Can you take Biguanides with Vitamin B12 (Cobalamin)?

Metformin interferes with the absorption of vitamin B12 in the final section of the small intestine, and over years of use this measurably drains the body's B12. The evidence here is unusually strong for a supplement interaction: a 4.3-year randomised placebo-controlled trial found metformin lowered B12 levels by 19% on average and produced roughly one extra case of outright deficiency for every 14 people treated, and 13 years of follow-up in the Diabetes Prevention Program found the risk of deficiency grew with each additional year of metformin use, with nerve symptoms more common in metformin users whose B12 was low. A meta-analysis of 31 studies confirms about double the risk of B12 deficiency in metformin users, with the fall in B12 depending on both the dose and the duration of treatment. Because the liver stores B12, the shortfall develops silently over years rather than weeks. Taking a B12 supplement is a sensible safeguard here, and this is one of the clearest cases on the site where supplementing is the helpful move rather than the risky one. Two caveats. Metformin impairs absorption, so a standard oral dose relies on the small fraction that crosses the gut wall passively — which is an argument for a proper 1,000 mcg tablet rather than a token amount buried in a multivitamin. And if you take folic acid alongside it, whether separately or in a B-complex, the folate can correct the anaemia of B12 deficiency while nerve damage quietly continues, so supplementing is not a substitute for testing. If you take metformin long-term, ask your prescriber to check your B12 level periodically — the authors of the trial, the Diabetes Prevention Program follow-up, and the meta-analysis all recommend exactly this.

[1], [2], [3]