A molecule that binds to a receptor and blocks it — preventing the receptor from being activated by anything else.An antagonist occupies the receptor without activating it, effectively silencing that signal. GnRH antagonists (cetrorelix, ganirelix, degarelix) block GnRH receptors in the pituitary, immediately suppressing LH and FSH secretion and therefore testosterone or estrogen — used in fertility protocols and prostate cancer treatment. NK1 receptor antagonists block substance P signaling. Contrast with agonist.