Does Metformin Lower Testosterone in Men? What Human Studies Actually Show

Short answer: metformin may lower testosterone in some men, particularly in certain studies of men with type 2 diabetes, but the human evidence is not consistent enough to say that it universally lowers testosterone. Diabetes, obesity, insulin resistance, weight loss and glucose control can themselves change sex-hormone levels, so the effect is difficult to isolate.

This matters because metformin is widely used for type 2 diabetes and is also discussed off-label in longevity circles. The useful question is therefore not “is metformin good or bad for testosterone?” but which men were studied, for how long, compared with what, and what was actually measured?

The strongest direct evidence: a randomized study in men with type 2 diabetes

A randomized study included 70 men with newly diagnosed, previously untreated type 2 diabetes and HbA1c above 9%. All participants first received intensive insulin treatment for five days to normalize glucose. They were then randomized either to continue insulin alone or to receive metformin in addition for one month.

After that month, the metformin group had lower total testosterone than the control group: 12.7 vs 15.3 nmol/L. Free and bioavailable testosterone were also lower. Because glucose had initially been normalized in both groups, the authors concluded that metformin may decrease testosterone independently of glucose control.

Primary source: Rapid Changes in Serum Testosterone in Men With Newly Diagnosed Type 2 Diabetes With Intensive Insulin and Metformin, Diabetes Care, 2021.

A three-month study points in the same direction

Another study followed men with type 2 diabetes for three months. Testosterone rose in the control group as metabolic control improved, while the change in total and bioavailable testosterone was significantly lower in the metformin group. The authors interpreted this as metformin counteracting the testosterone increase that accompanied improved blood glucose.

Primary source: Effect of Metformin on Testosterone Levels in Male Patients With Type 2 Diabetes Mellitus Treated With Insulin, 2021.

But the result is not universal

A one-year double-blind randomized trial studied 106 men aged 18–50 with obesity and low testosterone but without diabetes. Participants received metformin, testosterone, both, or placebo. Metformin improved insulin resistance, but metformin alone did not produce a clear clinically important fall in testosterone or erectile function compared with placebo. Only the combination of metformin plus testosterone significantly increased total and free testosterone versus placebo.

Primary source: Metformin, testosterone, or both in men with obesity and low testosterone: a double-blind randomized controlled trial, Metabolism, 2022.

Why the literature looks contradictory

Testosterone is strongly entangled with metabolic health. Type 2 diabetes and obesity are themselves associated with lower testosterone in many men. Better glucose control, reduced insulin resistance, changes in body fat, calorie restriction and changes in SHBG can all move measured testosterone. A drug can therefore have a direct endocrine effect while simultaneously improving metabolic variables that push testosterone in the opposite direction.

This is why a 2022 review of metformin and male reproductive function described the human evidence on testosterone synthesis as controversial and contradictory, with studies reporting both potentially protective and potentially harmful effects on testicular function.

Review: The Effect of Metformin on Male Reproductive Function and Prostate: An Updated Review, World Journal of Men’s Health, 2022.

What about libido and erectile function?

Low testosterone can contribute to low libido and sexual dysfunction, but erectile function in diabetes is also affected by vascular disease, neuropathy, medications, psychological factors and overall metabolic control. It is therefore too strong to infer that metformin causes erectile dysfunction simply because some studies found lower testosterone in metformin-treated groups.

If a man taking metformin develops persistent symptoms such as reduced libido, fewer spontaneous erections, fatigue or loss of muscle mass, the useful next step is not to assume the drug is responsible. Morning testosterone, repeat confirmation when appropriate, SHBG/free testosterone in selected cases, and the broader metabolic and medication context matter.

What about metformin for longevity?

Metformin has biological effects that make it interesting to aging researchers, but interest is not the same as proof that it extends lifespan in healthy people. Using a prescription drug solely for longevity creates a different risk-benefit calculation from using it to treat diabetes or another established indication.

Personal note: I originally became interested in metformin because of longevity research and at one point considered taking it for that purpose. I chose not to. That is a personal decision, not evidence that others should stop or avoid metformin, particularly when it is prescribed for diabetes.

Bottom line

The most defensible conclusion is narrower than the original claim: several human studies suggest that metformin can lower or blunt recovery of testosterone in some men with type 2 diabetes, but other randomized data do not show a universal testosterone-lowering effect. The signal is real enough to investigate, but not settled enough to treat as a general rule.

Key references

Hu et al., 2021 — randomized one-month study in newly diagnosed T2DM
Cai et al., 2021 — three-month study in men with T2DM
Fernández-García et al., 2022 — one-year double-blind RCT in men with obesity and low testosterone
Tseng, 2022 — review of metformin and male reproductive function