Anabolic steroids are synthetic derivatives of testosterone modified to enhance the anabolic rather than the androgenic actions of the hormone. The anabolic effects are considered to be those promoting protein synthesis, muscle growth and crythopoiesis. There are numerous side-effects to anabolic steroids, including hypertension and atherosclerosis, blood clotting, jaundice, hepatic carcinoma, tendon damage, psychiatric and behavioural effects and, in males, reduced fertility and gynaccomastia. Anabolic steroids were added to the International Olympic Committee's list of banned substances in 1975. The majority of 'evidence' concerning the efficacy of anabolic steroids as performance enhancing agents is anecdotal. In the main, experimental investigations have been poorly designed scientifically, clinically and statistically. The percentage of positive test results from IOC accredited laboratories has remained consistently low. However, athletes take their steroids during training and out-of-competition testing is not conducted in all countries, although international co-operation is now under consideration. Despite the lack of conclusive evidence, steroids users will continue to hold the view that their effects are efficacious and they are therefore unlikely to be persuaded to curtail their use.