Antiestrogens, also known as estrogen antagonists or estrogen blockers, are a class of drugs which 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.
Types and examples
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.
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.
Antiestrogens are used for:
Side effects of antiestrogens include hot flashes, osteoporosis, breast atrophy, vaginal dryness, and vaginal atrophy. In addition, they may cause depression and reduced libido.
Ethamoxytriphetol (MER-25) was the first antagonist of the ER to be discovered, followed by  clomifene and tamoxifen. 
"Definition of antiestrogen - NCI Dictionary of Cancer Terms, Definition of antiestrogen - NCI Dictionary of Cancer Terms". ,
Dorland's Medical Dictionary
Judi Lindsley Nath (2006). . Lippincott Williams & Wilkins. pp. Using Medical Terminology: A Practical Approach 977–. ISBN . 978-0-7817-4868-1
Eckhard Ottow; Hilmar Weinmann (8 September 2008). . John Wiley & Sons. pp. 164–165. Nuclear Receptors as Drug Targets ISBN . 978-3-527-62330-3
Bruce A. Chabner; Dan L. Longo (8 November 2010). . Lippincott Williams & Wilkins. pp. 660–. Cancer Chemotherapy and Biotherapy: Principles and Practice ISBN . 978-1-60547-431-1
Riggins RB, Bouton AH, Liu MC, Clarke R (2005). "Antiestrogens, aromatase inhibitors, and apoptosis in breast cancer". Vitam. Horm. Vitamins & Hormones. 71: 201–37. doi: 10.1016/S0083-6729(05)71007-4. ISBN . 9780127098715 PMID 16112269.
Thiantanawat, Apinya; Long, Brian; Brodie, Angela (2003-11-15). "Signaling Pathways of Apoptosis Activated by Aromatase Inhibitors and Antiestrogens". The Journal of Cancer Research . Retrieved . 2015-04-13
Philipp Y. Maximov; Russell E. McDaniel; V. Craig Jordan (23 July 2013). . Springer Science & Business Media. pp. 7–. Tamoxifen: Pioneering Medicine in Breast Cancer ISBN . 978-3-0348-0664-0
V Craig Jordan (27 May 2013). . World Scientific. pp. 7, 112. Estrogen Action, Selective Estrogen Receptor Modulators and Women's Health: Progress and Promise ISBN . 978-1-84816-959-3
Walter Sneader (23 June 2005). . John Wiley & Sons. pp. 198–199. Drug Discovery: A History ISBN . 978-0-471-89979-2
External links This article incorporates public domain material from the U.S. National Cancer Institute document: "Dictionary of Cancer Terms".