Vitamin B12 and Medications: What Depletes It and Why
Vitamin B12 deficiency is more common than most people expect, partly because it can take years to develop — the body stores several years' worth — and partly because two of the most widely prescribed drug classes in the world interfere with how B12 is absorbed. If you take metformin for diabetes, or a proton pump inhibitor for acid reflux or ulcers, your B12 absorption may be lower than it would otherwise be. How much lower depends on the medication, the dose, and how long you've been taking it.
Metformin and B12 depletion
Metformin, the first-line oral medication for type 2 diabetes, is one of the most prescribed drugs in the world. Its main mechanism — reducing hepatic glucose output and improving insulin sensitivity — is well understood. Less commonly discussed is its effect on vitamin B12 absorption.
The absorption of dietary B12 depends on a specific transport system in the small intestine. B12 from food binds to intrinsic factor (a protein produced by stomach cells), and the resulting complex attaches to calcium-dependent receptors at the ileal membrane for uptake. Metformin appears to interfere with this calcium-dependent step, impairing how much B12 gets taken in. The effect is dose-related and accumulates gradually — it doesn't cause a sudden drop in B12, but over months and years of treatment, levels can drift downward.
The NIH ODS Vitamin B12 fact sheet documents this interaction directly, noting that long-term metformin use has been associated with reduced B12 absorption in clinical studies, and that B12 blood levels should be monitored in people on long-term metformin therapy. The risk is higher in people who already have limited B12 intake — vegans and vegetarians, for instance — or in older adults, who absorb B12 less efficiently even without medication interference.
Because B12 deficiency affects nerve function and can cause symptoms that overlap with diabetic neuropathy (tingling, numbness, cognitive changes), it can be easy to misattribute deficiency symptoms to the underlying disease rather than the medication. Monitoring B12 levels periodically is a reasonable precaution for anyone on long-term metformin.
Proton pump inhibitors and B12 absorption
Proton pump inhibitors — omeprazole, lansoprazole, esomeprazole, pantoprazole, and similar drugs — are prescribed for acid reflux, peptic ulcers, and related conditions. They work by blocking the gastric proton pump (H+/K+ ATPase), substantially reducing gastric acid production. That's useful for treating acid-related conditions, but gastric acid plays a necessary role in B12 absorption: it cleaves B12 from the proteins in food, releasing it so it can bind to intrinsic factor.
When acid secretion is suppressed, protein-bound B12 from food isn't released efficiently. Crystalline B12 — the form used in most supplements and fortified foods — doesn't require acid for absorption and isn't affected the same way. So PPIs can impair B12 absorption from food while leaving supplement absorption largely intact.
The NIH ODS fact sheet notes that several studies have found reduced B12 levels in people on long-term PPI therapy, with the effect becoming more pronounced at higher doses and longer durations. As with metformin, the concern is most relevant for people who take these medications for years rather than the weeks typically recommended for acute treatment.
Other medications with B12 interactions
H2 blockers (famotidine, ranitidine, cimetidine) also reduce gastric acid, though less potently than PPIs, and have a similar but generally smaller effect on B12 absorption from food. Cholestyramine, the bile acid sequestrant, can bind B12 in the gut and reduce absorption, though this is less commonly flagged than the metformin and PPI interactions.