Tamoxifen is an antagonist of the estrogen receptor in breast tissue via its active metabolite, 4-hydroxytamoxifen. In other tissues such as the endometrium, it behaves as an agonist, and thus may be characterized as a selective estrogen-receptor modulator. Tamoxifen is the usual endocrine (anti-estrogen) therapy for hormone receptor-positive breast cancer in pre-menopausal women, and is also a standard in post-menopausal women although aromatase inhibitors are also frequently used in that setting.Some breast cancer cells require estrogen to grow. Estrogen binds to and activates the estrogen receptor in these cells. Tamoxifen is metabolized into compounds that also bind to the estrogen receptor but do not activate it. Because of this competitive antagonism, tamoxifen acts like a key broken off in the lock that prevents any other key from being inserted, preventing estrogen from binding to its receptor, blocking cancer cell growth.Tamoxifen was discovered by pharmaceutical company Imperial Chemical Industries (now AstraZeneca) and is sold under the trade names Nolvadex, Istubal, Valodex, and Genox. However, the drug has been widely referred to by its generic name ""tamoxifen"", even before its patent expiration.It is on the World Health Organization's List of Essential Medicines, a list of the most important medication needed in a basic health system.