Effects of Clenbuterol on Body Composition and Strength Following Resistance Training, Detraining, and Retraining in Healthy Adults: A Randomized Controlled Trial.

BACKGROUND: Clenbuterol, a potent β2-adrenoceptor agonist, is commonly used off-label in conjunction with resistance training to increase muscle mass and decrease fat mass. However, the long-term effects on body composition and strength with use and following discontinuation of use remain unexplored. OBJECTIVE: The aim of the study was to investigate the effects of clenbuterol on body composition and strength during a resistance training intervention, as well as the long-term effects of prior use of clenbuterol during detraining and subsequent retraining in healthy young men and women. METHODS: This randomized, double-blinded, placebo-controlled trial was conducted at the University of Copenhagen, Denmark, between March 2023 and March 2025. Thirty-two healthy men (26 ± 3 years; mean ± SD; n = 16) and women (25 ± 5 years; n = 16) completed 8 weeks of resistance training with daily ingestion of clenbuterol (80 μg) (n = 16) or placebo (n = 16). This was followed by 16 weeks of detraining and another 8 weeks of resistance training. Before the intervention and after each training and detraining period, body composition was assessed by dual-energy X-ray absorptiometry (DXA) and strength was assessed by leg press and bench press 1-repetition maximum (1RM). RESULTS: Clenbuterol increased gains in lean mass (+ 1.7 kg, p < 0.001) and 1RM bench press (+ 6 kg, p < 0.001) and leg press (+ 16 kg, p = 0.001) during 8 weeks of resistance training compared with placebo, with a more pronounced effect in males than females (lean mass: p = 0.003; 1RM bench press: p < 0.001), whereas the decrease in fat mass did not differ between clenbuterol and placebo (p = 0.614). Following cessation of clenbuterol, detraining decreased lean mass (p < 0.001), 1RM bench press (p < 0.001), and 1RM leg press (p = 0.026) to a greater extent in the clenbuterol group, such that initial changes in lean mass (p = 0.363) and 1RM bench press (p = 0.834) no longer differed from placebo, whereas 1RM leg press remained elevated in the clenbuterol group (p = 0.041). Retraining increased lean mass and 1RM leg press with no differences between the clenbuterol and placebo group (lean mass: p = 0.393; 1RM leg press: p = 0.662), whereas the increase in 1RM bench press was greater in the clenbuterol group (+ 3 kg, p = 0.047). CONCLUSION: Clenbuterol markedly augments training-induced gains in lean mass and strength. After discontinuation, prior clenbuterol exposure does not enhance retraining-induced lean mass gains, but some strength-related advantages persist. TRIAL REGISTRATION: ClinicalTrials.gov identifier, NCT05692856.