GLP-1 Side Effects in Men: Stomach, Muscle, Testosterone and Fertility

Semaglutide and tirzepatide work well for weight loss, but they come with side effects most men will notice at some point. This guide covers the common ones, the questions men ask most about muscle, testosterone and fertility, and the rarer risks to know before you start.

The common side effects: your stomach

Stomach and gut symptoms are by far the most common. In pooled data from the STEP 1 to 3 trials of semaglutide 2.4 mg, nausea affected 43.9% of people on the drug vs 16.1% on placebo, diarrhea 29.7% vs 15.9%, vomiting 24.5% vs 6.3% and constipation 24.2% vs 11.1%. Most were mild to moderate (98.1%), short-lived and most frequent during or just after dose increases. Only 4.3% stopped the drug because of them.

Tirzepatide has a similar profile: in the SURMOUNT-5 head-to-head trial, the most common side effects of both drugs were gastrointestinal and mostly occurred during dose escalation.

• Eat smaller meals and stop when you feel full.
• Cut back on greasy, fried and very sweet food in the first weeks.
• Drink enough water, especially if you have diarrhea or vomiting.
• Do not raise your dose faster than prescribed. FDA has received reports of side effects, some needing hospital care, linked to dosing errors with compounded injections.

Muscle loss

Part of the weight lost on GLP-1 drugs is lean tissue, not just fat. A 2026 review notes it is still unknown whether the share of muscle lost is greater than with other weight-loss methods, or whether it causes real harm. Resistance exercise and enough protein are the first-line ways to protect muscle. For men who lift, telling your clinician about your training helps them pace the dose.

Testosterone: usually up, not down

Excess weight is one of the most common reasons men have low testosterone, so losing it often helps. In a records study of 110 men with obesity or type 2 diabetes taking semaglutide, dulaglutide or tirzepatide, presented at ENDO 2025, the share with normal total and free testosterone rose from 53% to 77% over 18 months, alongside about 10% weight loss. A 2025 meta-analysis of four studies also found higher bioavailable testosterone on GLP-1 drugs, while effects on free testosterone were inconclusive.

GLP-1 drugs are not a testosterone treatment. If your levels are still low after the weight comes off, a clinician can look at options such as enclomiphene or TRT.

Fertility and sperm

In a small randomized trial of 25 men with obesity, type 2 diabetes and low testosterone, semaglutide 1 mg a week raised total testosterone and increased the share of normally shaped sperm over 24 weeks. Men given testosterone injections instead saw sperm concentration and count fall, which is a known effect of TRT. The trial was small and open label, but it is reassuring for men who want children.

Sex drive and erections

The evidence here is mixed. In that same trial, erectile function scores improved significantly only in the TRT group. Weight loss and better blood sugar can help erections over time, but if your sex drive or erections change on a GLP-1, tell your clinician.

Serious risks to know

• Thyroid tumors: semaglutide and medicines like it caused thyroid tumors in rodent studies, and the labels carry a warning. Do not use them if you or a family member has had medullary thyroid cancer or MEN 2.
• Pancreatitis and gallbladder problems: tell your clinician if you have a history of either, and get help for severe stomach pain that does not go away.
• Low blood sugar: more likely if you also take insulin or sulfonylureas for diabetes.
• Faster resting heart rate: tell your clinician if a racing or pounding heartbeat lasts several minutes.
• Dehydration and kidney strain: from heavy vomiting or diarrhea.

What happens when you stop

Most people regain weight after stopping. One year after semaglutide was stopped in the STEP 1 extension, participants had regained about two-thirds of their weight loss, and their heart and metabolic improvements largely reversed. Plan for treatment as a long-term commitment.

Where to go next: Compare programs in our GLP-1 guide for men, or see how losing weight affects low testosterone.

FAQ

How long does GLP-1 nausea last?

For most people it is worst in the days after starting or raising the dose and eases within a few weeks at a steady dose. In the STEP trials, stomach side effects were mostly mild and short-lived.

Does semaglutide lower testosterone?

Studies point the other way: in men with excess weight, testosterone tends to rise as weight comes off. A small trial found semaglutide raised total testosterone, and unlike TRT it did not lower sperm counts.

Can I take a GLP-1 with TRT or enclomiphene?

Many men do, but only with one clinician who knows about both. Retest your testosterone after you lose weight, since your needs may change.