A GnRH agonist is a synthetic peptide that acts like the gonadotropin releasing hormone (GnRH) but has a much longer biological half life. As a result initially there is an increase in FSH and LH secretion (so-called flare effect), however after about 10 days a profound hypogonadal effect is achieved through downregulation. Generally this induced and reversible hypogonadism is the therapeutic goal. Thus GnRH agonists are usefull in the treatment of cancers that are hormonally sensitive, i.e. prostate cancer and breast cancer. GnRH agonists are also useful in the medical treatment of estrogen-dependent lesions such as endometriosis, adenomyosis, and uterine leiomyoma. Further, they find application in IVF therapy as they allow for better control of ovarian stimulation. GnRH agonists are derivatives of the natural GnRH decapeptide with specific amino acid substitutions. Current agonists are used by injection or intranasally.