GLP-1 Medications and Testosterone in Men: The Trade Nobody Explains

A man of 46 starts Mounjaro. Eight months later he has lost twenty-two kilograms, his bloods look better than they have in a decade, and his testosterone has climbed from 9.2 to 15.8 nmol/L without a single injection.

He also cannot do the press-ups he could manage last year, and his deadlift has fallen off a cliff.

Both of those things came from the same drug, and neither is a side effect in the usual sense. They are two halves of the same physiology, and which half dominates depends almost entirely on what the man does while taking it.

What these drugs are

The names overlap confusingly, so it helps to separate them.

• Semaglutide — Ozempic for diabetes, Wegovy for weight management
• Tirzepatide — Mounjaro for diabetes, Zepbound for weight management
• Liraglutide — Victoza and Saxenda, older and less potent

Semaglutide acts on one receptor. Tirzepatide acts on two and produces greater weight loss. That difference matters more than the branding suggests, and it returns below in a way most men will not expect.

In trials, men lose roughly 15 per cent of body weight on semaglutide and 18 to 21 per cent on tirzepatide. Those numbers previously required surgery.

They do raise testosterone, and the mechanism is not the drug

The effect is real and larger than most men anticipate.

A study following 110 men with obesity and type 2 diabetes over 18 months found that a 10 per cent reduction in body weight was accompanied by a 53 to 77 per cent rise in total and free testosterone. None of the men were taking testosterone or any hormonal therapy.

Read that carefully, because the mechanism matters for what follows. These drugs do not act on your testes. They do not stimulate the pituitary, they do not block oestrogen, and they have no direct effect on the hypothalamic-pituitary-gonadal axis at all.

What they do is remove fat. Fat tissue contains aromatase, the enzyme converting testosterone into oestradiol, so carrying less of it means converting less. Excess fat also drives insulin resistance and inflammation, both of which suppress the signal from brain to testes. Take away enough fat and the whole loop unwinds.

This is the same mechanism that raises testosterone after bariatric surgery, and the same one that raises it in men who lose substantial weight through diet alone. Why your belly fat will not budge covers the loop in detail, and what causes low testosterone lists visceral fat as the single largest contributor in middle-aged men.

The drug is not a hormone treatment. It is a very effective way of removing the thing that was suppressing your hormones.

The part that gets left out

Now the other half.

All substantial weight loss costs lean tissue. These drugs produce substantial weight loss. The arithmetic follows.

In the body-composition substudy of STEP 1, men and women on semaglutide lost about 15 per cent of body weight, and lean mass accounted for roughly 40 per cent of what came off. In SURMOUNT-1, tirzepatide produced greater weight loss with a more favourable split: about 25 per cent from lean tissue.

Across published trials the range sits at 25 to 39 per cent of total weight lost being fat-free mass.

That proportion is not unusual. Diet-only weight loss produces a similar ratio. The difference is the absolute amount, because the total weight loss is so much larger. A man of 110 kilograms losing 22 per cent of his body weight may shed six to eight kilograms of lean tissue if he does nothing to prevent it.

"Lean tissue" is not all muscle. It includes water, glycogen, connective tissue and organ mass, and some of that reduction is appropriate for a smaller body. But a meaningful share is skeletal muscle, and skeletal muscle is what you use to lift things, climb stairs, and stay independent at eighty.

The difference between the two drugs

Here is the finding worth knowing before you choose, and it did not come from a trial.

A 2026 analysis of routine care covering 670,422 first-time GLP-1 users compared the two drugs directly on body composition. Tirzepatide was associated with greater relative lean mass loss than semaglutide at every measured point, with excess losses of 1.1, 1.5, 1.3 and 2 percentage points at three, six, nine and twelve months.

The researchers also defined what they called a depletive pattern: more than 20 per cent total weight loss combined with more than 5 per cent lean mass loss. It occurred in 10.3 per cent of tirzepatide users against 6.7 per cent on semaglutide.

The same analysis found the strongest predictor of heavy lean-tissue loss was not age or starting weight. It was baseline musculoskeletal pain. Men with neck or knee pain lost substantially more lean mass, presumably because pain limits the training that would have protected it.

If your knees hurt and you cannot train, you are in the group most likely to lose muscle on these drugs. That is precisely backwards from where you want to be, and it is the strongest argument for sorting out the training question before you start rather than after.

Why this matters more for a man than for the scales

Muscle is not decoration. It is where you dispose of glucose, and losing it works against the metabolic improvement you are taking the drug to achieve.

There is also a circularity worth naming. Testosterone builds and maintains muscle. Muscle mass supports insulin sensitivity. Insulin sensitivity supports testosterone. Strip out enough muscle and you weaken one of the mechanisms that raised your testosterone in the first place.

So the man who loses twenty-five kilograms with no attention to protein or training gets a better testosterone reading and a worse body. The man who does the same weight loss while lifting keeps both.

The drug does not decide which of those you become.

The two things that protect you

Neither is complicated, and both are the same things that protect muscle during any deficit.

