Anabolic steroids, officially known as anabolic-androgen steroids (AAS), are drugs which mimic the effects of the male sex hormones testosterone and dihydrotestosterone. They increase protein synthesis within cells, which results in the buildup of cellular tissue (anabolism), especially in muscles. Anabolic steroids also have androgenic and virilizing properties, including the development and maintenance of masculine characteristics such as the growth of the vocal cords and body hair. The word anabolic comes from the Greek ἀναβολή anabole, "that which is thrown up, mound", and the word androgenic from the Greek ἀνδρός andros, "of a man" + -γενής -genes, "born".

Anabolic steroids were first isolated, identified and synthesized in the 1930s, and are now used therapeutically in medicine to stimulate bone growth and appetite, induce male puberty, and treat chronic wasting conditions, such as cancer and AIDS. The American College of Sports Medicine acknowledges that AAS, in the presence of adequate diet, can contribute to increases in body weight, often as lean mass increases, and that the gains in muscular strength achieved through high-intensity exercise and proper diet can be additionally increased by the use of AAS in some individuals.

Some health risks can be produced by long-term use or excessive doses of anabolic steroids. These effects include harmful changes in cholesterol levels (increased low-density lipoprotein and decreased high-density lipoprotein), acne, high blood pressure, liver damage (mainly with oral steroids), and dangerous changes in the structure of the left ventricle of the heart.

Ergogenic uses for anabolic steroids in sports and bodybuilding are controversial because of their adverse effects and the potential to gain an advantage conventionally considered "cheating." Their use is referred to as doping and banned by all major sporting bodies. For many years AAS have been by far the most detected doping substances in IOC-accredited laboratories. In countries where AAS are controlled substances, there is often a black market in which smuggled or even counterfeit drugs are sold to users.