A GnRH antagonist is a synthetic peptide that competes with the gonadotropin releasing hormone (GnRH) for its receptor thus decreasing or blocking GnRH action. As a result endogenous pituitary output of FSH and LH is inhibited. Such a peptide is a derivative of the natural GnRH decapeptide with specific amino acid substitutions. Currently antagonists (i.e. Ganirelix) are used in IVF therapy to prevent endogenous ovulation during the exogenous administration of gonadotropins and allow for better control of ovarian stimulation.