Metformin is what lowers the B12, not the people who take it
Over 4.3 years against placebo, metformin cut B12 by 19 percent and left 7.2 percentage points more people deficient. Because people were randomised, it is the drug doing it.
What the study found
Metformin added one B12 deficiency for every fourteen users
Over 4.3 years against a placebo, metformin cut B12 by 19 percent and left 7.2 percentage points more people frankly deficient. Pooling shorter trials put the drop at about 57 pmol/L. Because people were randomised, this is the drug doing it rather than something about who takes it.
The caveat · Everyone was also on insulin
Everyone in the trial was on insulin for type 2 diabetes, so the absolute numbers may not carry to metformin alone. Deficiency was called on the blood level alone, not on markers of whether cells were short. The homocysteine rise that would prove a functional cost fell short of significance.