q=\"2\" data-answer=\"b\" data-title=\"What does the metformin label say about B12, and how has it changed?\" data-section=\"#what-does-the-metformin-label-say-about-b12-and-how-has-it-c\">\n<p class=\"msq-question\">2. How did the MHRA classify reduced vitamin B12 on metformin in June 2022?<\/p>\n<div class=\"msq-options\" role=\"group\" aria-label=\"Question 2 options\"><button type=\"button\" class=\"msq-opt\" data-opt=\"a\">Rare, affecting fewer than 1 in 1,000<\/button><button type=\"button\" class=\"msq-opt\" data-opt=\"b\">Common, affecting up to 1 in 10<\/button><button type=\"button\" class=\"msq-opt\" data-opt=\"c\">Not a recognised side effect<\/button><\/div>\n<p class=\"msq-explain\" hidden><span class=\"msq-verdict\"><\/span>The <a href=\"https:\/\/www.gov.uk\/drug-safety-update\/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk\" target=\"_blank\" rel=\"noopener\">MHRA Drug Safety Update of 20 June 2022<\/a> classed reduced B12 as common, meaning up to 1 in 10 people, with risk rising with dose and years on the medicine. <a class=\"msq-section\" href=\"#what-does-the-metformin-label-say-about-b12-and-how-has-it-c\">Read the section<\/a><\/p>\n<\/li>\n<li class=\"msq-q\" data-q=\"3\" data-answer=\"a\" data-title=\"How often does B12 fall on metformin, and does it matter for thinking?\" data-section=\"#how-often-does-b12-fall-on-metformin-and-does-it-matter-for\">\n<p class=\"msq-question\">3. In the Diabetes Prevention Program Outcomes Study, what share of the metformin group had low or borderline low B12 at 5 years?<\/p>\n<div class=\"msq-options\" role=\"group\" aria-label=\"Question 3 options\"><button type=\"button\" class=\"msq-opt\" data-opt=\"a\">19.1 percent, against 9.5 on placebo<\/button><button type=\"button\" class=\"msq-opt\" data-opt=\"b\">4.3 percent, against 4.3 on placebo<\/button><button type=\"button\" class=\"msq-opt\" data-opt=\"c\">50 percent, against 25 on placebo<\/button><\/div>\n<p class=\"msq-explain\" hidden><span class=\"msq-verdict\"><\/span><a href=\"https:\/\/pmc.ncbi.nlm.nih.gov\/articles\/PMC4880159\/\" target=\"_blank\" rel=\"noopener\">Aroda and colleagues, 2016<\/a> reported 19.1 percent low or borderline low at 5 years on metformin against 9.5 percent on placebo. The study did not test memory or concentration. <a class=\"msq-section\" href=\"#how-often-does-b12-fall-on-metformin-and-does-it-matter-for\">Read the section<\/a><\/p>\n<\/li>\n<li class=\"msq-q\" data-q=\"4\" data-answer=\"a\" data-title=\"Does metformin make thinking worse or better over the years?\" data-section=\"#does-metformin-make-thinking-worse-or-better-over-the-years\">\n<p class=\"msq-question\">4. What did the 2022 pooled analysis of 28 observational studies find about cognitive impairment in people with diabetes on metformin?<\/p>\n<div class=\"msq-options\" role=\"group\" aria-label=\"Question 4 options\"><button type=\"button\" class=\"msq-opt\" data-opt=\"a\">A slightly lower rate, with an adjusted hazard ratio of 0.92<\/button><button type=\"button\" class=\"msq-opt\" data-opt=\"b\">A doubled rate, matching the 2013 Moore study<\/button><button type=\"button\" class=\"msq-opt\" data-opt=\"c\">No study has ever looked at it<\/button><\/div>\n<p class=\"msq-explain\" hidden><span class=\"msq-verdict\"><\/span><a href=\"https:\/\/pmc.ncbi.nlm.nih.gov\/articles\/PMC9453211\/\" target=\"_blank\" rel=\"noopener\">Zhang and colleagues, 2022<\/a> found an adjusted hazard ratio of 0.92 for cognitive impairment and 0.90 for dementia, with no effect on Alzheimer&#8217;s disease, and asked for randomised trials because every study pooled was observational. <a class=\"msq-section\" href=\"#does-metformin-make-thinking-worse-or-better-over-the-years\">Read the section<\/a><\/p>\n<\/li>\n<li class=\"msq-q\" data-q=\"5\" data-answer=\"a\" data-title=\"What should you ask the prescriber about B12 monitoring?\" data-section=\"#what-should-you-ask-the-prescriber-about-b12-monitoring\">\n<p class=\"msq-question\">5. In the English GP practices studied, what share of metformin users had a B12 test in a given year after the June 2022 safety update?<\/p>\n<div class=\"msq-options\" role=\"group\" aria-label=\"Question 5 options\"><button type=\"button\" class=\"msq-opt\" data-opt=\"a\">38.2 percent, up from 34.5 before it<\/button><button type=\"button\" class=\"msq-opt\" data-opt=\"b\">95 percent, up from 60 before it<\/button><button type=\"button\" class=\"msq-opt\" data-opt=\"c\">10 percent, down from 34.5 before it<\/button><\/div>\n<p class=\"msq-explain\" hidden><span class=\"msq-verdict\"><\/span><a href=\"https:\/\/pmc.ncbi.nlm.nih.gov\/articles\/PMC12924288\/\" target=\"_blank\" rel=\"noopener\">Parsonage and colleagues, 2025<\/a> found annual testing rose from 34.5 percent to 38.2 percent across nine practices covering about 148,000 patients, so most metformin users still went untested in any given year. <a class=\"msq-section\" href=\"#what-should-you-ask-the-prescriber-about-b12-monitoring\">Read the section<\/a><\/p>\n<\/li>\n<\/ol>\n<div class=\"msq-result\" hidden><\/div>\n<\/div>\n<\/details>\n<script>(function(){\n  if (window.__msqInit) { return; }\n  window.