Anabolic steroids, also known as anabolic–androgenic steroids (AAS), are a class of drugs that are structurally related to testosterone, the main male sex hormone, and produce effects by binding to and activating the androgen receptor (AR). The term "anabolic steroid" is essentially synonymous with "steroidal androgen" or "steroidal androgen receptor agonist". Anabolic steroids have a number of medical uses, but are also used by athletes to increase muscle size, strength, and performance.