Metformin Side Effects in Men: What Is Common, What Is Rare and When to Call a Doctor

Short answer: The most common metformin side effects in men are the same as in women: diarrhea, nausea, gas and stomach upset, which usually show up when you start or raise the dose and often settle with time, food and a slower dose increase. Long-term use can lower vitamin B12, so periodic testing is worth asking about. Lactic acidosis is rare but serious, and kidney function and heavy drinking raise the risk. Research on metformin and testosterone or erections in men is small and mixed, so talk to your doctor before drawing conclusions, and never stop metformin on your own.

Key Takeaways

• Stomach and bowel symptoms are the main side effects. In the FDA label’s placebo-controlled trial, diarrhea was reported by 53% of people on immediate-release metformin versus 12% on placebo.^[2]
• Taking metformin with meals, starting low and increasing slowly, or switching to an extended-release tablet are the usual ways doctors improve tolerance.
• Long-term use is linked to lower vitamin B12. The label advises checking B12 every 2 to 3 years.^[2]
• Lactic acidosis is rare but life threatening. Poor kidney function, heavy alcohol use, dehydration and serious illness raise the risk.^[1]
• Studies on testosterone and sexual function in men are small and point in different directions. Diabetes itself affects both, so the drug is rarely the whole story.
• Do not stop or change your dose without your prescriber. Uncontrolled blood sugar carries its own risks for nerves, blood vessels and sexual health.

What Metformin Is and Why Side Effects Happen

Metformin is a prescription medicine used as first-line treatment for type 2 diabetes and, in some cases, prediabetes.^[4] It lowers blood sugar mainly by reducing the amount of glucose the liver releases and by helping the body respond better to insulin. It does not make the pancreas pump out extra insulin, which is why it rarely causes low blood sugar when taken alone.^[2]

Most of its side effects come from the gut. Metformin acts in the intestine as well as in the bloodstream, and the digestive tract is where people feel it first. There is no separate “male” side effect profile in the prescribing information: men and women report the same common reactions. What differs for men is the questions they tend to ask, especially about energy, testosterone and erections, so this article covers those carefully as well.

Common Side Effects and How They Are Managed

The table below shows the reactions that were reported by more than 5% of patients on immediate-release metformin, and more often than on placebo, in the trial summarized in the FDA prescribing information (141 people on metformin, 145 on placebo).^[2]

MedlinePlus also lists a metallic or changed taste, flushing, nail changes, muscle aches and rash among possible side effects.^[1] Many of these are mild, but tell your prescriber about anything that is bothersome or does not go away.

Practical ways doctors reduce stomach problems

• Take it with food. The label calls for metformin to be given with meals, and the patient information says this helps reduce stomach upset.^[2]
• Start low, go slow. A typical label schedule starts at 500 mg twice daily or 850 mg once daily, then increases by 500 mg per week or 850 mg every 2 weeks according to blood sugar and tolerability.^[2] If symptoms flare after a dose increase, your doctor may pause or step back.
• Split larger doses. The label notes that doses above 2,000 mg a day may be better tolerated when divided into three doses with meals.^[2]
• Ask about extended-release. Extended-release (ER or XR) tablets release the drug slowly and are usually taken once a day.

Does extended-release metformin cause fewer side effects?

Possibly, but the evidence is softer than many people assume. A systematic review and meta-analysis of head-to-head trials found extended-release metformin was linked to lower odds of digestive side effects than immediate-release (odds ratio 0.76), but the result just missed statistical significance. A delayed-release form showed a clearer reduction (odds ratio 0.45).^[8] In the label trial for one extended-release brand (Glumetza, added to glyburide), diarrhea was reported by 12.5% of patients versus 5.6% on placebo plus glyburide, and nausea by 6.7% versus 4.2%.^[3] Those figures come from a different trial design than the immediate-release table above, so they should not be compared number for number.

