The Short Answer
Yes, metformin can lower vitamin B12 levels over time. It appears to interfere with how your gut absorbs B12, and people taking metformin are more than twice as likely to develop B12 deficiency. You do not need to stop metformin, but regular B12 monitoring is essential.
How Common Is Metformin Vitamin B12 Deficiency?
The numbers are significant. A meta-analysis by Yang et al. (2019), which pooled data from 31 studies, found that metformin users had a relative risk of 2.09 for B12 deficiency (95% CI 1.49 to 2.93). Average B12 levels among metformin users were 63.70 pM lower than non-users. The UK MHRA classifies reduced vitamin B12 levels with metformin as a common side effect, meaning it may affect up to 1 in 10 people.
The risk grows with time on the drug. The Diabetes Prevention Program Outcomes Study (Aroda et al., 2016) was a landmark 13-year follow-up that tracked over 2,000 participants and found that each additional year of metformin use was associated with a 13% increase in B12 deficiency odds (OR 1.13 per year). At the 5-year mark, low B12 was nearly twice as common in metformin users as in the placebo group (4.3% vs. 2.3%). A real-world database study (EMBER, 2023) confirmed the pattern holds in everyday clinical settings, not just clinical trials.
People at highest risk:
- People taking metformin for more than 4 to 5 years
- People taking higher doses (1,500 mg/day or more)
- Adults aged 65 and older
- People who follow a vegan or vegetarian diet (lower dietary B12 to begin with)
- People who also take proton pump inhibitors (PPIs) like omeprazole
- People who have had gastric or small bowel surgery
How Metformin Lowers B12 Levels
Your body absorbs B12 through a two-step process. First, a protein called intrinsic factor (made in your stomach) grabs onto B12 from food. Then, this intrinsic factor-B12 complex travels to the end of your small intestine, where it binds to the cubam receptor and gets absorbed into your bloodstream.
Metformin appears to interfere with absorption of the vitamin B12–intrinsic factor complex in the ileum. Bell (2022) describes this as involving blocked calcium-dependent binding at the cubam receptor in the terminal ileum — when that binding step fails, B12 passes through without being absorbed.
Your liver stores enough B12 to last roughly 2 to 5 years under normal conditions. That buffer is why problems rarely show up in the first year or two. After long-term metformin use, those stores run low and deficiency becomes detectable. The FDA prescribing label for metformin already flags this: in a 29-week clinical trial, approximately 7% of patients showed a drop to subnormal B12 levels.
B12 level comparison: metformin vs. no metformin
| Group | Finding |
|---|---|
| Average B12 difference | -63.70 pM vs. non-users |
| Risk of deficiency | 2.09x higher than non-users |
| Affected patients (MHRA estimate) | Up to 1 in 10 |
Symptoms of B12 Deficiency While on Metformin
Many B12 deficiency symptoms look like diabetes symptoms. Tingling feet or fatigue can get chalked up to diabetes when falling B12 is the real cause.
Symptoms to watch for:
- Fatigue and weakness that seems worse than usual
- Tingling or numbness in the hands or feet
- Balance problems or difficulty walking
- Memory issues or trouble concentrating
- Sore, red or swollen tongue
- Pale or slightly yellowish skin
- Shortness of breath
An important note from Bell (2022): once B12-induced neuropathy has developed, nerve damage may stabilize but not fully reverse even after B12 levels are corrected. Catching low B12 before symptoms appear is the only real protection.
If you are on metformin and notice any of these symptoms, bring them up with your doctor. Do not assume they are unrelated to your medication.
Metformin B12 Monitoring: How Often to Test
Monitoring guidelines have tightened over the past decade as long-term data accumulated.
The FDA metformin label states: measure B12 levels every 2 to 3 years and manage any abnormalities.
The ADA 2026 Standards of Care support more frequent monitoring in long-term metformin users, with annual B12 testing suggested for people taking metformin long term, especially beyond 4 years.
| Risk level | How often to test |
|---|---|
| Taking metformin less than 4 years, no other risk factors | Every 2 to 3 years (FDA label) |
| Taking metformin 4+ years, OR high-risk (vegan, PPIs, older age) | Annually (ADA 2026) |
| Symptoms present | Discuss with doctor promptly |
Most labs flag below 200 to 250 pg/mL as deficient. Borderline-low is generally 200 to 300 pg/mL. The Aroda et al. study used 203 pg/mL as the cutoff for deficiency and 298 pg/mL as borderline in their 13-year analysis. Your doctor can tell you what your specific lab uses.
