Anti-estrogens represent substances that block the harmful action estrogen may have in some women and men. These chemicals that are closed enough in structure to estrogen fit into the estrogen receptor site, so that any estrogen produced by the body or exogenous estrogen cannot find an open receptor site to attach to. The free-floating estrogen then presents far less problems to homeostasis.

Estrogens are hormones that promote the development and maintenance of female sex characteristics. In men, excessive estrogen can cause gynecomastia, water retention (edema), impotence (in men) and acne.

Estrogen may also interfere with testosterone production in men and may cause libido problems.

Anti-estrogens work by binding to the estrogen receptors and by doing so, they prevent harmful estrogen from binding to these receptor sites.

Aromatase Inhibitors represent substances that inhibit the formation of estrogen in the body. Athletes and bodybuilders who use anabolic steroids frequently use aromatase inhibitors for decreasing the amount of estrogen that result as a side effect from using these drugs. Although aromatase inhibitors could be considered anti-estrogens by some definitions, they are often considered to be a distinct class. Aromatase inhibitors reduce the production of estrogen, while the term "antiestrogen" is usually reserved for agents reducing the response to estrogen.