Gonadotropin-releasing hormone agonist
A gonadotropin-releasing hormone agonist (GnRH agonist, GnRH–A) is a synthetic peptide modeled after the hypothalamic neurohormone GnRH that interacts with the gonadotropin-releasing hormone receptor to elicit its biologic response, the release of the pituitary hormones FSH and LH.
GnRH agonists are pregnancy category X drugs.
Flare effect and downregulation 
Agonists do not quickly dissociate from the GnRH receptor. As a result initially there is an increase in FSH and LH secretion (so-called "flare effect").
However after about ten days a profound hypogonadal effect (i.e. decrease in FSH and LH) is achieved through receptor downregulation by internalization of receptors. Generally this induced and reversible hypogonadism is the therapeutic goal.
Agonists with double and single substitutions 
GnRH agonists are synthetically modeled after the natural GnRH decapeptide with specific amino acid substitutions typically in position 6 and 10. These substitutions inhibit rapid degradation. Agonists with two substitutions include: leuprolide (Lupron, Eligard), buserelin (Suprefact, Suprecor), histrelin (Supprelin LA, Vantas), goserelin (Zoladex), and deslorelin (Suprelorin, Ovuplant). The agents nafarelin (Synarel) and triptorelin are agonists with single substitutions at position 6.
GnRH agonists are useful in:
- Treatment of cancers that are hormonally sensitive and where a hypogonadal state decreases the chances of a recurrence. Thus they are commonly employed in the medical management of prostate cancer and have been used in patients with breast cancer.
- Treatment of delaying puberty in individuals with precocious puberty.
- Delaying puberty pending treatment decisions in children with gender incongruency
- Management of female disorders that are dependent on estrogen productions. Women with menorrhagia, endometriosis, adenomyosis, or uterine fibroids may receive GnRH agonists to suppress ovarian activity and induce a hypoestrogenic state.
- Suppressing hormone levels in transsexual people, especially trans women.
- IVF therapy: they allow for better control of ovarian hyperstimulation during the administration of exogenous FSH. Typically, after GnRH agonists have induced a state of hypoestrogenism, exogenous FSH is given to stimulate ovarian follicle, followed by human chorionic gonadotropins (hCG) to trigger oocyte release. GnRH agonists routinely used for this purpose are: buserelin, leuprorelin, nafarelin, and triptorelin. GnRH agonists also provide a valid alternative to oocyte release triggering with HCG, resulting in an elimination of the risk of ovarian hyperstimulation syndrome. The delivery rate is approximately 6% less than with hCG triggering.
- Severe cases of congenital adrenal hyperplasia
- Temporary Suppression of Fertility in Male Dogs
- Induction of Ovulation in Mares
Women of reproductive age who undergo cytotoxic chemotherapy have been pretreated with GnRH agonists to reduce the risk of oocyte loss during such therapy and preserve ovarian function. Further studies are necessary to prove that this approach is useful.
Side effects 
Side effects of the GnRH agonists are signs and symptoms of hypoestrogenism, including hot flashes, headaches, and osteoporosis. In patients under long-term therapy, small amounts of estrogens could be given back (“add-back regimen”) to combat such side effects and to prevent bone wastage. Generally, long-term patients, both male and female, tend to undergo annual DEXA scans to appraise bone density.
See also 
- Use of agonists in endometriosis
- Lupron, by manufacturer
- Buserelin website
- Information of use of Zoladex in prostate cancer
- 10th International Symposium on GnRH
- SupprelinLA, by Endo Pharmaceuticals, Inc.
- van Loenen AC, Huirne JA, Schats R, Hompes PG, Lambalk CB (November 2002). "GnRH agonists, antagonists, and assisted conception". Semin. Reprod. Med. 20 (4): 349–64. doi:10.1055/s-2002-36713. PMID 12536358.
- Humaidan P, Kol S, Papanikolaou EG (2011). "GnRH agonist for triggering of final oocyte maturation: time for a change of practice?". Hum. Reprod. Update 17 (4): 510–24. doi:10.1093/humupd/dmr008. PMID 21450755.
- http://www.genengnews.com/news/bnitem.aspx?name=63455946&source=genwire "Researchers Suggest Hormone Therapy for Prostate Cancer Can Cause Serious Heart Problems and Death"