__msqInit = true;\n  var root = document.getElementById('ms-quiz');\n  if (!root) { return; }\n  var meta = { slug: root.getAttribute('data-slug'), category: root.getAttribute('data-category'),\n               list: root.getAttribute('data-list') || '1' };\n  function qs(el, s){ return el.querySelector(s); }\n  function qsa(el, s){ return Array.prototype.slice.call(el.querySelectorAll(s)); }\n  function track(name, params){\n    try { if (typeof gtag === 'function') { var p = params || {}; p.article = meta.slug; gtag('event', name, p); } } catch (e) {}\n  }\n  function jumpList(missed){\n    if (!missed.length) { return ''; }\n    return '<ul class=\"msq-jump\">' + missed.map(function(q){\n      return '<li><a href=\"' + q.getAttribute('data-section') + '\">' + q.getAttribute('data-title') + '<\/a><\/li>';\n    }).join('') + '<\/ul>';\n  }\n  function emailForm(){\n    return '<div class=\"msq-sub\"><p><strong>Get the next one.<\/strong> We publish new articles on ' + meta.category\n      + ' most weeks and recheck this page on Sundays. Email me when a new article goes up or this page changes.<\/p>'\n      + '<form class=\"msq-form\" method=\"post\" action=\"\/mail\/form.php\" novalidate>'\n      + '<input type=\"hidden\" name=\"list\" value=\"' + meta.list + '\">'\n      + '<input type=\"hidden\" name=\"name\" value=\"\">'\n      + '<label class=\"msq-hp\" aria-hidden=\"true\">Leave this empty<input type=\"text\" name=\"website\" tabindex=\"-1\" autocomplete=\"off\"><\/label>'\n      + '<input type=\"email\" name=\"email\" placeholder=\"you@example.com\" required aria-label=\"Email address\" autocomplete=\"email\">'\n      + '<button type=\"submit\" class=\"msq-btn\">Email me the updates<\/button>'\n      + '<a href=\"#\" class=\"msq-no\">No thanks<\/a>'\n      + '<p class=\"msq-privacy\">We store your email and this page\\'s topic, nothing else. Unsubscribe any time.<\/p>'\n      + '<\/form><p class=\"msq-msg\" aria-live=\"polite\"><\/p><\/div>';\n  }\n  \/* The autoresponder always answers 200 with its own page. A \".msg.err\" box means the\n     address was rejected, \"You're subscribed\" means accepted, and a redirect only happens\n     after a successful subscribe. Same reading as the landing pages on the other sites. *\/\n  function sendForm(form){\n    return fetch(form.getAttribute('action'), { method: 'POST', body: new FormData(form), credentials: 'same-origin', redirect: 'manual' })\n      .then(function(res){\n        if (res.type === 'opaqueredirect') { return { status: 'ok' }; }\n        if (res.status === 404) { return { status: 'notlive' }; }\n        if (!res.ok) { throw new Error('HTTP ' + res.status); }\n        return res.text().then(function(t){\n          var doc = new DOMParser().parseFromString(t, 'text\/html');\n          var err = doc.querySelector('.msg.err');\n          if (err) {\n            var m = err.textContent.trim();\n            if (\/something went wrong\/i.test(m)) { throw new Error(m); }\n            return { status: 'rejected', message: m };\n          }\n          if (\/you.re subscribed\/i.test(doc.body.textContent)) { return { status: 'ok' }; }\n          return { status: 'unknown' };\n        });\n      });\n  }\n  function wireForm(sub){\n    var form = qs(sub, '.msq-form'), msg = qs(sub, '.msq-msg');\n    qs(form, '.msq-no').addEventListener('click', function(e){\n      e.preventDefault();\n      sub.innerHTML = '<p class=\"msq-msg\">No problem. The Sunday refresh keeps this page current either way.<\/p>';\n      track('quiz_email', { result: 'declined' });\n    });\n    form.addEventListener('submit', function(e){\n      e.preventDefault();\n      var input = qs(form, 'input[name=email]'), btn = qs(form, '.msq-btn');\n      var email = input.value.trim(); input.value = email;\n      if (!\/^[^\\s@]+@[^\\s@]+\\.[^\\s@]+$\/.test(email)) { msg.textContent = 'That email does not look right.'; return; }\n      btn.disabled = true; btn.textContent = 'Sending';\n      function reset(){ btn.disabled = false; btn.textContent = 'Email me the updates'; }\n      sendForm(form).then(function(r){\n        if (r.status === 'ok' || r.status === 'unknown') {\n          sub.innerHTML = '<p class=\"msq-msg\"><strong>Done.<\/strong> We will email you when the next article on ' + meta.category\n            + ' goes up and when this page changes. If a confirmation email arrives, click it.<\/p>';\n          track('quiz_email', { result: r.status });\n          if (r.status === 'ok') { track('generate_lead', { method: 'quiz' }); }\n        } else if (r.status === 'notlive') {\n          msg.textContent = 'Sign up is not switched on yet for this site. Nothing was stored.'; reset();\n          track('quiz_email', { result: 'notlive' });\n        } else {\n          msg.textContent = r.message || 'Please check the email address and try again.'; reset();\n          track('quiz_email', { result: 'rejected' });\n        }\n      }).catch(function(){\n        msg.textContent = 'Could not reach the sign up service just now. Nothing was stored.'; reset();\n        track('quiz_email', { result: 'error' });\n      });\n    });\n  }\n  function finish(container, mode, score, total, missed){\n    var result = qs(container, '.msq-result');\n    track('quiz_complete', { mode: mode, score: score, total: total });\n    var h;\n    if (mode === 'pre') {\n      h = '<h3>You knew ' + score + ' of ' + total + ' before reading.<\/h3>';\n      h += (score === total) ? '<p>All of them. Read on anyway; the sections below carry the detail behind each answer.<\/p>'\n                             : '<p>The rest is below. Jump straight to what you missed:<\/p>' + jumpList(missed);\n      try { sessionStorage.setItem('msq-pre-' + meta.slug, String(score)); } catch (e) {}\n    } else {\n      var pre = null;\n      try { pre = sessionStorage.getItem('msq-pre-' + meta.slug); } catch (e) {}\n      h = '<h3>You got ' + score + ' of ' + total + '.<\/h3>';\n      if (pre !== null) { h += '<p>Before reading you had ' + pre + ' of ' + total + '.