What to Know About Taking Metformin and Vitamin B12
Key takeaways:
Metformin is a first-choice medication for managing Type 2 diabetes. But it can sometimes lower your vitamin B12 levels.
You can increase the amount of vitamin B12 in your diet to help keep your levels up while taking metformin. But it’s best to avoid taking a B12 supplement unless your prescriber recommends it.
Your prescriber will likely check your vitamin B12 levels at least once a year while you’re taking metformin. They’ll recommend the right form and dose of vitamin B12 based on your individual needs if your levels become too low.
Metformin is a first-choice option to lower blood glucose (sugar) levels in people living with Type 2 diabetes. It’s also often prescribed off-label for conditions such as prediabetes and polycystic ovarian syndrome (PCOS). Metformin is considered a safe and effective medication. But it does have some side effects to consider — and low vitamin B12 levels is one of them.
Here’s what you should know about metformin and vitamin B12 deficiency.
How does metformin affect vitamin B12 levels?
Scientists first discovered that metformin was associated with low vitamin B12 levels in 1969. It happens because metformin can lower the absorption of vitamin B12 from the food you eat.
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Health experts are still studying exactly how metformin lowers vitamin B12 levels, but some of the main theories are:
By causing changes in how calcium binds to vitamin B12 (calcium is required for certain parts of the intestine to absorb vitamin B12)
By causing more vitamin B12 to be held by the liver, so less is available in rest of the body
By lowering amount of intrinsic factor that’s released in your stomach (intrinsic factor is a type of protein needed to absorb vitamin B12)
By slowing the movement of food and increasing the amount of bacteria in the small intestine
It’s also good to note that metformin doesn’t always cause vitamin B12 deficiency. Various studies reported low vitamin B12 levels in a wide range of people taking metformin (from 6% to 50%). But it does seem that taking high doses of metformin for a long time can make low vitamin B12 levels more likely. Other factors can also increase your risk — more on that later.
Does vitamin B12 interfere with metformin?
No, vitamin B12 doesn’t interact with metformin. This means that you can safely increase the amount of vitamin B12 in your diet or take vitamin B12 supplements with metformin if your prescriber recommends it.
However, it’s always a good idea to review your current list of prescription and over-the-counter (OTC) medications with your healthcare team before making any changes. This helps your prescriber and pharmacist make sure there aren’t any interactions or other issues of concern.
What happens if your vitamin B12 levels are too low?
If vitamin B12 levels drop too low, it can lead to health problems, such as nerve damage and anemia (low red blood cells).
Vitamin B12 — also known as cobalamin — helps support your body’s blood and nerve cells. It also helps your body make DNA. In most cases, you can get enough vitamin B12 through your diet from food and drinks, such as:
Beef
Seafood
Poultry
Dairy products
Nutritional yeast
Fortified breakfast cereals
Plant-based milks, such as almond, coconut, and soy milk
In addition to taking metformin, your risk of low vitamin B12 levels goes up if you:
Are age 65 or older
Eat a vegan or vegetarian diet
Have had weight-loss surgery
Have had intestinal surgery
Take acid-reducing medications, such as famotidine (Pepcid) or omeprazole (Prilosec)
Metformin alternatives: If metformin isn’t a good choice for you, several other medications are available to help you manage Type 2 diabetes.
Metformin side effects: Some side effects are common when taking metformin. Learn more about what you can expect while taking this medication.
Is your B12 level low? Learn about the symptoms of low vitamin B12 levels in the body to help you know when to seek medical care.
How do you know if your vitamin B12 levels are low?
Blood tests are the most reliable way to tell if your vitamin B12 levels are low.
Vitamin B12 levels lower than 200 pg/mL generally suggest a vitamin B12 deficiency. Your healthcare team may also check methylmalonic acid (MMA) levels in your blood or urine. Your body makes extra MMA if it doesn’t have enough vitamin B12. So a high MMA level can also indicate vitamin B12 deficiency.
