Does vitamin B12 interact with any medications?

Metformin may reduce B12 absorption and significantly lower blood B12 levels.

Acid-reducing drugs (proton pump inhibitors and H2 blockers) can interfere with absorbing B12 from food and lead to deficiency.

What this means for you

Two very common long-term medicines interfere with getting B12 out of food, and neither of them is usually discussed in those terms.

Metformin can reduce absorption and lower blood levels measurably. A review of 26 papers put the size of it at about 57 pmol/L within six weeks to three months of starting the drug, which is enough to push some people with type 2 diabetes below the deficiency line. Read that figure as an order of magnitude. The pooled number rests on four trials inside a review that is otherwise observational, and the source reports its confidence interval inconsistently.

Acid-reducing drugs, both proton pump inhibitors and H2 blockers, interfere with releasing B12 from food, because that step needs stomach acid.

Not for everyone. Supplement B12 does not depend on stomach acid the way food B12 does, which is why a tablet often works for someone who cannot absorb the vitamin from a meal. That is a decision for a clinician with a blood result in front of them, not a reason to start on your own.

Established. If treatment turns out to be needed, a tablet is usually as good as a needle. Three randomised trials comparing swallowed B12 with injections found no clinically relevant difference in blood levels at 1,000 micrograms a day, and at 2,000 micrograms a day the tablets came out ahead by 680 pg/mL. Pooling 16 studies in 6,098 people, the oral, sublingual and injected routes all raised serum B12 by around 400 pg/mL with no significant difference between them.

Two limits sit on that. None of those three trials recorded whether anyone's symptoms changed, only what the blood showed, and the pooled analysis mixed observational studies in with the randomised ones. If a clinician has chosen injections for a reason, that is a conversation to have rather than a decision to take on your own.

What to do is not to stop either drug. It is to ask for a B12 level if you have been on one of them for years, especially if you are over 50 or eat little meat, fish or dairy. Between 3 and 43 per cent of older adults living in the community are already low before any medicine is added.

The reason this matters more than most interactions is the shape of the damage. B12 deficiency harms nerves, and nerve damage caught late does not fully reverse.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Full guide: Vitamin B12: who actually needs to worry, and why the damage can be permanent