About Ondansetron Selective serotonin 5-HT3 receptor antagonist, Imidazole derivative, Antiemetic. Mechanism of Action of Ondansetron Ondansetron is a selective 5HT3 receptor antagonist. 5HT3 receptors are present in vagal nerve terminals and chemoreceptor trigger zone of area postrema of the brain. Cytotoxic chemotherapy is associated with release of Serotonin from Enterochromaffin cells in the small intestine. Serotonin triggers vomiting through 5HT3 vagal receptor that activates the vomiting reflex. Since Ondansetron is a 5HT3 receptor antagonist, it is indicated for the prevention of nausea and vomiting associated in chemotherapy. Anaesthetic adjuncts: It is used preoperatively in order to reduce the post operative vomiting.