Ondansetron is a serotonin 5-HT3 receptor antagonist used to prevent nausea and vomiting caused by cancer chemotherapy, radiation therapy, and surgery. It has little effect on vomiting caused by motion sickness[citation needed], and does not have any effect on dopamine receptors or muscarinic receptors. Ondansetron is a highly specific and selective serotonin 5-HT3 receptor antagonist and with low affinity for Dopamine receptors. The 5-HT3 receptors are present both peripherally on vagal nerve terminals and centrally in the chemoreceptor trigger zone of the area postrema. Serotonin is released by the enterochromaffin cells of the small intestine in response to chemotherapeutic agents and may stimulate vagal afferents (via 5-HT3 receptors) to initiate the vomiting reflex. It is not certain whether ondansetron's antiemetic action is mediated centrally, peripherally, or in both sites.
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