<\/p>'; }\n      if (missed.length) { h += '<p>Worth a second look:<\/p>' + jumpList(missed); }\n      h += emailForm();\n    }\n    result.innerHTML = h; result.hidden = false;\n    if (mode !== 'pre') { wireForm(qs(result, '.msq-sub')); }\n    result.scrollIntoView({ behavior: 'smooth', block: 'nearest' });\n  }\n  function setup(container, mode){\n    var qEls = qsa(container, '.msq-q'), answered = 0, score = 0, missed = [];\n    qEls.forEach(function(q){\n      var answer = q.getAttribute('data-answer'), opts = qsa(q, '.msq-opt'), explain = qs(q, '.msq-explain');\n      q.removeAttribute('data-done'); explain.hidden = true; qs(explain, '.msq-verdict').textContent = '';\n      opts.forEach(function(b){ b.disabled = false; b.classList.remove('is-right', 'is-wrong'); });\n      opts.forEach(function(b){\n        b.addEventListener('click', function(){\n          if (q.getAttribute('data-done')) { return; }\n          q.setAttribute('data-done', '1');\n          var ok = b.getAttribute('data-opt') === answer;\n          opts.forEach(function(o){ o.disabled = true; if (o.getAttribute('data-opt') === answer) { o.classList.add('is-right'); } });\n          if (ok) { score += 1; } else { b.classList.add('is-wrong'); missed.push(q); }\n          qs(explain, '.msq-verdict').textContent = ok ? 'Right. ' : 'Not quite. ';\n          explain.hidden = false;\n          answered += 1;\n          track('quiz_answer', { mode: mode, question: q.getAttribute('data-q'), correct: ok ? 1 : 0 });\n          if (answered === qEls.length) { finish(container, mode, score, qEls.length, missed); }\n        });\n      });\n    });\n  }\n  var box = qs(root, '.msq-box');\n  setup(root, 'pre');\n  if (box) { var skipFirst = box.hasAttribute('data-default-open'); box.addEventListener('toggle', function(){ if (!box.open) { return; } if (skipFirst) { skipFirst = false; return; } track('quiz_open', { mode: 'pre' }); }); }\n  \/* The end block sits far below this script, so it does not exist yet while this runs\n     during parsing. A delegated listener finds it at click time. *\/\n  document.addEventListener('click', function(e){\n    var startBtn = (e.target && e.target.closest) ? e.target.closest('.msq-start-test') : null;\n    if (!startBtn) { return; }\n    var end = startBtn.closest('.msq-end');\n    if (!end) { return; }\n    var mount = qs(end, '.msq-mount');\n    var clone = qs(root, '.msq-body').cloneNode(true);\n    var intro = qs(clone, '.msq-intro');\n    if (intro) { intro.textContent = 'The same five questions, now that you have read the page.'; }\n    var res = qs(clone, '.msq-result'); if (res) { res.hidden = true; res.innerHTML = ''; }\n    mount.innerHTML = ''; mount.appendChild(clone);\n    setup(mount, 'test');\n    startBtn.hidden = true;\n    track('quiz_open', { mode: 'test' });\n    mount.scrollIntoView({ behavior: 'smooth', block: 'start' });\n    var first = qs(mount, '.msq-opt'); if (first) { first.focus({ preventScroll: true }); }\n  });\n  \/* Shared tool behaviour: checklists remember their ticks on this device, and a chooser\n     shows the result for the option picked. Calculators carry their own small script. *\/\n  function initTools(){\n    qsa(document, '.msq-check').forEach(function(box){\n      var key = 'msq-check-' + meta.slug + '-' + (box.getAttribute('data-id') || 'x');\n      var boxes = qsa(box, 'input[type=checkbox]'), count = qs(box, '.msq-check-count');\n      var yes = qs(box, '.msq-verdict-yes'), no = qs(box, '.msq-verdict-no');\n      var saved = {};\n      try { saved = JSON.parse(localStorage.getItem(key) || '{}'); } catch (e) {}\n      boxes.forEach(function(cb, i){ if (saved[i]) { cb.checked = true; } });\n      function refresh(){\n        var n = boxes.filter(function(cb){ return cb.checked; }).length, state = {};\n        boxes.forEach(function(cb, i){ if (cb.checked) { state[i] = 1; } });\n        try { localStorage.setItem(key, JSON.stringify(state)); } catch (e) {}\n        if (count) { count.textContent = n + ' of ' + boxes.length + (box.getAttribute('data-verb') || ' done'); }\n        if (yes && no) { yes.hidden = n === 0; no.hidden = n !== 0; }\n      }\n      boxes.forEach(function(cb){ cb.addEventListener('change', function(){ refresh(); track('tool_check', { tool: box.getAttribute('data-id') }); }); });\n      refresh();\n    });\n    qsa(document, '.msq-choose').forEach(function(box){\n      var results = qsa(box, '.msq-choice-result');\n      qsa(box, 'input[type=radio]').forEach(function(r){\n        r.addEventListener('change', function(){\n          results.forEach(function(el){ el.hidden = el.getAttribute('data-for') !== r.value; });\n          track('tool_choose', { tool: box.getAttribute('data-id'), choice: r.value });\n        });\n      });\n    });\n  }\n  \/\/ read progress: how far down the article the reader has scrolled, fired once per quarter\n  (function(){\n    var art = root.closest('article') || root.parentNode, fired = {};\n    function check(){\n      var top = art.getBoundingClientRect().top + window.pageYOffset, h = art.offsetHeight || 1;\n      var seen = (window.pageYOffset + window.innerHeight - top) \/ h * 100;\n      [25, 50, 75, 100].forEach(function(m){ if (seen >= m && !fired[m]) { fired[m] = true; track('read_progress', { percent: m }); } });\n    }\n    window.addEventListener('scroll', check, { passive: true }); window.addEventListener('load', check);\n  })();\n  if (document.readyState === 'loading') { document.addEventListener('DOMContentLoaded', initTools); } else { initTools(); }\n})();<\/script>\n<\/div>\n\n\n\n<p class=\"wp-block-paragraph\">People search this phrase from two directions. Some have felt slow since starting the tablets. Others have been on them for years and have just read that they can lower B12. The label, two regulators, two guidelines and a 13 year trial each have something to say, and they do not say the same thing. This page sets them side by side with their dates, then reports what people on the forums describe, which is a third thing again.