The use of anabolic drugs by athletes who wish to increase lean body mass and improve muscular strength is widespread, especially among elite weight-trained athletes. The current regimens used for steroid doping include combinations of injectable and oral preparations of steroids at doses 10 to 40 times greater than those prescribed therapeutically. Most of the scientific studies of steroid use by healthy male athletes have used steroid doses substantially lower than those used by many athletes. Analysis of these studies suggests that most persons will gain an average of 2.2 kg of lean body weight during steroid administration but that there exist great individual differences in strength changes induced by steroids. Approximately 50% of the investigations show significant improvements in strength measurements with steroid treatment, whereas the remainder show indefinite effects. There is no substantial evidence to support the use of anabolic steroids for improving aerobic work capacity. Anabolic steroids cause interrupted growth and virilization in children, birth defects in the unborn, severe virilization in women, and testicular atrophy and reduced blood levels of gonadotropins and testosterone in adult males. In addition, the oral preparations of anabolic steroids are associated with liver dysfunction, including carcinoma and peliosis hepatis, and a number of other disorders including unpredictable changes in mood, aggression, and libido. Although there have been only rare reports of severe or life-threatening side effects in athletes who have abused steroids, such side effects may not appear obvious until 20 years or more of widespread steroid abuse.