Tamoxifen Citrate is a Selective Estrogen Receptor Modulator (SERM). Tamoxifen is known mainly for its antagonistic (inhibitory) effects on the estrogen receptor at the breast. As a SERM, it has varying effects on the estrogen receptor in other locations. It has agonistic (activating) effects on the estrogen receptor at the endometrium, part of the female reproductive system.
Tamoxifen is a prodrug that is metabolized by the liver’s cytochrome P450 system into its main active metabolite: 4-hydroxytamoxifen (4-OHT). After binding to the estrogen receptor in the breast, estrogen-responsive genes are inhibited. 4-OHT binds to the estrogen receptor with a much greater affinity than estradiol (main human endogenous estrogen), therefore, it can be used to prevent or treat gynecomastia.