Antiemetics · Gastrointestinal
Ondansetron
Zofran®(brand names vary by market)
Excellent for the vomiting it fits, useless for the vomiting it does not.
Also: oral liquid, injection
Mechanism
Blocks 5-HT3 receptors on vagal afferents in the gut and in the chemoreceptor trigger zone. It has no action at dopamine, histamine or muscarinic receptors, which is exactly why it fails in vestibular nausea.
Indications
- Chemotherapy- and radiotherapy-induced nausea
- Post-operative nausea and vomiting
- Gastroenteritis in selected patients
Formulations
Dosing concepts
Match the mechanism
Choose the antiemetic by the cause. Motion, vestibular disease and vertigo need antihistamines or anticholinergics; this drug will do nothing for them.
QT
Dose- and route-dependent QT prolongation, with intravenous administration carrying more risk than oral.
Concepts, not prescribing instructions. Practice numbers live in current references and local protocols.
Pharmacokinetics
- Bioavailability
- ~60%
- Half-life
- 3 to 6 h
- Elimination
- hepatic: CYP3A4, 2D6 and 1A2; reduce in significant hepatic impairment
- Protein binding
- ~73%
Adverse effects
- Headachecommon
- ConstipationcommonPredictable given the receptor distribution in the gut, and additive with opioids.
- QT prolongation and torsadesserious
- Serotonin syndrome in combinationseriousUncommon but described alongside other serotonergic drugs.
Contraindications & precautions
Contraindications
- Congenital long QT syndrome
- Concurrent apomorphine
Precautions
- Electrolyte disturbance
- Concurrent QT-prolonging drugs
- Hepatic impairment
Interactions
Monitoring
Whether it is working
If it has not helped after a dose or two, the cause is probably not one it addresses
ECG and electrolytes where risk factors stack
QT prolongation
Bowel function
Constipation, especially alongside opioids
Counselling points
- Dissolvable forms are placed on the tongue and not swallowed whole.
- Constipation is common; tell us if it becomes a problem.
- Report palpitations or fainting.
Clinical pearls
- The classic error is prescribing it for vertigo or motion sickness, where the receptors it blocks are not involved at all.
- In a post-operative patient already on opioids, it doubles down on constipation. Laxatives are part of the same prescription.
References & review
- Ondansetron product monograph (consult the current version for your market) (monograph)
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. (reference-work)
Last reviewed 2026-08-16 · jurisdiction: global · drug information changes; verify against current references before practice use.