Clenbuterol built extra muscle but no extra fat loss in an 8-week trial

Clenbuterol is a potent β2-adrenoceptor agonist that people commonly take off-label alongside resistance training, believing it builds muscle and strips fat at the same time. Despite that reputation, its long-term effects on body composition and strength had never been properly tested in a controlled trial, and nobody knew what happens after someone stops taking it. Two questions hang over any drug used this way: does it actually deliver both promised effects while you are on it, and does prior use leave any lasting advantage once you quit? This trial set out to answer both in healthy young men and women.

Researchers at the University of Copenhagen ran a randomised, double-blinded, placebo-controlled trial between March 2023 and March 2025. Thirty-two healthy young adults, 16 men and 16 women in their mid-twenties, completed 8 weeks of resistance training while taking either 80 μg of clenbuterol daily or a placebo. That was followed by 16 weeks of detraining with no drug, and then another 8 weeks of resistance training. Body composition was measured by DXA scanning and strength by one-repetition maximum tests on the leg press and bench press, assessed before the intervention and after each training and detraining period.

During the first 8 weeks, clenbuterol increased lean mass gains by 1.7 kg over placebo, and boosted bench press by 6 kg and leg press by 16 kg more than placebo, with clearly stronger effects in the men than the women. Crucially, fat loss did not differ between the clenbuterol and placebo groups. After the drug was stopped, the clenbuterol group lost more lean mass and strength during detraining, so their lean mass and bench press advantages disappeared entirely, though a leg press edge remained. During retraining, lean mass and leg press gains were no different between groups; only bench press rose 3 kg more in the prior clenbuterol group.

This is the first randomised trial to track what happens to clenbuterol users through training, discontinuation and retraining, so it fills a genuine gap: prior use was assumed by many to leave a lasting muscle advantage, and the drug's fat-burning reputation was largely untested in this setting. The findings complicate both beliefs. The muscle and strength gains were real and substantial while on the drug, but they largely evaporated after stopping, and no extra fat came off at all. Open questions remain about higher doses, longer use, trained lifters, and the drug's safety profile, none of which this trial addressed.

If you have ever seen clenbuterol promoted as a shortcut to a leaner, more muscular physique, this trial is a reason for scepticism on both counts. The extra fat loss that gives the drug its reputation simply did not show up here, and the genuine muscle and strength gains mostly disappeared within 16 weeks of stopping. Prior use also bought no advantage when training resumed, apart from a small lingering bench press edge. This is information about a drug used off-label, not an endorsement: the trial was too small and too short to say anything about safety.