A non-essential protein-like compound found in muscle and the brain. It is synthesized in the liver from the two amino acids arginine and glycine, and then taken up from the bloodstream into the muscle where it becomes phosphocreatine. During exercise, phosphocreatine donates its high-energy phosphate molecule to create adenosine triphosphate (ATP), which is used by the muscle for contraction.

Creatine is most commonly used for improving exercise performance and increasing muscle mass in athletes and older adults. There are numerous scientific studies supporting the use of creatine for improving the athletic performance of young, healthy people during brief high-intensity activity such as sprinting and weightlifting. Because of this, creatine is often used as a dietary supplement to improve muscle strength and overall athletic performance.