Metformin and Testosterone: What the Research Really Shows

Short answer: A few short studies in men with type 2 diabetes found that men taking metformin had lower testosterone after one to three months than men treated without it, but the drop was modest and the evidence is limited. Type 2 diabetes and excess body weight are themselves strongly linked to low testosterone, so metformin is rarely the whole story. If your testosterone looks low while you take metformin, get it rechecked and talk with your prescriber. Never stop or change metformin on your own.

Key Takeaways

• Two small studies from the same research group (a 3 month real world study and a 1 month randomized trial) found lower testosterone in men with type 2 diabetes who took metformin, compared with men who did not.
• The effect looked modest, long term data are lacking, and neither study measured symptoms, so the clinical meaning is still unclear.
• Type 2 diabetes is itself tied to low testosterone: in one well known study, 33% of patients with type 2 diabetes were hypogonadal.
• Metformin can lower vitamin B12 levels, and B12 deficiency causes fatigue and nerve symptoms that can feel like “low T”. Ask about B12 testing.
• Asian (Panax) ginseng may lower blood sugar, so men on diabetes medication should ask their doctor before taking any ginseng product.
• Do not stop metformin without talking to your doctor. MedlinePlus states this directly.

What Metformin Is and Why the Testosterone Question Comes Up

Metformin is a prescription medicine used to treat type 2 diabetes, alone or together with other medicines, including insulin. According to MedlinePlus, it works by decreasing the amount of glucose absorbed from food and the amount made by the liver, and by helping the body respond better to its own insulin.^[1]

The testosterone question arises because the men who take metformin are often the same men who are most likely to have low testosterone: men with type 2 diabetes, insulin resistance and extra abdominal weight. When a man on metformin feels tired, has less drive or notices changes in erections, it is natural to wonder whether the pill is to blame. The honest answer is that the research is small, recent and not yet settled.

What the Research on Metformin and Testosterone Shows

Most of the direct human evidence comes from two studies by a research group at Nanjing First Hospital in China. Both looked at men with type 2 diabetes who were also using insulin.

The 1 month randomized trial (Diabetes Care, 2021)

Researchers enrolled 70 men with newly diagnosed, drug naive type 2 diabetes and an HbA1c above 9.0%. Everyone first received 5 days of intensive insulin pump therapy to bring blood sugar down. During those 5 days, total, free and bioavailable testosterone all rose significantly, which shows how strongly high blood sugar itself is tied to lower testosterone. The men were then randomized to insulin alone or insulin plus metformin for one month.^[2]

After one month, the metformin group had lower levels than the insulin only group: total testosterone 12.7 vs 15.3 nmol/L, free testosterone 0.20 vs 0.24 nmol/L and bioavailable testosterone 4.56 vs 5.31 nmol/L. The authors concluded that one month of metformin may lower serum testosterone independent of glucose control and that longer term effects need more study.^[2]

The 3 month real world study (Frontiers in Endocrinology, 2021)

This study was a secondary analysis of 80 men (40 taking metformin, 40 matched controls) on premixed insulin, followed for 3 months. It was not randomized. In the control group, total testosterone rose over time as blood sugar improved. In the metformin group it did not rise. The change in total testosterone was minus 0.82 nmol/L with metformin versus plus 0.99 nmol/L in controls, and bioavailable testosterone showed a similar pattern. The difference in free testosterone change between groups was not statistically significant.^[3]

The authors concluded that 3 months of metformin can reduce testosterone, mainly by blunting the rise that normally follows better blood sugar control. The paper itself notes limits: the groups were not randomized and changes in insulin dose could not be fully ruled out as a cause.^[3]

How to read these results

Put together, the signal is real enough to take seriously but too limited to say that metformin “causes low testosterone” in a meaningful way for most men.

Diabetes Itself Is a Major Cause of Low Testosterone

Before blaming the medicine, it helps to look at the condition it treats. In a frequently cited 2004 study in the Journal of Clinical Endocrinology and Metabolism, 33% of 103 patients with type 2 diabetes were hypogonadal, based on low free testosterone. Their LH and FSH (the pituitary signals that tell the testes to make testosterone) were lower than in patients with normal levels, and higher BMI was linked to lower free and total testosterone. The authors concluded that hypogonadotropic hypogonadism occurs commonly in type 2 diabetes.^[4]

The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) adds that men with diabetes, especially those who are older or overweight, are more likely to have low testosterone, and that low testosterone may be the cause of some men’s erectile problems.^[5] In other words, a man with type 2 diabetes can have low testosterone whether or not he takes metformin. You can read more about symptoms in our guide to the signs of low testosterone.

Metformin Side Effects in Men: What Is Actually Documented

MedlinePlus lists the common side effects of metformin as diarrhea, nausea, stomach discomfort, gas, indigestion, constipation, low energy or weakness, taste changes, headache, flushing, nail changes, muscle pain and rash. It does not list low testosterone or sexual side effects.^[1]

Lactic acidosis (rare but serious)

Metformin may rarely cause lactic acidosis, a life threatening condition. Warning signs include extreme tiredness, weakness, nausea, vomiting, stomach pain, fast or difficult breathing, dizziness, an abnormal heartbeat, muscle pain or feeling cold. MedlinePlus advises calling your doctor immediately if these appear. Kidney disease, age over 65, heart or liver disease, dehydration and heavy drinking raise the risk.^[1]

Vitamin B12: the overlooked “low T” look alike

The NIH Office of Dietary Supplements notes that metformin might reduce the absorption of vitamin B12 and significantly lower blood B12 levels.^[6] Symptoms of B12 deficiency include fatigue, weakness, numbness and tingling in the hands and feet, and, in some studies, low mood. Because the body stores B12, these symptoms can take years to show up.^[6] Fatigue and low mood are exactly the complaints many men attribute to low testosterone, so a B12 check is worth asking about if you have been on metformin for a long time. MedlinePlus also asks patients to tell their doctor if they have ever had low B12.^[1]

Does metformin cause erectile dysfunction?