<\/p>\n\n\n\n<h2 id=\"does-metformin-cause-brain-fog\" class=\"wp-block-heading\">Does metformin cause brain fog?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Not by that name, and not directly in any document found for this page. The current US <a href=\"https:\/\/dailymed.nlm.nih.gov\/dailymed\/drugInfo.cfm?setid=0ff1ba75-3c86-4baf-a6b1-1bb883839866\" target=\"_blank\" rel=\"noopener\">metformin label, December 2025<\/a>, read on DailyMed on 23 October 2026, lists the common reactions from a 141 patient trial: diarrhoea in 53 percent against 12 on placebo, nausea or vomiting in 26 against 8, and asthenia, which is the medical word for weakness or tiredness, in 9 percent against 6. There is no row for memory, concentration or slowed thinking, and the postmarketing section mentions liver injury and nothing about the brain.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The route the label does describe runs through vitamin B12. Metformin can interfere with how B12 is absorbed, and B12 deficiency is a recognised reversible cause of poor concentration and memory, which is why it already has <a href=\"https:\/\/brainfogremedy.com\/articles\/b12-deficiency-brain-fog\/\">its own page on this site<\/a>. So the honest chain is: the medicine can lower B12 in some people over time, low B12 can affect thinking, and nobody has measured how often the whole chain happens in one person. Each link has a number below. The chain as a whole does not.<\/p>\n\n\n\n<h2 id=\"what-does-the-metformin-label-say-about-b12-and-how-has-it-c\" class=\"wp-block-heading\">What does the metformin label say about B12, and how has it changed?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">This is the part the ranking pages skip. The warning is old, but its tone has shifted, and the regulators have moved further than the label. The table reads each document in its own words, with the date of the version read.<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>Document<\/th><th>Date<\/th><th>What it says about B12<\/th><th>What it says about testing<\/th><\/tr><\/thead><tbody><tr><td><a href=\"https:\/\/www.accessdata.fda.gov\/drugsatfda_docs\/label\/2017\/020357s037s039,021202s021s023lbl.pdf\" target=\"_blank\" rel=\"noopener\">Glucophage label, April 2017<\/a><\/td><td>Revised April 2017<\/td><td>B12 fell below normal in about 7 percent of patients over 29 weeks, &#8220;without clinical manifestations&#8221;, and the fall is &#8220;very rarely associated with anemia&#8221;.<\/td><td>Annual blood counts. B12 tests every 2 to 3 years &#8220;may be useful&#8221; in people predisposed to low B12.<\/td><\/tr><tr><td><a href=\"https:\/\/dailymed.nlm.nih.gov\/dailymed\/drugInfo.cfm?setid=0ff1ba75-3c86-4baf-a6b1-1bb883839866\" target=\"_blank\" rel=\"noopener\">metformin label, December 2025<\/a><\/td><td>Revised December 2025<\/td><td>Same 7 percent figure. The fall &#8220;may be associated with anemia&#8221;. The phrase &#8220;without clinical manifestations&#8221; is gone.<\/td><td>&#8220;Measure hematologic parameters on an annual basis and vitamin B12 at 2 to 3 year intervals in patients on metformin.&#8221;<\/td><\/tr><tr><td><a href=\"https:\/\/www.gov.uk\/drug-safety-update\/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk\" target=\"_blank\" rel=\"noopener\">MHRA Drug Safety Update, 20 June 2022<\/a><\/td><td>20 June 2022<\/td><td>Reduced B12 is now a &#8220;common&#8221; side effect, meaning up to 1 in 10. Risk rises with dose and years on it. Lists cognitive impairment among the symptoms of deficiency.<\/td><td>Test B12 &#8220;if deficiency is suspected&#8221;. &#8220;Consider periodic vitamin B12 monitoring in patients with risk factors.&#8221;<\/td><\/tr><tr><td><a href=\"https:\/\/www.nice.org.uk\/guidance\/ng239\" target=\"_blank\" rel=\"noopener\">NICE guideline NG239, March 2024<\/a><\/td><td>6 March 2024<\/td><td>Metformin sits in the list of medicines that are a reason to consider testing for B12 deficiency, beside proton pump inhibitors and some anti seizure drugs.<\/td><td>Points clinicians to the MHRA advice. No fixed interval.<\/td><\/tr><tr><td><a href=\"https:\/\/diabetesjournals.org\/care\/article\/49\/Supplement_1\/S50\/163924\/3-Prevention-or-Delay-of-Diabetes-and-Associated\" target=\"_blank\" rel=\"noopener\">ADA Standards of Care 2026<\/a><\/td><td>January 2026<\/td><td>Metformin use is linked to B12 deficiency and can worsen neuropathy symptoms.<\/td><td>&#8220;Consider periodic assessment of vitamin B12 levels&#8221; on long term metformin, especially with anaemia or neuropathy; the older adults section says yearly after 4 years.<\/td><\/tr><\/tbody><\/table><figcaption class=\"wp-element-caption\">Read on 23 October 2026 from the documents linked in each row: the brand label as revised in April 2017, kept at the FDA archive, and a generic metformin label on DailyMed revised in December 2025. The ADA wording comes from the 2026 Standards sections on prevention and on older adults, which do not agree with each other on the interval.