Blood tests aside, symptoms you’re experiencing can also help point to low vitamin B12 levels. These include:
Pale skin
Feeling weak or more tired than usual
Dizziness
Fast or irregular heartbeat
Tingling in the feet and hands (nerve pain)
Balance or coordination problems
Contact your healthcare team if you’re experiencing any of these symptoms. They can check your vitamin B12 levels to help determine if that’s the cause.
How often should your vitamin B12 levels be monitored if you take metformin?
If you’ve been prescribed metformin, the American Diabetes Association recommends that your vitamin B12 levels should be checked at least once a year. But they may decide to check you more often if you’re at a higher risk for low B12 levels or develop symptoms.
Should you take vitamin B12 supplements if you take metformin?
It’s best to only take vitamin B12 supplements if your healthcare prescriber tells you to. They can recommend the right dosage and dosage form for your needs if your levels are low.
If you’re concerned about your vitamin B12 levels, increasing the amount of vitamin B12 in your diet is a good first step you can take on your own.
Vitamin B12 supplements and medications
If you're instructed to take a vitamin B12 supplement, that’s the time to take action.
Vitamin B12 is available OTC as a dietary supplement. It typically comes in the form of cyanocobalamin in oral pills and sublingual (under the tongue) tablets or liquid. B-complex products also typically contain vitamin B12 along with other B vitamins. You can find these products at your local pharmacy, grocery store, or online.
Vitamin B12 is also available by prescription in the form of a nasal spray (Nascobal) and an injection (Dodex).
Some studies suggest that oral and injectable forms of vitamin B12 have similar effectiveness. Results from one study showed that sublingual vitamin B12 works as well and may even be more effective than the injection. But vitamin B12 injections are typically absorbed better and work faster for people with severe deficiency caused by absorption issues.
Your healthcare team will help determine the best form and dose of vitamin B12 for you based on your individual needs.
The bottom line
Metformin is a first-choice medication for managing blood glucose levels in people with Type 2 diabetes. But in some cases, metformin can cause vitamin B12 deficiency. Your prescriber will likely monitor your vitamin B12 levels if you’re taking metformin. But be sure to let them know if symptoms appear, such as tingling in your hands and feet or unusually low energy levels.
Since metformin doesn’t always cause low vitamin B12 levels, you don’t always need to take a vitamin B12 supplement along with it. But increasing the amount of vitamin B12 in your diet is a good idea. If your levels become low, your prescriber will help find the best way to add vitamin B12 — through an over the counter supplement or prescription product — based on your individual needs.
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References
American Diabetes Association. (2020). Comprehensive medical evaluation and assessment of comorbidities: Standards of medical care in diabetes—2021. Diabetes Care.
American Diabetes Association Professional Practice Committee. (2024). Pharmacologic approaches to glycemic treatment: Standards of care in diabetes—2025. Diabetes Care.
Aroda, V. R., et al. (2016). Long-term metformin use and vitamin B12 deficiency in the diabetes prevention program outcomes study. The Journal of Clinical Endocrinology and Metabolism.
Carmel, R. (2008). How I treat cobalamin (vitamin B12) deficiency. Blood.
Dietary Supplement Label Database. (n.d.) B-complex. Office of Dietary Supplements.
Infante, M., et al. (2021). Long-term metformin therapy and vitamin B12 deficiency: An association to bear in mind. World Journal of Diabetes.
Johnson, L. E. (2024). Vitamin B12 deficiency. Merck Manual.
MedlinePlus. (2024). Methylmalonic acid (MMA) test.
Miller, J. W. (2018). Proton pump inhibitors, H2-receptor antagonists, metformin, and vitamin B-12 deficiency: Clinical implications. Advances in Nutrition.
Office of Dietary Supplements. (2023). Vitamin B12. National Institute of Health.
Wang, H., et al. (2018). Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database of Systematic Reviews.
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