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H2 blockers and vitamin B12: can acid reducers lower B12?

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

Ask your pharmacist or doctor before combining. NIH says H2 blockers such as cimetidine can interfere with absorbing vitamin B12 from food, because food B12 needs stomach acid in the first of two absorption steps, and that this can lead to low B12. B12 in supplements skips that acid step but still needs intrinsic factor.1,2

Key facts

  • NIH groups H2 blockers, such as cimetidine and ranitidine, with proton pump inhibitors as gastric acid inhibitors that can interfere with B12 absorption from food and lead to deficiency.1,2
  • Food B12 needs stomach acid to separate it from protein, which is the first of two absorption steps.2
  • B12 in supplements is not attached to protein, so it skips the acid step, but it still has to combine with intrinsic factor to be absorbed.2
  • The body stores 1,000 to 2,000 times as much B12 as people typically eat in a day, so signs of deficiency can take several years to appear.2
  • NIH says people who take these medicines regularly should discuss their B12 status with their health care providers.1

Your doctor may want to check on you

H2 blockers such as cimetidine also reduce stomach acid and can interfere with absorbing B12 from food, which NIH says can lead to low B12.1,2

Reported in studies

What do H2 blockers do to vitamin B12?

H2 blockers lower the amount of acid the stomach makes. MedlinePlus describes famotidine as an H2 blocker used for ulcers, acid reflux and heartburn, and says it works by decreasing the amount of acid made in the stomach.3

NIH's fact sheet for health professionals groups H2 blockers, such as cimetidine and ranitidine, with proton pump inhibitors as gastric acid inhibitors. It says they can interfere with vitamin B12 absorption from food by slowing the release of gastric acid, and so lead to vitamin B12 deficiency. The consumer sheet says the same.1,2

Does it matter whether the B12 comes from food or a supplement?

The body absorbs B12 from food in two steps: stomach acid separates it from the protein it is attached to, and it then combines with intrinsic factor, a protein made by the stomach.2

B12 in supplements is not attached to protein, so it does not need stomach acid to be released, but it still has to combine with intrinsic factor to be absorbed.2

That difference is why NIH words the concern as absorption from food. The sources on this page do not say that B12 supplements are untouched by acid reducers in every case, and they do not say whether a person using an H2 blocker needs a supplement. Whether to supplement, with what, and how to monitor are questions for the prescriber.1,2

How quickly does low B12 develop with acid-reducing medicines?

NIH gives no time for H2 blockers in particular. It does say the body stores 1,000 to 2,000 times as much vitamin B12 as you would typically eat in a day, so the symptoms of deficiency can take several years to appear. For people who take these medicines regularly, NIH's advice is to discuss their vitamin B12 status with their health care providers.1,2

NIH's consumer sheet lists signs of deficiency that include feeling tired or weak, numb or tingling hands and feet, balance problems, memory problems and a sore mouth or tongue, and says a doctor can test your B12 level. Those signs have many causes, so they are a reason to see a clinician, not a way to judge your own B12.2

Which acid reducers does NIH include?

NIH names the proton pump inhibitors omeprazole and lansoprazole, and the H2 blockers cimetidine and ranitidine. MedlinePlus calls famotidine an H2 blocker too, but the NIH sheets do not name it, so for famotidine the NIH wording applies by class rather than by name. The sources checked do not address antacids that neutralize acid, which is not a finding either way.1,3

NIH also names metformin as a medicine that might reduce B12 absorption, which matters if you take it as well as an acid reducer.2

What to ask your pharmacist or doctor about an H2 blocker and B12

  • Have I taken an acid reducer long enough that my B12 should be checked?
  • If I take a B12 supplement, does the form or amount matter with my medicine?
  • Do I take anything else, such as metformin, that could also affect my B12?
  • Does it change the answer if I eat little or no animal food?

Questions people ask

Do H2 blockers cause B12 deficiency?

NIH says gastric acid inhibitors, including H2 blockers, can interfere with B12 absorption from food and lead to deficiency. The NIH sheets do not say how often it happens or how long it takes.1,2

Is there an H2 blockers and vitamin B12 interaction?

NIH describes one, and it runs from the medicine to your B12 status rather than the other way around. Ask your pharmacist whether your B12 should be checked.1

Does vitamin B12 from supplements get absorbed when stomach acid is low?

NIH says B12 in supplements is not attached to protein and does not need stomach acid for the first step, but it still has to combine with intrinsic factor. Ask your clinician about your own situation.2

How long does it take for an H2 blocker to lower B12?

The sources give no time for H2 blockers. NIH says the body holds large stores of B12, so symptoms of deficiency can take several years to appear.2

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Sources

  1. National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. Updated July 2, 2025. Opened 2026-10-07.
  2. National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Consumers. Updated December 15, 2023. Opened 2026-10-07.
  3. MedlinePlus. Famotidine. MedlinePlus Drug Information, U.S. National Library of Medicine. Last revised January 15, 2022. Opened 2026-10-11.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.