01What did the 29-week Glucophage trials show on B12?
In clinical trials of 29-week duration, a decrease to subnormal levels of previously normal serum vitamin B12 was seen in approximately 7% of patients. That is a 29-week laboratory finding, not a first-week stool count. The people who fell had started with a normal level. The warning sits in the Glucophage prescribing information as vitamin B12 deficiency, next to the lactic acidosis box but not inside it.
The same 29-week US efficacy program also read glucose. This page does not reuse those glucose numbers. Seven percent here is B12 only.
02How often does the label ask for a B12 check?
Hematologic parameters are measured every year. Vitamin B12 is measured at 2- to 3-year intervals in people who stay on metformin. Abnormal results are managed; the label does not treat a low B12 as a trivia finding. Yearly blood counts and the 2- to 3-year B12 draw are two different clocks.
A clinician can draw B12 sooner when anemia, neuropathy symptoms, or a predisposing diet is already on the chart. The two-to-three-year interval is the labelled floor for people without that extra reason. It is not a 29-week repeat of the trial blood.
03Who is more likely to fall below the normal range?
The predisposed group on the label is anyone with inadequate vitamin B12 or calcium intake or absorption. That includes people already low before the first 500 mg, and people whose diet or gut disease already strains that pathway. Calcium sits next to B12 on that predisposition line.
A normal B12 on the start labs does not lock the 7% out. The 7% were people who started with a previously normal level and then fell during those 29-week trials.
04Does the 7% figure apply to the first week of diarrhea?
No. Seven percent is a 29-week B12 finding. First-week diarrhea is a different labelled reaction and lives on the diarrhea page. The 7% figure is a serum B12 change, not a stool count.
A loose stool in week one does not mean B12 has already fallen. B12 is a months-to-years watch on this tablet. Yearly blood counts and a 2- to 3-year B12 draw are that watch. People with poor B12 or calcium intake sit on the predisposed list for the 7% finding.
05If B12 is low, does metformin have to stop?
The fall appears rapidly reversible with discontinuation of Glucophage or with vitamin B12 supplementation. Stopping the tablet is one path. Replacing B12 while staying on metformin is the other path the label names. Both paths are labelled; this page does not pick one. Anemia that came with the low B12 is part of that same reverse.
Which path is used is a clinician call. The metformin shelf does not pick it. Glucose control and the B12 level both sit on that visit. A home B12 gummy does not close the yearly blood count.
06How does metformin interfere with B12 absorption?
The label's working idea is interference with B12 absorption from the B12-intrinsic factor complex. That is a gut-binding step, not a claim that metformin 'uses up' stored B12 in a week. The drop can be associated with anemia. Yearly blood counts are how that anemia is looked for while the tablet continues.
People who already absorb B12 poorly, or who take in little B12 or little calcium, appear predisposed to a subnormal level on this tablet. That predisposition is why a start B12 can still be worth knowing before the first 500 mg.