Mechanism of action
Testosterone and all anabolic steroids act through androgen receptors. Androgen receptors are located on the x chromosome of cells and are ubiquitous in the body. In addition to muscles, androgen receptors affect the heart, immunity, and nervous system.

Testosterone's popularity as a doping drug is attributed to its strong effect on muscle strength and mass. Testosterone also affects lipolysis, which is the breakdown of fat cells. Both short-term and long-term use of anabolic steroids lead to increased cell growth due to increased protein synthesis. Testosterone-induced muscle cell growth is mediated by the activation of satellite cells and the growth of muscle cell nuclei.

The number of androgen receptors is limited, and usually normal testosterone levels are sufficient. Hence, muscle growth is not necessarily attributable to excess anabolic steroids. Another mechanism that explains muscle growth may be the effect of anabolic steroids on cortisol. Cortisol is a catabolic hormone and anabolic steroids can reduce its effect. The inhibitory effect of anabolic steroids on the myostatin gene is also thought to be a muscle growth mechanism. Myostatin regulates muscle growth. Testosterone also increases the secretion of growth hormone and insulin-like growth factor.