Metformin is the first therapy for almost everyone with type 2 diabetes, but studies show that the drug creates a risk of vitamin B12 deficiency. Your clinician should test your B12 levels regularly and monitor for symptoms of this side effect.
If your levels are low, taking vitamin B12 supplements is an easy, reliable, and safe way to treat or prevent B12 deficiency. In some cases, injections of vitamin B12 may be recommended to rapidly replenish your reserves of this vital nutrient, according to a review published in 2021.
Metformin and Vitamin B12
Metformin is prescribed to people with type 2 diabetes to help manage their blood sugar levels. The drug is typically taken with a meal, unless your doctor instructs you otherwise. Adverse effects include muscle pain, numbness of your extremities, difficulty breathing, and upset stomach.
Another significant metformin side effect: vitamin B12 deficiency. Experts don’t know exactly why it happens, but metformin can block the absorption of vitamin B12 in the intestine, according to a 2022 article. This side effect occurs in about 7 percent of people who use the drug, according to the Food and Drug Administration, and it is more likely on higher doses and after prolonged use. Most do not experience serious symptoms.
Though this side effect is well established, it is still underrecognized, according to a 2024 case report. Though diabetes authorities recommend annual B12 screening for people who have diabetes and use metformin, many clinics fail to test B12 levels every year, according to Clinical Diabetes.
The Symptoms of Vitamin B12 Deficiency
Vitamin B12 is vital for good health and is essential for many body functions, including creating new red blood cells and converting food into energy, according to the National Institutes of Health Office of Dietary Supplements. It’s important to treat low levels.
The symptoms of vitamin B12 deficiency often develop slowly and gradually strengthen over time, according to Harvard Health, though in some cases they may come on quickly. Symptoms include:
- Tingling or numbness in the extremities
- Difficulty walking, loss of balance, and falling
- Brain fog
- Swollen tongue
- Tiredness
- Weakness
A lack of vitamin B12 can also lead to anemia, according to Johns Hopkins Medicine, because a lack of the nutrient impairs your body’s ability to make red blood cells. B12 anemia can also lead to gastrointestinal issues such as nausea and increased heart rate.
How Much Vitamin B12 Do You Need?
According to Yale Medicine, the recommended vitamin B12 dose for an adult is 2.4 micrograms (mcg).
The most important source of vitamin B12 for most people is food. The vital nutrient is found in the highest concentrations in meat, seafood, and eggs, and it is sometimes added to fortified foods such as breakfast cereals, according to Harvard Health. While many people get enough vitamin B12 naturally from their diet, vegetarians and vegans will often need a supplement because they eat few or no animal protein sources.
B12 deficiency is also more common in people who are over the age of 50, who are pregnant, or who have medical conditions that inhibit nutrient absorption, such as celiac disease and Crohn’s disease.
People with diabetes who have an enhanced risk of vitamin B12 deficiency or insufficiency may wish to ask their clinicians if they should be using a B12 supplement, and they should be especially careful to have their levels checked regularly.
Treating B12 Deficiency
If you have diabetes, use metformin, and have vitamin B12 deficiency, your doctor has two major options to help get your levels back into a safe range, according to the World Journal of Diabetes:
- Intramuscular injections: You might need injections every one to two days at first, gradually tapering off to injections every three to four weeks.
- Oral pills: High dose vitamin B12 tablets (up to 2,000 mcg) easily satisfy your daily B12 requirements. Though this is far above the recommended daily value, only a small percentage of the vitamin B12 in a pill is actually absorbed into the bloodstream.
Doctors are more likely to recommend injections when B12 levels are critically low, or if you have a persistent issue that causes nutrient malabsorption. Experts do not generally recommend discontinuing the use of metformin for people with new cases of vitamin B12 deficiency.
