Is an antiemetic whic selectively acto on 5-HT3 receptor antagonist. It works by blocking serotonin, both peripherally on vagal nerve terminals and centrally in the chemoreceptor trigger zone (CTZ) located in the area postrema of the medulla oblongata. The CTZ is outside of the blood-brain barrier, so it can respond to changes in blood chemistry as well as direct neural input from other parts of the brain. This makes it an important site for antiemetic drugs like ondansetron.