The use of performance-enhancing substances by athletes is nearly as old as sport itself. There are two primary categories of substances available to modern athletes: anabolic androgenic steroids (AAS) and nutritional supplements. All AAS and many of the nutritional supplements are used to increase testosterone levels in the body, thereby enhancing the athlete's ability to build lean muscle mass. Other nutritional supplements are used to increase the amount of energy available for workouts or competition. Although steroids are available in the US via physician prescription, nutritional supplements are widely available to all consumers with relatively scant regulation. Steroids are associated with a variety of side effects that can lead to physical changes, psychological disturbances, morbidity, and even mortality. The side effects of nutritional supplements are not as well studied but are presumed to be similarly dangerous. However, for many athletes at all levels facing pressure to excel, the potential benefits of taking these substances appear to be outweighing the associated risks. Increased testing at all levels is recommended.