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Does metformin lower vitamin B12?

Yes, for some people over time. The warning is on the drug's label: metformin may bring vitamin B12 levels down, and the label advises checking B12 every 2 to 3 years. In a trial lasting 4.3 years, B12 deficiency was 7.2 percentage points more common with metformin than with placebo. Low B12 can cause tiredness, anemia and numb or tingling feet. Ask your prescriber whether you need a B12 test or a supplement.

How well documented: On the drug’s label. The medicine’s FDA-approved label names this supplement, or the kind of product it is.

Last reviewed

How we grade evidenceEditorial policy

What's known

  • On the label. The metformin label says it may bring vitamin B12 levels down, possibly by blocking how B12 is absorbed in the gut. In trials lasting 29 weeks, about 7% of people with normal B12 fell below normal. People who get too little B12 or calcium, or absorb them poorly, are more prone. The label tells prescribers to check blood counts every year and B12 every 2 to 3 years, and says the drop can reverse with B12 supplements. (metformin label)
  • Drug-level studies. In a trial of 390 adults with type 2 diabetes who used insulin, metformin or a placebo was added for 4.3 years. B12 ended 19% lower with metformin. B12 deficiency was 7.2 percentage points more common, about one extra case for every 14 people treated. (de Jager 2010)
  • Long-term use. In the Diabetes Prevention Program, low or borderline-low B12 was found in 19.1% of people on metformin and 9.5% on placebo after 5 years, and in 20.3% and 15.6% after 13 years. Each year of metformin use raised the odds of deficiency by about 13%. Nerve damage (neuropathy) was more common in metformin users with low B12. (Aroda 2016)
  • Why it matters. Low B12 can cause anemia, tiredness, a sore tongue, palpitations, and numbness and tingling in the hands and feet. The nerve symptoms can appear without anemia, and symptoms can take years to show. ODS sets no upper limit for B12, since even large amounts are generally considered safe. (ODS)

What to do

  • Ask your prescriber when your B12 was last checked. The label's interval is every 2 to 3 years.
  • Talk to your prescriber or pharmacist before you start B12, so a low level can be confirmed and the right form chosen.
  • See your doctor for numbness or tingling in your hands or feet, unsteady walking, a sore tongue, or tiredness with pale skin.
  • Don't stop or change a prescribed medicine on your own. Metformin's B12 effect is a reason to test, not to quit it.

Sources

  1. US National Library of Medicine. Metformin hydrochloride tablets (Zydus): prescribing information. DailyMed, revised 07/2026. DailyMed
  2. de Jager J, et al. Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency: randomised placebo controlled trial. BMJ. 2010. PubMed 20488910 · doi:10.1136/bmj.c2181
  3. Aroda VR, et al. Long-term Metformin Use and Vitamin B12 Deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab. 2016. PubMed 26900641 · doi:10.1210/jc.2015-3754
  4. National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. Updated 2 July 2025. ODS