Creatine is a compound made in the body from amino acids and stored mainly in muscle, where it helps regenerate ATP — the immediate energy currency used in short, hard efforts. Creatine monohydrate is the supplemental form used in nearly all of the research.
The International Society of Sports Nutrition (ISSN) position stand, published in the Journal of the International Society of Sports Nutrition in 2017, states that creatine monohydrate is the most effective ergogenic nutritional supplement currently available to athletes for increasing high-intensity exercise capacity and lean body mass during training. It reports that supplementation is not only safe but well tolerated in the short and long term in healthy populations, at studied doses (commonly a loading phase of about 5 g four times daily for 5–7 days, or 3–5 g daily without loading), and describes studied uses beyond performance, including in some clinical settings. The document also states that the greatest gains appear in repeated short-duration, high-intensity work, and that reports of harm to kidney function in healthy users are not supported by the trials it reviews.