Protein, deliberately. Around 1.2 to 1.6 grams per kilogram of body weight daily, spread across three or four meals with at least 25 to 30 grams in each. On a drug engineered to remove your appetite, this requires planning rather than hunger. Most men on these medications undereat protein badly, because they eat less of everything and protein is the first casualty.

Resistance training, two or three times weekly. This is the only intervention that directly opposes muscle loss, and a systematic review found it preserves lean mass during GLP-1 weight loss. Compound movements, progressive load, nothing exotic. How much muscle can you build naturally covers realistic expectations, and the free workout generator will build you a plan.

The pharmaceutical industry has noticed the problem. Trials combining semaglutide with myostatin inhibitors have prevented roughly half the lean mass loss, and one phase 2 study brought the lean fraction down to about 7 per cent. Those drugs are years from routine use. Protein and a barbell are available this week.

What to test, and when

If you are starting one of these drugs and testosterone is part of why:

Get a baseline first. Two morning samples before eleven, on separate days, with SHBG, free testosterone, LH and FSH. Without a baseline you will never know what the weight loss did. The blood test guide covers what to ask for.

Retest after six months, or after roughly 10 per cent weight loss, whichever comes first. That is when the change becomes measurable.

Track something besides the scales. Waist measurement, grip strength, or simply what you can lift. Weight alone cannot distinguish between the two outcomes described above, and it is the number most likely to mislead you.

If you are already on TRT

This combination is increasingly common and there is no formal contraindication, but two things follow from the mechanism.

Weight loss raises your own testosterone production. If you are on a fixed replacement dose and simultaneously restoring endogenous output, your levels can climb higher than intended, and haematocrit with them. TRT side effects covers why thickened blood is the finding that changes management fastest.

And for some men, particularly those whose low testosterone was driven mainly by obesity, substantial weight loss may reduce or remove the need for replacement entirely. That is worth knowing in advance, because it is a conversation about dose reduction rather than a permanent prescription. TRT versus natural optimisation covers where that line sits.

Either way, retest rather than assume. The dose that was right at 120 kilograms may not be right at 95.

The honest summary

GLP-1 medications raise testosterone in men with obesity, substantially, without touching the hormonal system directly. For a man whose low testosterone was driven by weight, that is a genuine route out, and the effect size beats anything a supplement has produced.

They also remove lean tissue, more so with tirzepatide, and most so in men who cannot train.

The useful framing is not whether to take them. It is that these drugs create a window during which your body composition is changing rapidly in both directions at once, and what you do with protein and training during that window determines which change you keep.

Nobody is going to tell you this at the prescription stage. The consultation covers nausea and dose escalation. The muscle conversation is one you have to start.

Common questions

Do GLP-1 drugs raise testosterone?

Indirectly, and substantially. One study of 110 men found a 10 per cent weight reduction accompanied by a 53 to 77 per cent rise in testosterone over 18 months. The drugs do not act on the testes; they remove the fat that was suppressing production.

Will I lose muscle on Ozempic or Mounjaro?

Some, unless you actively prevent it. Across trials, 25 to 39 per cent of total weight lost is fat-free mass. That proportion is similar to dieting, but the absolute amount is larger because the weight loss is larger.

Which is worse for muscle, semaglutide or tirzepatide?

Tirzepatide, in routine care. An analysis of 670,000 users found consistently greater lean mass loss on tirzepatide, and a depletive pattern in 10.3 per cent of users against 6.7 per cent on semaglutide.

How do I protect my muscle?

Protein at 1.2 to 1.6 g per kg of body weight daily, spread across meals, and resistance training two or three times a week. These are the only two interventions with real evidence behind them.

Can I take these with TRT?

There is no formal contraindication, but weight loss raises your own production, so levels and haematocrit can climb higher than intended on a fixed dose. Retest rather than assume, and expect a dose conversation.

Will my testosterone stay up if I stop?

Only if the weight stays off. The testosterone rise came from losing fat, not from the drug, so regaining the weight reverses the mechanism. Muscle built during treatment is what makes maintenance easier afterwards.

Sources: Wilding JPH et al., Once-weekly semaglutide in adults with overweight or obesity (STEP 1), N Engl J Med 2021;384:989-1002 · Jastreboff AM et al., Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1), N Engl J Med 2022;387:205-16 · Portillo Canales S et al., Anti-obesity medication use and testosterone levels in men with obesity and type 2 diabetes, presented 2026 · Body-composition digital phenotyping of 670,422 first-episode GLP-1 receptor agonist users, medRxiv 2026 · Locatelli JC et al., Resistance training preserves lean mass during weight loss with GLP-1 receptor agonists: a systematic review, Obesity 2024;32:1234-47 · Corona G et al., Body weight loss reverts obesity-associated hypogonadotropic hypogonadism, Eur J Endocrinol 2013;168(6):829-43 · Educational only, not medical advice. Never stop or change a prescribed medicine without speaking to your doctor.

Keep reading: What causes low testosterone · Why your belly fat will not budge · How much muscle can you build naturally · TRT side effects · Blood test: what to check · Free workout generator