Ondansetron / Zofran

Ondansetron is a selective serotonin 5-HT3 receptor antagonist. It works in two places: it blocks 5-HT3 receptors on vagal afferents in the GI tract and in the chemoreceptor trigger zone (CTZ) of the medulla. When chemotherapy, radiation, or anesthesia triggers massive serotonin release from enterochromaffin cells in the gut, ondansetron intercepts that signal before it reaches the vomiting center. This makes it first-line for chemotherapy-induced nausea and vomiting (CINV), radiation-induced nausea, and postoperative nausea and vomiting (PONV). It can be given PO, IV, or ODT (orally disintegrating tablet — useful when the client is already vomiting). The critical safety concern is QT prolongation, which can lead to torsades de pointes; ondansetron is contraindicated in congenital long QT syndrome. A baseline ECG should be reviewed in clients with electrolyte imbalances (hypokalemia, hypomagnesemia) or heart failure, and electrolyte imbalances should be corrected before administration. The maximum single IV dose is 16 mg. Common side effects are headache, constipation, and dizziness — far milder than the sedation and extrapyramidal symptoms seen with older antiemetics like promethazine or metoclopramide.