Supplements and metformin: what to ask about vitamin B12

September 26, 2026By VitOptics Staff

Short answer: Metformin (Fortamet, Glumetza) may reduce vitamin B12 absorption and lower B12 levels in the blood. Ask your doctor about your B12 status before choosing a supplement. Do not stop or change metformin without talking to your doctor.

NIH advises people who take metformin regularly to discuss their B12 status with their health care providers. The next step depends on your history, symptoms and test results.

Metformin and vitamin B12: what the research shows

Vitamin B12 helps the body make red blood cells and supports the nervous system. Food sources of B12 include fish, meat, eggs, dairy products and foods with B12 added, such as some breakfast cereals.

In a randomized trial in the Netherlands, 390 adults with type 2 diabetes who were already using insulin took metformin or a placebo for 4.3 years. Over the study, average B12 in the metformin group was about 19% lower than in the placebo group, and the gap grew over time. These group results do not predict what will happen to every person taking metformin.

Metformin's prescribing information says it may lower B12 and tells prescribers to check B12 every two to three years. Ask how that schedule applies to you.

Tell your doctor if you have, or have ever had, low B12. MedlinePlus lists it among the precautions for metformin, and the FDA-approved patient information says metformin may lower B12, especially if your levels have been low before. The evidence is a reason to ask about your own status, rather than assume everyone taking metformin needs the same supplement.

Ask about testing before picking a dose

A vitamin B12 blood test measures the amount of B12 in your blood. Laboratories can use different reference ranges, so ask your doctor to explain your result using the range on your report.

Sometimes a clinician uses a methylmalonic acid (MMA) test, on blood or urine, to help evaluate B12 status. Its result is considered alongside symptoms, medical history and other tests.

Useful questions include:

Have I had B12 checked, and when?
Would testing be appropriate now?
Who will explain the result, and when should we discuss it again?

Bring the date and number from any older B12 result. Do not select a supplement dose from another person's lab report or an online comment. Follow the laboratory's preparation instructions, and ask before stopping any medicine for a blood test.

Symptoms to report, and when to call right away

Low B12 can be associated with fatigue; prolonged low levels can also cause numbness or tingling in the hands or feet. Some people have no symptoms. If you notice a change, contact your doctor and describe when it started, how often it happens and whether it is getting worse. Seek a professional assessment rather than using a supplement purchase as your answer, and don't decide in advance that metformin, B12 or a particular product explains it.

Some symptoms need a call right away. Metformin can rarely cause a serious condition called lactic acidosis, and MedlinePlus's metformin page says to stop taking metformin and call your doctor immediately if you have extreme tiredness, weakness or discomfort; nausea; vomiting; stomach pain; decreased appetite; deep and rapid breathing or shortness of breath; dizziness; lightheadedness; a fast or slow heartbeat; muscle pain; or feeling cold, especially in the hands or feet.

Include food, other medicines and past surgery

B12 status is not only about one prescription. Factors associated with a higher chance of low B12 include being over 50, vegetarian or vegan diets, certain stomach or intestinal surgeries, and digestive conditions such as celiac or Crohn's disease.

Some acid-reducing medicines can also interfere with B12 absorption from food. These include proton pump inhibitors such as omeprazole (Prilosec) and H2 blockers such as cimetidine (Tagamet). Include occasional use as well as daily use when you discuss these medicines, your usual eating pattern and any relevant surgery with your clinician.

Keep your medication list together

Bring a list of your prescription medicines, nonprescription medicines and supplements, including each product's name, amount and timing. Include full labels or photographs and anything you are considering but have not started. Use our guide to what to review before starting a new supplement to prepare.

If your list includes a beta-blocker such as metoprolol or carvedilol, our guide to metoprolol, carvedilol and supplements explains what their labels say about low blood sugar. If you also take insulin or a sulfonylurea such as glipizide, see our guide to insulin, sulfonylureas and supplements before you add a product sold for blood sugar.

If your tablet combines metformin with an SGLT2 inhibitor, such as Synjardy, Xigduo XR or Invokamet, our guide to SGLT2 inhibitors and supplements covers what those labels say about low blood sugar, fluids and supplements.

Check for repeated ingredients

B12 can appear in a multivitamin, a B-complex and a separate B12 product. Check all the labels together; our guide explains how to check for duplicate supplement ingredients.

The FDA cautions that combining supplements or taking too much of some ingredients can create problems. Ask what each proposed ingredient is for in your situation.

A product described as natural is not automatically safe to take with your medicines.

Leave with an agreed next step

Write down the product, amount, timing and follow-up you agreed on, including who to contact between appointments. If no supplement is recommended, record that too, and keep the dated plan available for your next medication review.

Where VitOptics fits

VitOptics builds a personalized daily supplement pack from a free, three-minute assessment that asks for your medicines, including metformin, your conditions and the supplements you already take. Every pack is checked against the medications and conditions you enter, by a second automated check or, if your answers need a closer look, by a person on our team. Your result lists every formula, the amount of each ingredient and the reason it is there, including any B12, so you can compare it with what you already take and show it to your doctor or pharmacist before you start.

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Every VitOptics pack is checked against the medications you enter before anything goes in, and your result explains why each item is there.

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Read more in the VitOptics learning hub.

This article is for educational purposes and is not medical advice. Supplements are not intended to diagnose, treat, cure or prevent any disease.

Prepared with AI assistance using the sources linked in this article.

Last updated: September 26, 2026

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