The 7% is a later story than the toilet
In 29-week Glucophage trials, about 7% of people who started with a normal B12 fell into the subnormal range. The mechanism is interference with B12 absorption from the B12-intrinsic-factor complex.
Some of those people become anemic. Some do not. The fall often reverses if metformin stops or B12 is replaced. People with poor intake or absorption sit at higher risk.
Brynza's 500 mg lock is a start strength. Long-term totals of 1000-2000 mg a day are where this sidebar still applies - the absorption problem is not unique to 500 mg, but 500 mg is the column this site will not abandon.
Who should not wait three years
Vegan diet, prior gastric surgery, metformin plus a PPI, and existing anemia are reasons to check sooner than the 2-3 year default.
Metformin plus pregabalin for pain is a reminder to name both causes of numb feet at the same visit.
- Do not stop 500 mg on a forum B12 scare
- Do not skip B12 because stools settled
- Do not treat numb feet as only glucose
- Acidosis symptoms still override this page
B12 is not the boxed warning
Lactic acidosis remains the rare emergency on the GI sidebar. Low B12 is commoner and slower.
People mix them because both live on the metformin page. One is weeks of stool. One is years of absorption. One is a night in resuscitation.
Numb toes have two authors on a 500 mg chart
About 7% of people with previously normal B12 fell low in a 29-week Glucophage program. Interference with the B12-intrinsic-factor complex is the labelled mechanism. Some become anemic. Some do not. The fall often reverses if metformin stops or B12 is replaced. Annual CBC and B12 every two to three years is the labelled pair. Sooner if numb, vegan, post-bariatric, or on a PPI. Numb toes have two authors: diabetes and B12. Pregabalin 75 mg on the same chart treats symptoms; it does not fill a hole.
XR versus IR does not retire the absorption note. Long metformin is long metformin. The 500 mg lock is a start strength. Long totals of 1000-2000 mg still belong on this sidebar. Do not stop 500 mg on a forum scare. Do not ignore a B12 of 140 because A1c looks pretty. Acidosis symptoms override this page. Fast breathing is not a B12 clinic.
Most desks replace B12 and keep metformin if A1c still needs it. Stopping without a plan can bounce glucose. Injections versus tablets are a route after a real deficit. Weekly gym shots without a level are not a 500 mg tax. Glossitis, pale fatigue, and rising MCV are the hematology version. Metformin is not a typical GI bleeder. Still look for blood loss. Folate can travel with the same meals. The prescriber picks the panel.
A B12 of 180 with numb toes is treat-and-recheck. A B12 of 180 with no symptoms still deserves a plan. Family who blame 500 mg for stomach blood need both sentences. GI week-one diarrhea is the other sidebar. Do not mix day-three stool with year-five pins. PPI plus five years of 500-1000 mg is a sooner-check pair. Parent: metformin column. Lyrica fog: pregabalin column.
Seven percent in 29 weeks is why we still draw
About 7% of people with previously normal B12 fell low in a 29-week Glucophage program. Interference with the B12-intrinsic-factor complex is the labelled mechanism.
Some become anemic. Some do not. The fall often reverses if metformin stops or B12 is replaced.
Annual CBC and B12 every two to three years is the labelled pair. Sooner if numb, vegan, post-bariatric, or on a PPI.
Numb toes have two authors: diabetes and B12. Pregabalin 75 mg on the same chart treats symptoms; it does not fill a hole.
XR versus IR does not retire the absorption note. Long metformin is long metformin.
The 500 mg lock is a start strength. Long totals of 1000-2000 mg still belong on this sidebar.
Do not stop 500 mg on a forum scare. Do not ignore a B12 of 140 because A1c looks pretty.
Acidosis symptoms override this page. Fast breathing is not a B12 clinic.
Oral versus injection is a clinic choice
The label says the decrease appears rapidly reversible with discontinuation or supplementation. It does not force a route in this sidebar.
Do not stop 500 mg yourself because a forum fears neuropathy. Do not ignore a B12 of 140 because A1c looks pretty.
Folate can travel with the same meal problems. The prescriber picks the panel.
Replace and keep glucose unless the chart says stop
Most desks replace B12 and keep metformin if A1c still needs it. Stopping without a plan can bounce glucose.
