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Medicines and supplements

Vitamin B12 and metformin: what the label and NIH say

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. Metformin may lower vitamin B12 levels: in 29-week trials, about 7% of people with normal levels fell below normal. The metformin label tells prescribers to check B12 every 2 to 3 years, and NIH lists metformin among medicines that can reduce B12 absorption.1,2

Key facts

  • The label suggests the drop comes from interference with absorbing B12 bound to intrinsic factor.1
  • The label says the fall appears to reverse quickly when metformin is stopped or B12 is given.1
  • People with low B12 or calcium intake or absorption appear more likely to develop low B12 on metformin.1
  • MedlinePlus asks you to tell your doctor before starting metformin if you have ever had low B12.3

Your doctor may want to check on you

Metformin may lower vitamin B12 levels, possibly by interfering with its absorption. In 29-week trials described on the metformin label, about 7% of people with normal B12 dropped below normal, and the label tells prescribers to measure B12 every 2 to 3 years.1,2,3,4

On the medicine's label

What is documented

On the medicine's label: the metformin prescribing information lists vitamin B12 deficiency among its warnings. In clinical trials lasting 29 weeks, about 7% of people whose B12 was normal at the start dropped below normal. The label says this may be linked to anemia, and tells prescribers to measure blood counts every year and B12 every 2 to 3 years.1

From NIH: its vitamin B12 fact sheets name metformin as a medicine that might reduce B12 absorption and significantly lower blood B12 levels.2,4

Why it happens

B12 is absorbed at the end of the small intestine after it binds to intrinsic factor, a protein made in the stomach. The label suggests metformin interferes with absorbing that B12 and intrinsic factor pair. People who already take in or absorb little B12 or calcium appear more prone to low levels.1,2

Because the body holds a large store of B12, a drop can build slowly over years of treatment, and the label asks for repeat checks every 2 to 3 years.1,4

Signs of low B12 to mention

Low B12 can cause tiredness, weakness, numbness or tingling in the hands and feet, balance problems and memory trouble, and nerve damage can happen even without anemia. Tell your doctor about any of these rather than guessing the cause.2,4

Acid-reducing medicines, such as omeprazole, can lower B12 as well, so mention them too if you take both.4

What to ask your pharmacist or doctor

  • Has my vitamin B12 been checked, and when is the next test due?
  • Do I need a B12 supplement, and if so, how much and in what form?
  • Does my diet give me enough B12, especially if I eat little meat, fish, eggs or dairy?
  • Do my other medicines, such as acid reducers, affect B12 too?

Bring the labels of any multivitamin or B12 supplement you take, so your doctor can see how much B12 you already get.

Questions people ask

Does metformin cause B12 deficiency?

It can lower B12 in some people. In the label's 29-week trials, about 7% of people with normal levels fell below normal, and the label calls for B12 checks every 2 to 3 years.1

Should I take B12 if I am on metformin?

Ask your doctor. The label calls for regular B12 checks and for managing any abnormal result, and whether you need a supplement, and how much, depends on that test.1

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Sources

  1. Metformin hydrochloride tablets and extended-release tablets: prescribing information (Laurus Labs). DailyMed, U.S. National Library of Medicine. Published September 28, 2026. Opened 2026-10-07.
  2. National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. Updated July 2, 2025. Opened 2026-10-07.
  3. MedlinePlus. Metformin. U.S. National Library of Medicine. Last revised February 15, 2024. Opened 2026-10-07.
  4. National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Consumers. Updated December 15, 2023. Opened 2026-10-07.

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