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5-HT3 Receptor Antagonist (Antiemetic — Hyperemesis Gravidarum) Pregnancy: Caution in first trimester (uncertain orofacial cleft risk); generally accepted from 10 weeks for severe hyperemesis

Ondansetron (Hyperemesis Gravidarum)

Brand names: Zofran

Adult dose

Dose: 4–8 mg oral or IV every 8 hours (max 32 mg/day). Start with 4 mg; increase to 8 mg TDS if inadequate response
Route: Oral (tablet or ODT) or IV slow injection
Frequency: Every 8 hours (three times daily)
Max: 32 mg/day
RCOG Green-top 69 (Hyperemesis Gravidarum, 2024): ondansetron recommended as second-line after first-line antiemetics (cyclizine, prochlorperazine, promethazine) if no response. Use preferably after 10 weeks gestation. The MHRA has noted possible but uncertain orofacial cleft risk in first trimester — discuss benefits and risks with patient. Available off-label for hyperemesis.

Paediatric dose

Route: N/A
Frequency: N/A
Max: N/A
Not applicable in obstetric indication

Dose adjustments

Renal

No dose adjustment required

Hepatic

Maximum 8 mg/day in severe hepatic impairment

Clinical pearls

  • RCOG Green-top 69 (2024): ondansetron is second-line antiemetic for hyperemesis gravidarum — after cyclizine, promethazine, prochlorperazine
  • First trimester safety: MHRA noted uncertain orofacial cleft signal (Zambelli-Weiner 2019) — informed consent important; benefit likely outweighs risk in severe hyperemesis
  • Preferred route: IV if unable to tolerate oral; 4 mg oral ODT (Zofran Melt) dissolves on tongue — useful if swallowing difficult
  • QTc monitoring: ECG if using high doses or concomitant QT-prolonging drugs
  • Hyperemesis management: correct dehydration + electrolytes (K+, Mg2+) first; ondansetron adjunct to rehydration, not substitute

Contraindications

  • Congenital long QT syndrome
  • Concomitant apomorphine (severe hypotension)
  • First trimester use — avoid if possible due to uncertain orofacial cleft risk

Side effects

  • Constipation (common)
  • Headache
  • QT prolongation (dose-dependent — avoid >32 mg/day)
  • Dizziness
  • Transient LFT elevation
  • Possible orofacial cleft risk (uncertain — first trimester signal)

Interactions

  • QT-prolonging drugs — additive QT prolongation
  • Apomorphine — severe hypotension (contraindicated)
  • Tramadol — serotonin syndrome risk

Monitoring

  • Electrolytes (K+, Mg2+) — correct before initiating
  • ECG (QTc) if risk factors
  • Weight and hydration status
  • Nausea/vomiting severity score (PUQE scale)

Reference: BNFc; RCOG Green-top 69 Hyperemesis Gravidarum (2024); BNF; MHRA Drug Safety Update (ondansetron pregnancy). Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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