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