Creatine is an amino acid produced naturally by the liver and kidneys and stored in muscles.
Athletes take creatine supplements to gain extra energy, train longer and harder - and bulk up. The supplement is popular among pro and college athletes in the United States, where it is considered a legal alternative to steroids. The U.S. Food and Drug Administration lists it as a food supplement and allows it to be sold with no more restrictions than those on vitamins. Creatine is not listed as a banned substance by the International Olympic Committee, but its sale is illegal in France. High-profile athletes who have admitted to using creatine include the French tennis player Mary Pierce and U.S. baseball star Mark McGwire.