Tamoxifen is an oral selective estrogen receptor modulator which is used in breast cancer, and is currently the world's largest selling breast cancer treatment. It is used for the treatment of early and advanced breast cancer in pre- and post-menopausal women. It is also approved by the FDA for the reduction of the incidence of breast cancer in women at high risk of developing the disease. It has been further approved for the reduction of contralateral (in the opposite breast) breast cancer.

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. Interestingly enough, in the United States, the drug was almost always referred by its generic name even before patent expiration.

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.