Use of performance-enhancing supplements occurs at all levels of sports, from professional athletes to junior high school students. Although some supplements do enhance athletic performance, many have no proven benefits and have serious adverse effects. Anabolic steroids and ephedrine have life-threatening adverse effects and are prohibited by the International Olympic Committee and the National Collegiate Athletic Association for use in competition. Blood transfusions, androstenedione, and dehydroepiandrosterone are also prohibited in competition. Caffeine, creatine, and sodium bicarbonate have been shown to enhance performance in certain contexts and have few adverse effects. No performance benefit has been shown with amino acids, beta-hydroxy-beta-methylbutyrate, chromium, human growth hormone, and iron. Carbohydrate-electrolyte beverages have no serious adverse effects and can aid performance when used for fluid replacement. Given the widespread use of performance-enhancing supplements, physicians should be prepared to counsel athletes of all ages about their effectiveness, safety, and legality.