What this means in practice
If you have been on metformin for years, this is a reason to have B12 checked, and to ask for methylmalonic acid rather than accept a normal serum B12 as reassurance. It is not by itself a reason to buy a supplement, and it is not a reason to stop metformin. Anyone on metformin who develops numbness or tingling should raise B12 specifically, because that symptom is easily written off as diabetic neuropathy.
What people say
Metformin depletes your B12, so anyone on it should be taking a B12 supplement. Some versions go further: the neuropathy blamed on diabetes is really metformin-induced B12 deficiency in disguise.
What the evidence supports
The depletion itself is well established and comes from randomised trials, not just observation. Over 4.3 years, metformin cut blood B12 by 19 percent against placebo and produced 7.2 percentage points more frank deficiency, meaning about one extra deficient person for every fourteen treated. In the Diabetes Prevention Program follow-up, each additional year on metformin raised the odds of deficiency by 13 percent, and a meta-analysis put the average drop at 57 pmol per litre within six weeks to three months. The neuropathy strand is thinner but not empty: in that same follow-up, neuropathy was more common among metformin users whose B12 was low. What is missing is the last step. No trial in this collection shows that routinely supplementing B12 alongside metformin prevents symptoms or improves outcomes, as opposed to correcting a laboratory number.
How it is supposed to work
Metformin reduces how much vitamin B12 the body takes up from food, and the shortfall accumulates with years of use rather than appearing at once. The precise step it interferes with is not settled by the sources collected here, which establish that absorption falls without pinning down the mechanism; impaired uptake of the vitamin and intrinsic factor complex in the last part of the small intestine is the usual proposal, and it should be treated as a proposal.
Which way it runs
Runs one way only. Metformin lowers vitamin B12 status; vitamin B12 has no established effect on metformin or on how well it controls blood sugar. Stopping metformin is not the intervention this supports, and nothing here argues against taking metformin.
What is still missing
The popular version and the evidence agree on the depletion and part company on what follows from it. What is proven is a falling number in people who mostly feel fine; what is sold is a supplement that stops that number causing harm, and that second step has not been tested.
Sources
- Long-term Metformin Use and Vitamin B12 Deficiency in the Diabetes Prevention Program Outcomes StudyAroda VR, Edelstein SL, Goldberg RB, Knowler WC, Marcovina SM, Orchard TJ, Bray GA, Schade DS, Temprosa MG, White NH, Crandall JP, 2016
- Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency: randomised placebo controlled trialde Jager J, Kooy A, Lehert P, Wulffelé MG, van der Kolk J, Bets D, Verburg J, Donker AJM, Stehouwer CDA, 2010
- Association between metformin and vitamin B12 deficiency in patients with type 2 diabetes: A systematic review and meta-analysisChapman LE, Darling AL, Brown JE, 2016