Injections versus tablets are a route after a real deficit. Weekly gym shots without a level are not a 500 mg tax.
Glossitis, pale fatigue, rising MCV are the hematology version. Metformin is not a typical GI bleeder.
Folate can travel with the same meals. The prescriber picks the panel.
A B12 of 180 with numb toes is treat-and-recheck. A B12 of 180 with no symptoms still deserves a plan.
Family who blame 500 mg for stomach blood need both sentences: look for blood loss and look for B12-deficient anemia.
GI week-one diarrhea is the other sidebar. Do not mix day-three stool with year-five pins.
Parent: metformin column. Lyrica fog: pregabalin column.
Who checks B12 before year three
Vegan intake, bariatric surgery, and a PPI plus metformin are reasons to draw earlier than the 2-3 year default. The 7% note in 29 weeks already showed the mechanism can move in months.
Annual CBC still catches anemia. MCV up plus fatigue is a hematology visit, not a 'push the 500 mg'.
Glossitis and pale skin are the old textbook pair. Numb feet are the diabetes-overlap pair. Order the B12.
Folate can travel with the same meals. The prescriber picks the panel. Do not buy a random B-complex megadose as a 500 mg tax.
XR versus IR does not retire the absorption note. Long metformin is long metformin.
If pregabalin 75 mg is on the chart for pain, name both causes of numb feet at the same visit. Fog is Lyrica. Deficiency is B12 until the number says otherwise.
A gym shot is not a measured 500 mg tax
Measure, then treat a real deficit. Unsupervised megadose is not protection. The labelled habit is a number every two to three years, sooner if numb. Hemoglobin that dropped is a CBC plus B12 plus a look for blood loss, not a kitchen stop of Glucophage for bleeding. Metformin is not a typical GI bleeder. B12-deficient anemia is the labelled hematology story.
Vegan households, bariatric surgery, and long PPIs move the draw forward. So does a rising MCV on an annual CBC that nobody read. This desk will not dose cyanocobalamin from a selfie of the tongue.
If Accutane 40 mg lipids are also running, diabetes marks TG risk on that sister page. It does not cancel the B12 cadence here. If 80 mg Lasix cramps are new, ions come first; B12 still gets a date.
Lock remains 500 mg IR. Parent: metformin column. This page stays on the 7% note and the draw.
Replace B12 and keep glucose unless told to stop
The label describes reversal with supplementation or discontinuation. Most desks replace and keep metformin if A1c still needs it.
Stopping 500 mg because a forum fears neuropathy can bounce glucose. That is a clinic trade, not a sidebar dare.
Injections versus tablets are a route choice after a real deficit. Weekly gym shots without a level are not protection.
A B12 of 180 with numb toes is a treat-and-recheck story. A B12 of 180 with no symptoms still deserves a plan.
Family who blame 500 mg for 'stomach blood' need the hematology sentence: metformin is not a typical GI bleeder. Look for blood loss and for B12-deficient anemia.
Acidosis symptoms still override this entire page. Fast breathing is not a B12 clinic.
Numb toes have two authors
Diabetic neuropathy and B12-deficient neuropathy can look like the same socks. Treating only the glucose and ignoring a low B12 on five years of metformin is a missed side effect.
Pregabalin 75 mg (see the Lyrica sidebars) may be on the same chart for pain. That does not excuse a never-checked B12.
Glossitis, pale fatigue, and a rising MCV are the hematology version. Annual blood counts are the labelled habit; B12 every two to three years is the other habit.
| Check | Label instinct | Do not |
|---|---|---|
| CBC yearly | Catch anemia | Wait for a transfusion |
| B12 every 2-3 years | Catch the 7% pattern | Assume 'diabetes feet' only |
| Earlier if numb or vegan | Higher pretest risk | Start megadose B12 from a gym |
500 mg can be quiet and still tax B12
Ask when the last B12 was drawn if you have been on metformin longer than a football season.
GI week-one: GI sidebar. Parent: metformin column.
Sources
- Glucophage PI 5.2 - ~7% subnormal B12 in 29-week trials; intrinsic-factor interference
- Label - hematologic parameters annually; B12 every 2-3 years
- Reversible with stop or B12 replacement in the labelled description
Checked against the current label and reviewed by Dr. Tomasz Krajewski. See Brief, Set, Proof, Issue.