If your B12 is low, do not stop or reduce metformin on your own. The MHRA guidance is explicit: "You can keep taking metformin while vitamin B12 levels are being corrected." Stopping diabetes medication without medical supervision can cause serious blood sugar problems.
- Talk to your doctor about B12 supplementation. Supplements and injections are both commonly used. If absorption is compromised, high-dose oral B12 or injections may work better than standard-dose capsules. Your doctor or pharmacist can advise on the right form and dose.
- Some research suggests that taking calcium supplements alongside metformin may help partially restore B12 absorption, though this should be discussed with your doctor before trying it.
- If you eat a vegan or vegetarian diet, mention this, as your dietary B12 intake is already lower.
Foods high in B12 (general dietary awareness; food sources alone are not a substitute for treating deficiency):
- Beef, liver, and shellfish (clams, oysters)
- Fish (salmon, tuna, trout)
- Dairy products (milk, yogurt, cheese)
- Eggs
- Fortified cereals and plant-based milks (important for vegans)
For tips on timing a B12 supplement alongside your other medications, see the supplement timing chart or the guide on the best time to take B12.
If you want to understand how your other medications may affect nutrient absorption, the best drug interaction checkers article covers tools that screen for these interactions.
How Pillo Helps
Pillo's built-in Safety Checker includes a Drug-Induced Nutrient Loss Checker. You can look up metformin directly and see which nutrients it depletes, including B12. Worth checking when you start a new medication or change your dose.
Beyond the checker, Pillo is built for the reality of long-term medication management. You can set a recurring annual reminder for B12 lab work, so it does not fall through the cracks between appointments. The persistent alarm system means reminders actually get acknowledged, whether for your daily metformin dose or a once-a-year lab checkup.
For more on managing metformin day to day, read about the best time to take metformin, what to do if you have a missed dose of metformin, or a full overview of metformin side effects.
Download Pillo on Google Play to use the Drug-Induced Nutrient Loss Checker and set up your medication and lab reminders.
Frequently Asked Questions
Does everyone on metformin get B12 deficiency?
No. The Yang et al. 2019 meta-analysis found a relative risk of 2.09, meaning metformin users are about twice as likely to develop deficiency. But most people, especially those on lower doses for shorter periods, will not develop a clinically significant deficiency if they are monitored regularly. Higher doses, longer duration, and certain dietary patterns increase the risk.
How long before metformin depletes B12?
Because the body stores several years' worth of B12, deficiency often develops gradually and becomes more likely with longer-term metformin use. The FDA label also observed drops in short 29-week trials, so the timeline varies by individual. This is why consistent monitoring matters more than a single test.
Can metformin-induced B12 deficiency cause nerve damage?
Yes. Bell (2022) describes metformin-induced B12 deficiency as a cause or aggravating factor for peripheral neuropathy: tingling, numbness, and balance problems in the hands, feet, and legs. This matters especially for people with diabetes, who may already have diabetic neuropathy. B12-related nerve symptoms can be mistaken for worsening diabetes. The concern is that once nerve damage develops from low B12, it may stabilize but not fully reverse even after levels are corrected. That is why routine monitoring matters.
Should I take a B12 supplement with metformin?
Not necessarily, and not without talking to your doctor first. Your doctor should check your actual B12 level before recommending supplementation. Some people will benefit from a daily supplement, but others may not need one. The dose and form that works best can vary. Standard oral doses may not work well if absorption is impaired. Do not start supplementing based on this article alone.
What are the symptoms of B12 deficiency from metformin?
Common symptoms include fatigue, tingling or numbness in the hands or feet, balance problems, memory issues, a sore or red tongue, and pale or yellowish skin. The challenge is that many of these overlap with diabetes symptoms, so they are often missed. If you have been on metformin for several years and notice any of these, mention them to your doctor and ask about a B12 test.
How often should B12 be checked if I take metformin?
The FDA label recommends every 2 to 3 years. The ADA 2026 guidelines recommend annually if you have been on metformin for 4 or more years, or if you have other risk factors such as a vegan diet, older age, use of PPIs, or a history of GI surgery. Ask your doctor which schedule applies to you.
Medical Disclaimer: This article is for general informational purposes only and does not constitute medical advice. Vitamin B12 deficiency and its management depend on your individual health situation. Do not stop or adjust your metformin dose based on this article. Consult your doctor or pharmacist for advice specific to your medications, health history, and lab results. If you have symptoms of B12 deficiency, contact your healthcare provider.