<\/figcaption><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">Three changes are worth noticing. The 2017 brand label called the B12 fall a laboratory finding &#8220;without clinical manifestations&#8221; and said testing &#8220;may be useful&#8221; in people already inclined to. The 2025 generic label drops the reassurance and tells prescribers to measure B12 every 2 to 3 years in everyone on the drug. And the <a href=\"https:\/\/www.gov.uk\/drug-safety-update\/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk\" target=\"_blank\" rel=\"noopener\">MHRA Drug Safety Update, 20 June 2022<\/a> went further than either: reduced B12 became common, reduced cognitive function was named among the symptoms of deficiency, and patients were told to seek advice for &#8220;extreme tiredness, a sore and red tongue, pins and needles, or pale or yellow skin&#8221;. The same advice says &#8220;do not stop your treatment without first discussing this with your doctor&#8221;.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1376\" height=\"768\" src=\"https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/reading-patient-leaflet-about-metformin-brain-fog.jpg\" alt=\"Close up of hands holding a paper pamphlet, researching metformin brain fog risks.\" class=\"wp-image-850\" srcset=\"https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/reading-patient-leaflet-about-metformin-brain-fog.jpg 1376w, https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/reading-patient-leaflet-about-metformin-brain-fog-300x167.jpg 300w, https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/reading-patient-leaflet-about-metformin-brain-fog-1024x572.jpg 1024w, https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/reading-patient-leaflet-about-metformin-brain-fog-768x429.jpg 768w\" sizes=\"auto, (max-width: 1376px) 100vw, 1376px\" \/><figcaption class=\"wp-element-caption\">Checking the patient information leaflet can reveal important updates about vitamin levels.<\/figcaption><\/figure>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1600\" height=\"900\" src=\"https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/metformin-brain-fog-chart.png\" alt=\"Timeline of metformin brain fog B12 advice from the 1995 approval to the December 2025 label.\" class=\"wp-image-857\" srcset=\"https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/metformin-brain-fog-chart.png 1600w, https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/metformin-brain-fog-chart-300x169.png 300w, https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/metformin-brain-fog-chart-1024x576.png 1024w, https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/metformin-brain-fog-chart-768x432.png 768w, https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/metformin-brain-fog-chart-1536x864.png 1536w\" sizes=\"auto, (max-width: 1600px) 100vw, 1600px\" \/><figcaption class=\"wp-element-caption\">Dates from the FDA approval record, the April 2017 and December 2025 labels, the MHRA update of 20 June 2022 and NICE NG239 of 6 March 2024, all linked in the table above. Read on 23 October 2026.<\/figcaption><\/figure>\n\n\n\n<h2 id=\"how-often-does-b12-fall-on-metformin-and-does-it-matter-for\" class=\"wp-block-heading\">How often does B12 fall on metformin, and does it matter for thinking?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">The best long run numbers come from the Diabetes Prevention Program Outcomes Study, where 2,155 adults with prediabetes were randomised to metformin or placebo and followed for 13 years. <a href=\"https:\/\/pmc.ncbi.nlm.nih.gov\/articles\/PMC4880159\/\" target=\"_blank\" rel=\"noopener\">Aroda and colleagues, 2016<\/a> reported the B12 results.<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>Measure in the DPP Outcomes Study<\/th><th>Metformin group<\/th><th>Placebo group<\/th><\/tr><\/thead><tbody><tr><td>Low B12 at 5 years<\/td><td>4.3%<\/td><td>2.3%<\/td><\/tr><tr><td>Low or borderline low B12 at 5 years<\/td><td>19.1%<\/td><td>9.5%<\/td><\/tr><tr><td>Low B12 at 13 years<\/td><td>7.4%<\/td><td>5.4%<\/td><\/tr><tr><td>Low or borderline low B12 at 13 years<\/td><td>20.3%<\/td><td>15.6%<\/td><\/tr><tr><td>Anaemia at 5 years<\/td><td>14.3%<\/td><td>10.5%<\/td><\/tr><\/tbody><\/table><figcaption class=\"wp-element-caption\">From <a href=\"https:\/\/pmc.ncbi.nlm.nih.gov\/articles\/PMC4880159\/\" target=\"_blank\" rel=\"noopener\">Aroda and colleagues, 2016<\/a> in the Journal of Clinical Endocrinology and Metabolism. Low meant 203 pg\/mL or under, borderline low 204 to 298. B12 was measured in 858 people on metformin and 857 on placebo at 5 years, and 764 and 756 at 13 years. The 13 year low B12 difference did not reach statistical significance; the others did.<\/figcaption><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">The risk was highest when borderline cases were included, reaching twice the placebo rate after five years. The longer people took the medicine, the higher their risk of deficiency. However, the study did not test memory or concentration.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The one study that asked that question directly is small and old. <a href=\"https:\/\/europepmc.org\/article\/MED\/24009301\" target=\"_blank\" rel=\"noopener\">Moore and colleagues, 2013<\/a> looked at 126 people with diabetes inside a larger Australian memory study and found worse cognitive test scores in those on metformin, with an odds ratio of 2.23 and a confidence interval that only just cleared 1. The authors called for prospective trials, which is the polite way of saying the result needs checking. <\/p>\n\n\n\n<h2 id=\"does-metformin-make-thinking-worse-or-better-over-the-years\" class=\"wp-block-heading\">Does metformin make thinking worse or better over the years?