Creatine is used in muscle cells to store energy for sprinting and explosive exercise. Athletes can increase the amount of creatine in muscle by taking creatine supplements. Although some studies report no ergogenic effect, most indicate that creatine Supplementation (e.g. 20 g per day for 5 to 7 days) increases sprint performance by 1-5% and work performed in repeated sprints by up to 15%. These ergogenic effects appear to be related to the extent of uptake of creatine into muscle. Creatine supplementation for a month or two during training has been reported to promote further gains in sprint performance (5-8%), as well as gains in strength (5-15%) and lean body mass (1-3%). The only known side effect is increased body weight. More research is needed on individual differences in the response to creatine, periodic or cyclical use of creatine, side effects, and long-term effects on endurance.