Tamoxifen competes with oestrogen in the body for oestrogen receptors in breast tissue so that transcription of oestrogen-responsive genes is inhibited.

Tamoxifen was invented by ICI Pharmaceuticals (now AstraZeneca) and is sold under the brand names Nolvadex, Istubal, and Valodex. It is also available as a generic drug in a number of countries. In the United States and other countries, Tamoxifen was almost always referred to by its generic name even before its patents expired.

A rare condition occasionally treated with tamoxifen is retroperitoneal fibrosis.

Tamoxifen is sometimes used to treat gynaecomastia in men. Tamoxifen is also used by bodybuilders in a steroid cycle to try and prevent or reduce drug-induced gynaecomastia caused by steroids that are used in the same cycle.

Tamoxifen is also used to treat infertility in women with anovulatory disorders. A dose of 10-40 mg per day is administered in days 3-7 of a woman's cycle.

On April 17, 2006, it was announced that raloxifene is equally effective in reducing the incidence of breast cancer, but caused fewer side effects.

Side effects
Tamoxifen is a selective oestrogen receptor modulator. Even though it is an antagonist in breast tissue it acts as partial agonist on the endometrium. Therefore endometrial changes, including cancer, are among tamoxifen's side effects. For some women, tamoxifen can cause a rapid increase in triglyceride concentration in the blood.

4-hydroxytamoxifen
4-hydroxytamoxifen is a form of the drug tamoxifen that is made by the body after taking tamoxifen. It can also be made in the laboratory, and may help decrease breast density. A topical form of 4-hydroxytamoxifen is being studied in breast cancer screening.

Pharmacogenetics
Patients with variant forms of the gene CYP2D6 (also called simply 2D6) may not receive full benefit from tamoxifen. On Oct 18, 2006 the Subcommittee for Clinical Pharmacology recommended relabeling tamoxifen to include information about this gene in the package insert.