antiestrogen, any substance that blocks the synthesis or action of the hormone estrogen.
Estrogen can be both a beneficial and a harmful hormone. It maintains skeletal strength by preventing the loss of bone and enhancing calcium retention. However, estrogen causes the proliferation of cells in the breast and the uterus, which can increase a woman’s chance of developing cancer at these sites.
Selective estrogen-receptor modulators (SERMs), such as tamoxifen and raloxifene, produce estrogen action in those tissues (e.g., bone, brain, liver) where that action is beneficial and have either no effect or an antagonistic effect in tissues, such as the breast and uterus, where estrogen action may be harmful. Tamoxifen is used in the prevention and treatment of breast cancer. Raloxifene, used in the prevention and treatment of osteoporosis (the loss of bone mass) in postmenopausal women, also acts as an estrogen agonist in reducing total and low-density lipoprotein (LDL) cholesterol. Adverse effects of raloxifene include hot flashes, leg cramps, and increased risk of deep-vein thrombosis and pulmonary embolism.
Antiestrogens are antagonists at all estrogen receptors. Clomiphene can be used as a fertility drug to stimulate ovulation in some women who are otherwise unable to become pregnant. It interferes with the inhibitory feedback of estrogens on the pituitary gland. This results in an increase in follicle-stimulating hormone and luteinizing hormone release; these hormones in turn stimulate ovarian function.