Tamoxifen citrate is a selective estrogen receptor modulator. Selective estrogen receptor modulators can act as estrogen receptor agonists or antagonists. This activity of tamoxifen citrate is tissue selective, effecting those estrogen receptors located in the liver, breast, and bone. When the tamoxifen molecule binds to this receptor, the estrogen is blocked and can not have any influence, thereby remaining inactive in that tissue. By doing so, an "anti-estrogenic" effect is achieved.