Metformin has been used safely for decades and remains one of the most effective medications for type 2 diabetes. It is also prescribed for some people with prediabetes, insulin resistance, and other conditions.
Most discussions about metformin focus on stomach upset or diarrhea. However, one of the most important long-term considerations is vitamin B12 deficiency.
Not everyone who takes metformin develops low vitamin B12 levels. But because deficiency can develop gradually and may not cause symptoms at first, healthcare providers often recommend periodic monitoring—especially after several years of treatment. Recognizing vitamin B12 deficiency early can help prevent complications and ensure you continue to benefit from metformin safely.
What Is Vitamin B12?
Vitamin B12 is an essential nutrient that helps your body produce healthy red blood cells, maintain normal nerve function, make DNA, and support healthy brain function.
Your body cannot produce vitamin B12 on its own. Instead, it comes from foods such as meat, fish, poultry, eggs, milk, cheese, and fortified breakfast cereals and plant-based products. After you eat foods containing vitamin B12, your digestive system absorbs the vitamin and stores it, primarily in the liver.
How Does Metformin Affect Vitamin B12?
Researchers do not completely understand every mechanism, but metformin appears to reduce the absorption of vitamin B12 in the small intestine.
This effect develops gradually. Because the body stores several years' worth of vitamin B12, deficiency usually does not occur soon after starting metformin. Instead, the risk increases with longer duration of metformin therapy, higher daily doses, and other conditions that reduce vitamin B12 absorption. Many people continue taking metformin for years without any problems, while others eventually develop low vitamin B12 levels.
Who Is at Higher Risk?
Several factors increase the likelihood of vitamin B12 deficiency:
- Taking metformin for many years
- Higher doses of metformin
- Older age
- Following a vegetarian or vegan diet
- Previous stomach or intestinal surgery
- Certain digestive disorders
- Long-term use of acid-reducing medications such as proton pump inhibitors (PPIs)
Having one or more of these risk factors does not mean you will develop deficiency, but it may make monitoring more important.
What Are the Symptoms of Vitamin B12 Deficiency?
Vitamin B12 deficiency often develops slowly, and early symptoms may be subtle and easy to overlook. Possible symptoms include:
- Fatigue and weakness
- Difficulty concentrating and memory problems
- Numbness or tingling in the hands or feet
- Burning or painful feet
- Difficulty with balance
- Depression or mood changes
- Pale skin
Not everyone experiences symptoms, which is one reason routine testing may be recommended.
Why Can Vitamin B12 Deficiency Be Confusing?
Vitamin B12 deficiency can sometimes resemble diabetic neuropathy. Both conditions may cause tingling, numbness, burning sensations, and changes in balance. If vitamin B12 deficiency is missed, these symptoms may be incorrectly attributed only to diabetes. Checking a vitamin B12 level is one way healthcare providers evaluate these symptoms.
Should Everyone Taking Metformin Be Tested?
Not necessarily. Current diabetes guidelines recommend considering periodic vitamin B12 testing for people taking metformin, particularly those who have been taking metformin for several years, take higher doses, develop symptoms suggestive of deficiency, have anemia, or have numbness or tingling. Your healthcare provider will decide how often testing is appropriate based on your individual risk factors.
How Is Vitamin B12 Deficiency Diagnosed?
Vitamin B12 deficiency is usually diagnosed with a blood test. In some situations, your healthcare provider may recommend additional laboratory tests if the diagnosis is uncertain. Testing is straightforward and can often be performed during routine laboratory monitoring.
How Is Vitamin B12 Deficiency Treated?
Treatment depends on the severity of the deficiency and the underlying cause. Options may include oral vitamin B12 supplements, high-dose oral vitamin B12, or vitamin B12 injections in certain situations. Many people respond well to treatment.
Importantly, developing vitamin B12 deficiency does not usually mean you need to stop metformin. Instead, your healthcare provider can often treat the deficiency while you continue receiving the benefits of metformin.
Can You Prevent Vitamin B12 Deficiency?
There is no guaranteed way to prevent deficiency. However, you can reduce your risk by attending regular follow-up appointments, completing recommended laboratory testing, eating a balanced diet that includes vitamin B12 sources if appropriate for you, and reporting new numbness, tingling, or fatigue promptly. For people with dietary restrictions or known deficiency, your healthcare provider may recommend vitamin B12 supplementation.
Should You Take Vitamin B12 Just Because You're on Metformin?
Not necessarily. Many people taking metformin maintain normal vitamin B12 levels for years, and routine supplementation is not automatically recommended for everyone. Supplementation is typically based on blood test results, symptoms, individual risk factors, and healthcare provider recommendations. Taking supplements without discussing them with your healthcare provider may not be necessary.
Frequently Asked Questions
Does metformin always cause vitamin B12 deficiency?
No. Many people take metformin for years without developing low vitamin B12 levels. The risk increases with long-term use, but deficiency is not inevitable.
How often should vitamin B12 be checked?
There is no single schedule that applies to everyone. Your healthcare provider will recommend testing based on your symptoms, risk factors, and how long you've been taking metformin.
Can vitamin B12 deficiency be reversed?
In many cases, yes. When identified early, treatment with vitamin B12 supplements or injections is often effective. Some nerve symptoms improve over time, although recovery may depend on how long the deficiency has been present.
Should I stop metformin if my vitamin B12 is low?
Usually not. Most people can continue metformin while receiving treatment for vitamin B12 deficiency. Your healthcare provider will recommend the best approach for your situation.
When Should You Contact Your Healthcare Provider?
Contact your healthcare provider if you develop numbness or tingling in your hands or feet, notice increasing fatigue or weakness, have problems with balance or walking, have memory or concentration concerns, have been taking metformin for several years and have never had your vitamin B12 level checked, or have questions about whether vitamin B12 testing is appropriate for you.
Seek Immediate Medical Care If
Seek emergency medical care immediately if you experience sudden weakness on one side of your body, difficulty speaking, sudden vision changes, loss of consciousness, severe confusion, or chest pain or difficulty breathing. These symptoms may indicate a medical emergency unrelated to vitamin B12 deficiency and require immediate evaluation.
The Bottom Line
Metformin remains one of the safest and most effective medications for managing type 2 diabetes and reducing diabetes risk in selected people with prediabetes. However, long-term use can reduce vitamin B12 absorption in some individuals.
Because vitamin B12 deficiency often develops slowly and may resemble diabetic neuropathy, periodic monitoring is an important part of long-term metformin therapy. Early detection allows deficiency to be treated before serious complications develop, and most people can continue taking metformin while correcting low vitamin B12 levels.
If you've been taking metformin for several years or have developed symptoms such as fatigue, numbness, or tingling, ask your healthcare provider whether checking your vitamin B12 level is appropriate. Routine monitoring is a simple step that can help protect your long-term health while allowing you to continue benefiting from one of the most well-studied medications in modern medicine.