5-HT3 serotonin receptor antagonist
ATC code: A04AA01 (Ondansetron)
A selective 5-HT3 receptor antagonist in the area postrema of the medulla and in vagal afferents of the gut. Blocks the emetic reflex triggered by serotonin release from enterochromaffin cells in response to cytotoxics, radiotherapy, and surgery. Metabolized by CYP3A4, CYP2D6, and CYP1A2.
First line
Standard prophylaxis for acute nausea and vomiting with moderately and highly emetogenic chemotherapy per MASCC/ESMO 2023 and NCCN 2024. For highly emetogenic regimens (cisplatin, AC), ondansetron 8–16 mg IV 30 minutes before chemo is part of a quadruple combination with NK1 antagonist, dexamethasone, and olanzapine. For moderately emetogenic regimens: ondansetron + dexamethasone ± NK1.
First line
Prevention and treatment of postoperative nausea and vomiting in high-risk patients (female sex, non-smoker, history of PONV or motion sickness). Standard dose 4 mg IV 30 minutes before surgery ends. In patients at risk of recurrence, repeat the dose or combine with dexamethasone and/or droperidol.
The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.
Not recommended
Ondansetron is a serotonin 5-HT3 receptor antagonist. It is prescribed for nausea and vomiting from chemotherapy or radiotherapy, postoperative nausea, and acute gastroenteritis in children (ASCO 2024 and 2018 guidelines). For stress nausea or anxiety vomiting, ondansetron is not used: no clinical studies of such use exist. The drug carries a risk of dose-dependent QT-interval prolongation and Torsades de pointes ( warning, 2012; doses above 16 mg IV are not recommended). If nausea is anxiety-related, other approaches work: breathing techniques, psychotherapy, and, when indicated, treatment of an anxiety disorder. If ondansetron was prescribed for nervous nausea, consider seeking a second opinion.
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A 2013 Danish cohort study described a small rise in cleft and cardiac defect risk in T1, but later larger studies (Swedish 2014, Canadian 2018) did not confirm the association. Acceptable in severe hyperemesis gravidarum when alternatives (doxylamine + pyridoxine, metoclopramide) are ineffective.
Limited data. Milk concentrations are low; infant risk with single doses is minimal.
Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.
Ondansetron is evaluated for the following indications with varying evidence strength: Postoperative nausea and vomiting (evidence tier A), Chemotherapy-induced nausea and vomiting (evidence tier A), Stress and anxiety (evidence tier F). See the full indication matrix with dosing and citations above on this page.
Common side effects of Ondansetron (≥ 1 in 100): Headache, Constipation, Warmth or flushing, Elevated liver enzymes. See the Safety section for uncommon and serious reactions.
FDA category B. A 2013 Danish cohort study described a small rise in cleft and cardiac defect risk in T1, but later larger studies (Swedish 2014, Canadian 2018) did not confirm the association. Acceptable in severe hyperemesis gravidarum when alternatives (doxylamine + pyridoxine, metoclopramide) are ineffective.
Limited data. Milk concentrations are low; infant risk with single doses is minimal.
Ondansetron is contraindicated in: Hypersensitivity to ondansetron or other 5-HT3 antagonists; Concurrent apomorphine; Congenital long-QT syndrome. Full list in the Safety section.