Anti-estrogens include two main types of products:
Selective Estrogen Receptor Modulators (SERMS)
Aromatase Inhibitor (AI's)
While both are prescribed for patients requiring a reduction in levels of circulating estrogen, SERMS such as tamoxifen and clomiphene, work by binding to the estrogen receptor and thereby displace estrogen from binding there. On the other hand, aromatase inhibitors work by preventing the formation of estrogen entirely such that there is no possibility of estrogen binding to estrogen receptor targets.
Anti-estrogens are often prescribed to women with breast cancer as well as to men who have gynecomastia. It is also used off-label by bodybuilders seeking to restore their testosterone levels after a cycle of anabolic steroids.