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

Abstract

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. 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. 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). 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). 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. ClinicalTrials.gov identifier, NCT05692856.