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Here the evidence points the other way, which the forums have noticed and the worried searcher has not. <a href=\"https:\/\/pmc.ncbi.nlm.nih.gov\/articles\/PMC9453211\/\" target=\"_blank\" rel=\"noopener\">Zhang and colleagues, 2022<\/a> pooled 28 observational studies and found people with diabetes on metformin had a slightly lower rate of cognitive impairment, with an adjusted hazard ratio of 0.92, and of dementia at 0.90, with no effect on Alzheimer&#8217;s disease. The studies disagreed widely, all were observational, and people who stay on metformin for years tend to be healthier than people who cannot, which flatters the drug. The authors asked for randomised trials. So did Moore. Nobody has run one large enough to answer either.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Reports to the FDA are a third kind of evidence, weaker than both. The <a href=\"https:\/\/open.fda.gov\/data\/faers\/\" target=\"_blank\" rel=\"noopener\">adverse event reporting system<\/a> holds voluntary reports from doctors, companies and patients, and the counts for terms near brain fog look like this.<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>Reaction term in reports naming metformin<\/th><th>Reports<\/th><th>Rank among all terms<\/th><\/tr><\/thead><tbody><tr><td>Fatigue<\/td><td>4,355<\/td><td>4th<\/td><\/tr><tr><td>Dizziness<\/td><td>3,091<\/td><td>12th<\/td><\/tr><tr><td>Confusional state<\/td><td>1,389<\/td><td>40th<\/td><\/tr><tr><td>Anaemia<\/td><td>1,240<\/td><td>49th<\/td><\/tr><tr><td>Somnolence<\/td><td>1,077<\/td><td>63rd<\/td><\/tr><tr><td>Feeling abnormal<\/td><td>1,055<\/td><td>64th<\/td><\/tr><\/tbody><\/table><figcaption class=\"wp-element-caption\">Counts pulled from the <a href=\"https:\/\/api.fda.gov\/drug\/event.json?search=patient.drug.openfda.generic_name:%22metformin+hydrochloride%22&amp;count=patient.reaction.reactionmeddrapt.exact&amp;limit=80\" target=\"_blank\" rel=\"noopener\">openFDA adverse event query<\/a> on 23 October 2026 across 89,999 reports naming metformin, data last updated 30 July 2026. Memory impairment is not in the top 80 terms. The FDA says every count is unvalidated: a report is not proof the medicine caused the problem, and most list other medicines too.<\/figcaption><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">Fatigue sits fourth, behind only diarrhoea, nausea and lactic acidosis, which matches the asthenia row on the label. Memory complaints, the thing the B12 warning is about, are not in the top 80. That could mean they are rare, or that nobody connects a slow memory to a tablet they have taken for eight years. The reports cannot tell those apart.<\/p>\n\n\n\n<h2 id=\"what-do-people-taking-metformin-report-about-brain-fog\" class=\"wp-block-heading\">What do people taking metformin report about brain fog?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Three threads and one comment search on Reddit were read for this page, posted between May and October 2026, carrying twelve people&#8217;s own accounts. They are accounts, not findings, and nearly all come from people prescribed the medicine for polycystic ovary syndrome or insulin resistance rather than type 2 diabetes. Nothing in them tells a reader what is happening to them.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The surprise is the direction. Seven of the twelve described their fog getting better on the medicine. In <a href=\"https:\/\/www.reddit.com\/r\/PCOS\/comments\/1w67958\/did_you_experience_any_differences_in_energy\/\" target=\"_blank\" rel=\"noopener\">a thread from September 2026<\/a> someone three months in said the clearing was gradual, until one day the heavy feeling behind the eyes had gone, with the energy following about six weeks later; a second reply reported a large change but only once they reached 1,500 mg. In <a href=\"https:\/\/www.reddit.com\/r\/Perimenopause\/comments\/1wq9ndj\/executive_function_in_the_toilet\/pc2bbly\/\" target=\"_blank\" rel=\"noopener\">a comment from September 2026<\/a> a person given metformin for migraines linked to insulin resistance said the migraines stopped eleven days in and the fog cleared by most of the way at the same time. In <a href=\"https:\/\/www.reddit.com\/r\/cfs\/comments\/1tefjd9\/pretty_significant_improvement_on_metformin\/\" target=\"_blank\" rel=\"noopener\">a thread from May 2026<\/a> a person with chronic fatigue and PCOS described the &#8220;concussed feeling&#8221; lifting within weeks, and creeping back whenever stomach side effects forced them to skip doses.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Four described the opposite, and all four were in the first weeks. In <a href=\"https:\/\/www.reddit.com\/r\/MetforminUsers\/comments\/1tydjij\/people_who_felt_tired_on_metformin_when_did_you\/\" target=\"_blank\" rel=\"noopener\">a thread from June 2026<\/a> the poster felt very tired ten days into an extended release tablet; a reply at two weeks felt sluggish, the poster came back a fortnight later to say they felt much better, a third person called the first two months awful, and a fourth at day thirteen wanted to give up. One account cut against both patterns: in <a href=\"https:\/\/www.reddit.com\/r\/PCOS\/comments\/1x9aaqs\/im_so_jealous_of_people_without_pcos\/phjojwz\/\" target=\"_blank\" rel=\"noopener\">a comment from October 2026<\/a> a person with PCOS listed exhaustion and fog among symptoms that metformin had done nothing for.