Trenbolone is a synthetic analogue of testosterone, belonging to the nandrolone group. It has both a strong anabolic effect and a limited androgenic effect (i.e. an androgen and anabolic steroid - AAS). It is used illegally by professional or amateur athletes, who want to improve their athletic performance and appearance by increasing their muscle mass. Trenbolone, like other AASs, are harmful, with 90% of users experiencing injurious side effects. It acts systemically on the body, and as such, its side effects can manifest as symptoms from different systems. Nevertheless, its popularity is increasing. This paper reviews the current state of knowledge regarding the adverse effects of trenbolone on the nervous, reproductive, immune systems and breast, muscular and adipose tissues. However, various other adverse consequences of trenbolone utilization are observed, with severe acne and gynaecomastia affecting approximately one-third of all users, as well as excessive body hair, stretch marks, hypertension and cardiac arrhythmia. The drugs are also subject to contamination, with use frequently resulting in local inflammation at the injection site, muscle adhesions and fibrosis, nerve damage or, in extreme cases, necrosis of the injection site. Additionally, due to the lack of available knowledge on the subject, many of the effects of trenbolone use remain unknown. Moreover, the fact that multiple AASs may be used simultaneously presents a significant problem in their study. Therefore, further research is necessary to better understand the effects of AAS on the body, and to expand our currently incomplete knowledge of their functional pathways.