Creatine Monohydrate

It is often used to improve exercise performance and muscle mass.

Creatine is involved in making energy for muscles. About 95% of it is found in skeletal muscle. The majority of sports supplements in the US contain creatine. People who have lower creatine levels when they start taking creatine seem to get more benefit than people who start with higher levels.
People commonly use creatine for improving exercise performance and increasing muscle mass. It is also used for muscle cramps, fatigue, multiple sclerosis (MS), depression, and many other conditions, but there is no good scientific evidence to support most of these uses.

Creatine use is allowed by the International Olympic Committee and National Collegiate Athletic Association (NCAA).