One practical point: extended-release tablets must be swallowed whole, never split, chewed or crushed. You may see the empty tablet shell in your stool. That is normal and does not mean the medicine was not absorbed.^[1]

Vitamin B12 Deficiency With Long-Term Use

The NIH Office of Dietary Supplements states that metformin might reduce absorption of vitamin B12 and significantly reduce blood B12 levels, and that people taking it regularly should discuss their B12 status with their health care provider.^[4] The FDA label reports that a drop to below-normal B12 levels was seen in about 7% of patients in controlled trials, and recommends measuring blood counts every year and B12 every 2 to 3 years.^[2]

The strongest long-term data come from the Diabetes Prevention Program Outcomes Study. Compared with the group originally given placebo, the metformin group had more low or borderline-low B12 (at or below 298 pg/mL) at 5 years (19.1% vs 9.5%) and at 13 years (20.3% vs 15.6%). Each additional year of metformin use was linked to higher odds of B12 deficiency (odds ratio 1.13). The metformin group also had more anemia, and among people with low B12, more neuropathy. The authors suggested considering routine B12 testing in people treated with metformin.^[5]

Why this matters for men: low B12 can cause fatigue, numbness or tingling in the hands and feet, balance problems and memory changes, symptoms that are easy to blame on age, stress or diabetes itself. Diabetic nerve damage and B12-related nerve damage can look similar, which is one more reason testing beats guessing. For a full walkthrough of symptoms and testing, see our guide to B12 deficiency symptoms in men. Do not start high-dose B12 on your own without a blood test and your doctor’s input, because it can mask what the numbers would have shown.

Lactic Acidosis, Kidney Function and Alcohol

Lactic acidosis: rare, but know the signs

Metformin carries a warning that it may rarely cause lactic acidosis, a dangerous buildup of acid in the blood. MedlinePlus lists warning signs that include extreme tiredness or weakness, nausea, vomiting, stomach pain, reduced appetite, fast breathing, dizziness, an abnormal heartbeat, muscle pain and feeling cold. If these appear, stop the medicine and get medical help right away.^[1]

Risk is higher with kidney disease, age over 65, heart or liver disease, a past heart attack or stroke, and a recent serious infection, severe diarrhea, vomiting, fever or poor fluid intake.^[1] If you are sick and cannot keep fluids down, call your doctor or pharmacist about whether to hold your dose.

Kidney function

Because the kidneys clear metformin, kidney function guides its use. The label says metformin should not be used when eGFR (a blood test estimate of kidney filtering) is below 30, starting it is not recommended between 30 and 45, and the benefits and risks should be reassessed if eGFR falls below 45 during treatment. Kidney function should be checked at least once a year, and more often in people at higher risk such as older adults.^[2]

Some imaging tests use iodinated contrast dye. Your doctor may ask you to stop metformin before the procedure and wait at least 48 hours before restarting, after kidney function is rechecked.^[1][2] Always tell the imaging team you take metformin.

Alcohol

Alcohol increases metformin’s effect on lactate, and drinking raises the risk of lactic acidosis and can also lower blood sugar.^[1][2] Binge drinking and regular heavy drinking are the main concerns. Ask your doctor what amount, if any, is reasonable for you. Our article on how much alcohol is too much for men explains standard drink sizes and limits.

Metformin, Testosterone and Sexual Function in Men

This is one of the most searched questions, and the honest answer is that the research is limited and not consistent. Erectile problems and low sex drive are not among the common side effects in the label’s adverse reaction table or on the MedlinePlus side effect list.^[1][2] What the studies do show is summarized below, with human and animal evidence kept separate.

Human study on testosterone

• Men on insulin (observational, 3 months): A 2021 real-world analysis in men with type 2 diabetes on premixed insulin compared men who also took metformin with matched men who did not. In the comparison group, total and bioavailable testosterone rose as blood sugar improved. In the metformin group, total testosterone fell slightly (change of minus 0.82 nmol/L versus plus 0.99 nmol/L in controls). The authors concluded that metformin offset the testosterone rise that usually follows better glucose control.^[6] This was not a randomized trial, it lasted only 3 months, and it did not measure symptoms.

This suggests metformin may have a modest effect on testosterone in some men, but it does not show that it causes clinically low testosterone. Diabetes, obesity, poor sleep and insulin resistance are all well known to lower testosterone on their own. Our deeper review of metformin and testosterone covers this research in more detail, and our guide to signs of low testosterone explains which symptoms justify a blood test.

Human and animal studies on erections

A 2017 review in Translational Andrology and Urology found only one human trial of metformin for erectile dysfunction: a small randomized, placebo-controlled pilot in 30 non-diabetic, insulin-resistant men who responded poorly to sildenafil. Adding metformin improved erectile function scores by an average of 5.5 points on a 25-point scale, versus 0.6 points with placebo, along with better insulin sensitivity.^[7] Animal studies in rats and mice suggest metformin may support the blood vessel lining (endothelium) in the penis.^[7] The authors called for larger trials, so this is a hypothesis, not a reason to take or avoid metformin.