None of the sources reviewed here list erectile dysfunction as a side effect of metformin. Diabetes, on the other hand, is one of the strongest known risk factors. NIDDK reports that more than half of men with diabetes will develop ED, that they are more than 3 times as likely to develop it as men without diabetes, and that it may appear 10 to 15 years earlier. The causes are nerve and blood vessel damage from high blood glucose, plus low testosterone in some men.^[5] Good diabetes control is part of protecting erectile function, and a doctor can treat ED with medicine or by adjusting the diabetes care plan.

What to Do if You Take Metformin and Suspect Low Testosterone

1. Keep taking metformin. MedlinePlus is clear: do not stop taking metformin without talking to your doctor. It controls diabetes but does not cure it.^[1]
2. Get a proper testosterone test. A single low value is not a diagnosis. Testing is usually repeated in the morning. See our guides on the best time to check testosterone levels and free vs total testosterone.
3. Ask about vitamin B12. Long term metformin users can discuss B12 status with their clinician.^[6]
4. Review blood sugar control and weight. In the randomized trial, testosterone rose within 5 days of bringing blood sugar down, and in the JCEM study higher BMI tracked with lower testosterone.^[2][4]
5. Bring up sexual symptoms openly. Erection changes in men with diabetes have medical causes that a doctor can evaluate and treat.^[5]
6. Do not self treat with testosterone or “T booster” products. NIDDK notes testosterone therapy may have serious risks and is not right for every man.^[5] Any change in medication, including swapping or adding diabetes drugs, is your prescriber’s decision.

A Note on Herbal Supplements and Diabetes Medication

Many men with low energy turn to herbal products. If you take metformin or any other blood sugar medicine, check with your doctor first. NCCIH reports that some evidence suggests Asian ginseng (Panax ginseng) might lower blood sugar levels, that it might interact with certain medications, and that people with diabetes should consult a health care provider before using it. NCCIH also notes that short term use, up to 6 months, appears safe for most people, while long term safety has been questioned.^[7]

This applies to our own product. Ultra Bann, a herbal supplement made by DL Bioproducts, contains Panax ginseng (110 mg per capsule) among its seven botanicals, and the full label is listed on our ingredients page. If you take metformin or other diabetes medication, show that label to your doctor or pharmacist and get their approval before you take it. A supplement is not a substitute for diabetes treatment or for a medical workup of low testosterone. For background on the herb itself, see Panax ginseng benefits for men.

Related reading: for the full list of common and serious effects, see metformin side effects in men.

Frequently Asked Questions

Does metformin lower testosterone?

Two small studies in men with type 2 diabetes on insulin found lower testosterone with metformin after 1 to 3 months compared with men not taking it.^[2][3] The effect was modest, the studies were short and from one research group, and symptoms were not measured. It is a possible effect, not an established one.

Should I stop metformin if my testosterone is low?

No. Do not stop or reduce metformin without talking to your doctor.^[1] Uncontrolled blood sugar is itself linked to low testosterone, and in one trial testosterone rose when blood sugar was brought under control.^[2] Your doctor can repeat the test, look for other causes and decide whether any change makes sense.

Can metformin cause erectile dysfunction?

MedlinePlus does not list erectile dysfunction as a metformin side effect.^[1] Diabetes, however, is a major cause of ED through nerve and blood vessel damage and, in some men, low testosterone.^[5] If you notice changes, talk to your doctor rather than assuming the medicine is responsible.

Why do I feel tired on metformin?

Low energy or weakness is listed among metformin’s common side effects, and long term use can lower vitamin B12, which causes fatigue and tingling.^[1][6] Rarely, extreme tiredness with breathing changes or muscle pain can signal lactic acidosis, which needs urgent care.^[1]

Can I take ginseng with metformin?

Ask your doctor first. NCCIH says Asian ginseng might lower blood sugar and might interact with certain medications, and advises people with diabetes to consult a health care provider before using it.^[7] That includes any supplement containing Panax ginseng.

References

1. MedlinePlus (U.S. National Library of Medicine). Metformin. medlineplus.gov/druginfo/meds/a696005.html
2. Hu Y, Ding B, Shen Y, et al. Rapid Changes in Serum Testosterone in Men With Newly Diagnosed Type 2 Diabetes With Intensive Insulin and Metformin. Diabetes Care, 2021. Diabetes Care supplementary record
3. 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. Frontiers in Endocrinology, 2021;12. frontiersin.org
4. Dhindsa S, Prabhakar S, Sethi M, Bandyopadhyay A, Chaudhuri A, Dandona P. Frequent Occurrence of Hypogonadotropic Hypogonadism in Type 2 Diabetes. J Clin Endocrinol Metab, 2004;89(11):5462 to 5468. academic.oup.com
5. NIDDK. Diabetes, Sexual, and Bladder Problems. niddk.nih.gov
6. NIH Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. ods.od.nih.gov
7. National Center for Complementary and Integrative Health. Asian Ginseng. nccih.nih.gov

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease. This article is for information only and is not medical advice.