Cetrorelix is a man-made hormone that blocks the effects of Gonadotropin Releasing Hormone (GnRH). GnRH controls another hormone that is called luteinizing hormone (LH), which is the hormone that starts ovulation during the menstrual cycle. When undergoing hormone treatment sometimes premature ovulation can occur, leading to eggs that are not ready for fertilization to be released. Cetrorelix does not allow the premature release of these eggs to occur.