An athlete may think that if a small amount of a chemical helps his or her performance, more will work better. The most appealing supplements are those that claim to help build muscle, improve endurance, and reduce body fat. Widespread acceptance of herbal of "natural" alternatives to mainstream medicine (especially nutritional supplements) is increasing, and the market is largely unregulated. The authors summarize the facts and fiction surrounding the use of popular products that may be found at the pharmacy and health food store that are being used in the locker rooms of high schools, colleges, and gyms in the U.S. They urge clinicians to stress the value of a well balanced diet to their active adolescent patients and not to encourage supplement use.