Serotonin, or 5-hydroxytryptamine (5-HT) is a neurotransmitter derived from the amino acid tryptophan. Systemic 5-HT affects the cardiovascular, respiratory, and gastrointestinal systems, with vasoconstriction being the typical vascular response. Thus, 5-HT antagonists will cause vasodilation. In the gastrointestinal system, most serotonin receptors are of the 5-HT3 type. Thus, 5-HT antagonists will cause vasodilation. In the gastrointestinal system, most serotonin receptors are of the 5-HT3 type. The strongest stimulus for emesis from both chemotherapy and postoperatively is serotonin release from the gut enterochromaffin cells. This release stimulates afferent vagal fibers via their 5-HT3 receptors that activate the vomiting center in the brainstem (chemoreceptor trigger zone). Thus, serotonin antagonists decrease nausea and cause vasodilation.

The three 5-HT3 antagonist drugs available in the United States are ondansetron (Zofran), granisetron (Kytril), and dolasetron (Anzemet). These 5-HT3 receptor antagonists are the most effective antiemetic drugs available. All three drugs are similar in effectiveness, cost, and side-effect profiles.