The bigger picture: high blood sugar over time damages the nerves and blood vessels that erections depend on. If you notice a change in erections or desire after starting metformin, raise it with your doctor. The cause may be blood sugar control, another medicine (such as some blood pressure drugs or antidepressants), low B12, low testosterone, stress or relationship factors, and each has a different fix.

When to Call a Doctor

Get urgent care if you have

• Signs of lactic acidosis: unusual muscle pain, fast breathing, extreme tiredness or weakness, stomach pain with nausea or vomiting, dizziness, feeling cold or an abnormal heartbeat.^[1]
• Chest pain, which MedlinePlus lists among serious symptoms.^[1]
• Severe vomiting or diarrhea that stops you from keeping fluids down.

Book a regular appointment if you have

• Stomach symptoms that last beyond the first weeks or after a dose change.
• Numbness, tingling, unusual fatigue or memory changes (ask about a B12 test).
• Shakiness, sweating or confusion, especially if you also take insulin or a sulfonylurea, since combining them can cause low blood sugar.^[2]
• Changes in erections, sex drive or energy that worry you.

Never stop metformin on your own. Stopping suddenly can let blood sugar climb without symptoms. If side effects are hard to live with, your doctor has options: a slower titration, extended-release tablets, a different dose schedule or another medicine. If you miss a dose, take it when you remember unless the next one is almost due, and never double up.^[1]

Frequently Asked Questions

Do metformin side effects go away?

Digestive side effects are most common when you start or increase the dose and often ease as your body adjusts, especially when the tablet is taken with food and the dose is raised slowly. If they persist, talk to your prescriber about an extended-release version or a different schedule rather than stopping.

Does metformin cause erectile dysfunction?

Erectile dysfunction is not listed among metformin’s common side effects in its FDA label or on MedlinePlus. The human evidence is very limited: one small pilot trial in insulin-resistant men actually found improved erectile scores when metformin was added to sildenafil. Diabetes itself is a major cause of erectile problems, so any change deserves a conversation with your doctor.

Does metformin lower testosterone in men?

A small number of studies suggest it may have a modest effect. One 3-month observational study in insulin-treated men found a slight drop in total testosterone compared with men not taking metformin. These studies are small and not randomized. If you have symptoms of low testosterone, ask for a morning blood test rather than changing your medicine yourself.

How often should B12 be checked on metformin?

The FDA label recommends measuring vitamin B12 every 2 to 3 years and blood counts every year. Your doctor may test sooner if you have symptoms such as numbness, tingling or unusual fatigue, or if you follow a diet low in animal foods.

Can I drink alcohol while taking metformin?

Heavy or binge drinking should be avoided, because alcohol increases the risk of lactic acidosis and low blood sugar. Whether light drinking is acceptable depends on your kidney and liver health and your other medicines, so ask your doctor.

References

1. MedlinePlus, U.S. National Library of Medicine. Metformin. medlineplus.gov/druginfo/meds/a696005.html
2. Metformin Hydrochloride Tablets: Prescribing Information (FDA label). Drugs.com. drugs.com/pro/metformin
3. GLUMETZA (metformin hydrochloride extended-release tablets) Prescribing Information. U.S. Food and Drug Administration, 2017. accessdata.fda.gov
4. NIH Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. ods.od.nih.gov
5. Aroda VR, Edelstein SL, Goldberg RB, et al. Long-term Metformin Use and Vitamin B12 Deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab. 2016;101(4):1754-1761. biostatcenter.gwu.edu
6. Cai T, Hu Y, Ding B, et al. Effect of Metformin on Testosterone Levels in Male Patients With Type 2 Diabetes Mellitus Treated With Insulin. Front Endocrinol. 2021;12. doi:10.3389/fendo.2021.813067. frontiersin.org
7. Patel JP, Lee EH, Mena CI, Walker CN. Effects of metformin on endothelial health and erectile dysfunction. Transl Androl Urol. 2017;6(3):556-565. tau.amegroups.org
8. Aiken C, Tarry-Adkins J, Grant I, Reynolds R, Ozanne S. An Update to the Article “Efficacy and Side Effect Profile of Different Formulations of Metformin: A Systematic Review and Meta-Analysis.” Diabetes Ther. 2021;12:2813-2816. link.springer.com

This article is for information only and is not medical advice. Statements on this site have not been evaluated by the Food and Drug Administration. Always talk with your doctor about your own test results and medicines.