Antiestrogens, also known as estrogen antagonists or estrogen blockers, are a class of drugs that prevent estrogens like estradiol from mediating their biological effects in the body. They act by blocking the estrogen receptor (ER) and/or inhibiting or suppressing estrogen production. Antiestrogens are one of three types of sex hormone antagonists, the others being antiandrogens and antiprogestogens.
Antiestrogens include selective estrogen receptor modulators (SERMs) like tamoxifen, clomifene, and raloxifene, the ER silent antagonist and selective estrogen receptor degrader (SERD) fulvestrant, aromatase inhibitors (AIs) like anastrozole, and antigonadotropins including androgens/anabolic steroids, progestogens, and GnRH analogues.
Although aromatase inhibitors and antigonadotropins can be considered antiestrogens by some definitions, they are often treated as distinct classes. Aromatase inhibitors and antigonadotropins reduce the production of estrogen, while the term "antiestrogen" is often reserved for agents reducing the response to estrogen.