Metformin causes vitamin B12 deficiency

Metformin, which is being used increasingly to treat polycystic ovary syndrome, significantly increases the likelihood of vitamin B₁₂ deficiency, according to the results of a nested case–control study.

Researchers from China found that as dose and treatment duration of metformin increase, so does the risk of the vitamin shortage. They calculated that each 1-g/d increase of metformin almost triples the risk of vitamin B₁₂ deficiency (OR 2.88; 95% CI; 2.15–3.87; P<0.001). Those using metformin for 3 years or more were almost two-and-a-half times as likely to develop the deficiency as those receiving the agent for less than 3 years (OR 2.39; 95% CI; 1.46–3.91; P=0.001).

The researchers admit that a shortcoming of the study was the use of serum B₁₂ concentrations alone to define the vitamin's deficiency without metabolite measurements for confirmation. Nevertheless, they believe the findings have important screening, monitoring, and prevention implications for patients taking the drug.