The tamoxifen molecule is an antagonist of the estrogen receptor in breast tissue via it's active metabolite, hydroxytamoxifen. In other tissues, such as the endometrium, it behaves as an agonist, so tamoxifen may be classified as both agonist or antagonist in it's function. Tamoxifen has been the standard endocrine (anti-estrogen) therapy for hormone receptor-positive early breast cancer in premenopausal women.
Some breast cancer cells require estrogen to grow and replicate. 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 competition, tamoxifen acts like a "key broken off in the lock", in that it prevents any other key from being inserted into the lock. This antagonistic blocking of binding to estrogen effectively blocks cancer cell growth.
Tamoxifen is metabolized by the liver into active metabolites, such as 4-hydroxytamoxifen and N-desmethyl-4-hydroxytamoxifen (endoxifen). These metabolites have 30-40 times more affinity for the estrogen receptor than tamoxifen itself. Tamoxifen is also used to treat sex offenders undergoing treatment by temporary chemical castration, people with bipolar disorder, angiogenesis (new blood vessel growth), and control of gene expression in genetically modified animals.