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Where the accounts agree with the label: tiredness early on, which matches the asthenia row and the fatigue count. Where they go beyond it: the improvement reports, which fit the observational studies above but which the label does not claim. Where they are silent: not one of the twelve mentioned B12, a blood test, or fog arriving after years on the drug. That is the gap the regulators worry about, and it is invisible from the inside, because a slowly falling blood level does not feel like anything until it does.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1376\" height=\"768\" src=\"https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/doctor-discussion-regarding-metformin-brain-fog.jpg\" alt=\"A patient discussing metformin brain fog symptoms with their doctor during a checkup.\" class=\"wp-image-851\" srcset=\"https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/doctor-discussion-regarding-metformin-brain-fog.jpg 1376w, https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/doctor-discussion-regarding-metformin-brain-fog-300x167.jpg 300w, https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/doctor-discussion-regarding-metformin-brain-fog-1024x572.jpg 1024w, https:\/\/brainfogremedy.com\/articles\/wp-content\/uploads\/2026\/10\/doctor-discussion-regarding-metformin-brain-fog-768x429.jpg 768w\" sizes=\"auto, (max-width: 1376px) 100vw, 1376px\" \/><figcaption class=\"wp-element-caption\">A simple conversation with your prescriber can clarify if B12 testing is necessary.<\/figcaption><\/figure>\n\n\n\n<h2 id=\"what-should-you-ask-the-prescriber-about-b12-monitoring\" class=\"wp-block-heading\">What should you ask the prescriber about B12 monitoring?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">The regulators agree the person to ask is whoever prescribes the tablets, and the evidence says the question often goes unasked. <a href=\"https:\/\/pmc.ncbi.nlm.nih.gov\/articles\/PMC12924288\/\" target=\"_blank\" rel=\"noopener\">Parsonage and colleagues, 2025<\/a> checked nine English GP practices covering about 148,000 patients and found the share of metformin users with a B12 test in a given year rose from 34.5 percent before the June 2022 safety update to 38.2 percent after it. A companion <a href=\"https:\/\/pmc.ncbi.nlm.nih.gov\/articles\/PMC12918417\/\" target=\"_blank\" rel=\"noopener\">survey of 124 primary care clinicians, 2026<\/a> found 87 percent knew of the link, 39 percent tested yearly, 55 percent tested only when a patient seemed symptomatic, and 36 percent thought the guidance was clear.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">So the useful questions are factual: when B12 was last measured, whether the result sat in the low or borderline range the trial used, how long you have been on the medicine and at what dose, and whether anything on the MHRA&#8217;s risk list applies, such as a vegan diet, a stomach operation, or a proton pump inhibitor taken alongside. None of that decides anything on its own. The MHRA says a person can keep taking metformin while a low level is corrected, that correction is for the clinician to manage, and that the symptoms it lists can also come from diabetes itself or from something unrelated. If fog started in the first weeks, say so, because the prescriber will want the timing. And since <a href=\"https:\/\/brainfogremedy.com\/articles\/brain-fog-causes\/\">the usual causes of brain fog<\/a> do not pause while a tablet is being blamed, it is worth saying what else has changed, including <a href=\"https:\/\/brainfogremedy.com\/articles\/brain-fog-lack-of-sleep\/\">how you are sleeping<\/a>, before the appointment becomes a conversation about the medicine alone.<\/p>\n\n\n\n<div class=\"msq-tool msq-check\" data-id=\"metformin-b12-questions\" data-verb=\" to ask\">\n<h3>Questions to bring to the prescriber about B12 monitoring<\/h3>\n<p>Tick the ones you want answered at the next review. This list asks for facts and decides nothing; it names no dose and no supplement.<\/p>\n<label><input type=\"checkbox\">When was my vitamin B12 last measured, and what was the result?<\/label>\n<label><input type=\"checkbox\">Was that result in the low or borderline range, or comfortably normal?<\/label>\n<label><input type=\"checkbox\">How long have I been on metformin, and at what daily dose?<\/label>\n<label><input type=\"checkbox\">Does anything on the MHRA risk list apply to me, such as a vegan diet, a stomach or bowel operation, or a proton pump inhibitor?<\/label>\n<label><input type=\"checkbox\">Have I had a full blood count in the last year, as the US label advises?<\/label>\n<label><input type=\"checkbox\">Which interval will you use for repeat B12 tests, and will it be added to my diabetes review?<\/label>\n<label><input type=\"checkbox\">If my fog started in the first weeks on the tablets, does the timing change what you think?<\/label>\n<label><input type=\"checkbox\">What else could explain my symptoms, including sleep, thyroid and mood?<\/label>\n<p class=\"msq-check-count\"><\/p>\n<p class=\"msq-note\">General information only. The MHRA says not to stop metformin without first discussing it with your doctor, and that any correction of a low B12 level is for the clinician to manage.<\/p>\n<\/div>\n\n\n\n<h2 id=\"frequently-asked-questions\" class=\"wp-block-heading\">Frequently asked questions<\/h2>\n\n\n<div id=\"rank-math-faq\" class=\"rank-math-block\">\n<div class=\"rank-math-list \">\n<div id=\"faq-q-1\" class=\"rank-math-list-item\">\n<h3 class=\"rank-math-question \">Is metformin brain fog a known side effect?