Antiemetic Pregnancy: B — considered safe after first trimester. Avoid in first trimester (possible cardiac malformation signal — risk small).

Ondansetron

Brand names: Zofran, Ondansetron 4mg

Adult dose

Dose: 4–8mg

Route: Oral / IV / IM

Frequency: 8-hourly (max 3 doses/day)

Max: 32mg/day (chemotherapy — only with ECG monitoring). Standard: 8mg TDS

PONV: 4mg IV at induction. Chemotherapy: 8mg IV slow bolus before treatment, then 8mg at 4h and 12h. Emergency/palliative: 4mg SL or IV every 4–8h.

Paediatric dose

Dose: 0.1 mg/kg

Route: IV (slow over 15 min) or Oral

Frequency: 8-hourly

Max: 4 mg per dose

Concentration: 2 mg/ml

BNF for Children: IV/oral 0.1 mg/kg (max 4 mg) every 8h. Not recommended in children <6 months. Oral (gastroenteritis): 0.1 mg/kg every 8h (max 4 mg). QTc monitoring in children with cardiac risk. Source: BNF for Children 2024; MHRA Ondansetron QT Alert

Dose adjustments

Renal

No adjustment required.

Hepatic

Max 8mg/day in severe hepatic impairment (reduced clearance).

Paediatric weight-based calculator

Patient weight (kg)

Clinical pearls

MHRA Alert 2013: IV 32mg single dose withdrawn from use — QTc prolongation risk. Max IV single dose now 16mg.
More effective for nausea due to 5-HT3 pathway (chemotherapy, PONV, opioid-induced) than for vestibular/motion sickness nausea.
Not effective for apomorphine-associated nausea (do not use together — severe hypotension).
Ondansetron ODT (Zofran Zydis): dissolves on tongue — useful when swallowing difficult or vomiting prevents tablets.

Contraindications

Congenital long QT syndrome
Apomorphine (risk of profound hypotension)
Known hypersensitivity

Side effects

Headache (most common)
Constipation
QTc prolongation (dose-dependent — dose-related, more significant with IV and high doses)
Flushing, injection site reactions (IV)
Serotonin syndrome (rare — with concomitant serotonergic drugs)

Interactions

Apomorphine: profound hypotension — contraindicated
QT-prolonging drugs: additive QTc prolongation
Serotonergic drugs (SSRIs, tramadol): increased serotonin syndrome risk
Phenytoin / carbamazepine / rifampicin: reduce ondansetron levels

Monitoring

QTc if IV route and high doses or known cardiac risk
symptom response

Reference: BNFc; NICE BNF 84; MHRA Ondansetron Safety Update 2013. Verify against your local formulary and the latest BNF before prescribing.

Curated clinical cross-links plus same-class fallbacks.

Calculators

Drugs

Metoclopramide · Antiemetic

Pathways