A substance that keeps the testicles and ovaries from making sex hormones by blocking other hormones that are needed to make them. In men, GnRHas cause the testicles to stop making testosterone. In women, they cause the ovaries to stop making estrogen and progesterone. Some GnRHas are used to treat prostate cancer. Also called GnRH agonist, gonadotropin-releasing hormone agonist, LHRH agonist, and luteinizing hormone-releasing hormone agonist.