<\/h3>\n<div class=\"rank-math-answer \">\n\n<p>Not under that name. The US label lists tiredness in 9 percent of trial patients against 6 on placebo and a fall in B12 in about 7 percent over 29 weeks. It has no row for memory or concentration. The UK regulator lists cognitive impairment as a symptom of the B12 deficiency the medicine can cause.<\/p>\n\n<\/div>\n<\/div>\n<div id=\"faq-q-2\" class=\"rank-math-list-item\">\n<h3 class=\"rank-math-question \">How long does it take for metformin to lower B12?<\/h3>\n<div class=\"rank-math-answer \">\n\n<p>The label figure comes from 29 week trials. In the 13 year Diabetes Prevention Program study the gap between metformin and placebo was already present at 5 years, and the odds of deficiency rose by about 13 percent for each year on the medicine.<\/p>\n\n<\/div>\n<\/div>\n<div id=\"faq-q-3\" class=\"rank-math-list-item\">\n<h3 class=\"rank-math-question \">Does the metformin label say to test B12?<\/h3>\n<div class=\"rank-math-answer \">\n\n<p>The current US label says to measure B12 at 2 to 3 year intervals in patients on metformin. The 2017 brand label said such tests may be useful in people already predisposed. The UK regulator says to test if deficiency is suspected and to consider periodic monitoring in people with risk factors.<\/p>\n\n<\/div>\n<\/div>\n<div id=\"faq-q-4\" class=\"rank-math-list-item\">\n<h3 class=\"rank-math-question \">Does metformin protect the brain instead?<\/h3>\n<div class=\"rank-math-answer \">\n\n<p>A 2022 pooled analysis of 28 observational studies found slightly lower rates of cognitive impairment and dementia in people with diabetes on metformin, and no effect on Alzheimer&#8217;s disease. The studies disagreed widely and none was randomised, so the authors asked for trials.<\/p>\n\n<\/div>\n<\/div>\n<div id=\"faq-q-5\" class=\"rank-math-list-item\">\n<h3 class=\"rank-math-question \">Should I stop metformin if I feel foggy?<\/h3>\n<div class=\"rank-math-answer \">\n\n<p>The MHRA patient advice says not to stop treatment without first discussing it with your doctor, and that a person can keep taking metformin while a low B12 level is being corrected. The decision belongs to the prescriber.<\/p>\n\n<\/div>\n<\/div>\n<div id=\"faq-q-6\" class=\"rank-math-list-item\">\n<h3 class=\"rank-math-question \">Does the extended release tablet cause less fog?<\/h3>\n<div class=\"rank-math-answer \">\n\n<p>No document found for this page compares the two forms on thinking. The label lists the same B12 warning for both. The accounts read were split on which form people were taking and did not compare them.<\/p>\n\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n\n\n<div class=\"msq-end\" id=\"ms-quiz-end\">\n<p><strong>Finished the page?<\/strong> The same five questions from the top, to see what stuck.<\/p>\n<button type=\"button\" class=\"msq-btn msq-start-test\">Test Your Understanding of This Article<\/button>\n<div class=\"msq-mount\"><\/div>\n<\/div>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<p class=\"wp-block-paragraph\"><em>This article is general information, not medical advice, and is not a substitute for care from a qualified healthcare professional. Speak to your doctor about your own situation.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"<p>The metformin label in 2017 and 2025, the MHRA, NICE and ADA positions on B12 with their dates, the 13 year trial numbers, and what twelve people on Reddit reported.<\/p>\n","protected":false},"author":1,"featured_media":849,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":"","rank_math_title":"Metformin Brain Fog: What Is Known About B12 and Thinking","rank_math_description":"Metformin brain fog is not a label term. The label puts a B12 fall at about 7 percent of trial patients, and the UK regulator now calls low B12 common.","rank_math_focus_keyword":"metformin brain fog","rank_math_canonical_url":"","rank_math_breadcrumb_title":"","rank_math_facebook_title":"","rank_math_facebook_description":"","rank_math_facebook_image":"","rank_math_twitter_title":"","rank_math_twitter_description":"","rank_math_twitter_image":"","rank_math_pillar_content":"","rank_math_robots":[],"rank_math_advanced_robots":[]},"categories":[2],"tags":[],"class_list":["post-852","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-what-causes-it"],"_links":{"self":[{"href":"https:\/\/brainfogremedy.com\/articles\/wp-json\/wp\/v2\/posts\/852","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/brainfogremedy.com\/articles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/brainfogremedy.com\/articles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/brainfogremedy.com\/articles\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/brainfogremedy.com\/articles\/wp-json\/wp\/v2\/comments?post=852"}],"version-history":[{"count":6,"href":"https:\/\/brainfogremedy.com\/articles\/wp-json\/wp\/v2\/posts\/852\/revisions"}],"predecessor-version":[{"id":876,"href":"https:\/\/brainfogremedy.com\/articles\/wp-json\/wp\/v2\/posts\/852\/revisions\/876"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/brainfogremedy.com\/articles\/wp-json\/wp\/v2\/media\/849"}],"wp:attachment":[{"href":"https:\/\/brainfogremedy.com\/articles\/wp-json\/wp\/v2\/media?parent=852"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/brainfogremedy.com\/articles\/wp-json\/wp\/v2\/categories?post=852"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/brainfogremedy.com\/articles\/wp-json\/wp\/v2\/tags?post=852"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}