Indications (Moderate–severe vomiting preventing oral rehydration therapy (ORT) — **not** routine for all gastroenteritis; consider when persistent vomiting fails ORT despite appropriate technique (ESPGHAN/NASPGHAN 2014, AAP, NICE NG84, CPS))
Timing (Administer **before** ORT attempt when antiemetic indicated — improves oral fluid tolerance; reassess hydration and continue ORT once vomiting controlled (ESPGHAN 2014, AAP, WHO Pocket Book))
First-line therapy
Ondansetron • p.o. • 0.1–0.15 mg/kg/dose • 3×/day
Max 8 mg/dose · Max 24 mg/day · Duration: PRN · 0.1–0.15 mg/kg/dose q8h — max 8 mg/dose, max 3 doses/24 h; give before ORT
• Standard PO/ODT dose: **0.15 mg/kg/dose** (range **0.1–0.15 mg/kg**) — max **8 mg/dose**; interval **q8–12 h**; max **2–3 doses/24 h** (ESPGHAN 2014, BNFC, AAP, CPS).
• Single-dose strategy (ESPGHAN): one dose 0.15 mg/kg (max 8 mg) before ORT in moderate dehydration with failed ORT — reassess at 4–8 h before repeat; avoids routine multi-dose use (ESPGHAN/NASPGHAN 2014).
• Maximum daily: **24 mg/day** if 3× 8 mg doses — calculator caps daily total; many centres limit to **1–2 doses** in uncomplicated AGE (ESPGHAN 2014, BNFC).
• Minimum effective: **0.1 mg/kg** acceptable at lower end of range — use 0.15 mg/kg when vomiting severe or prior inadequate response (BNFC, AAP, institutional ED protocols).
• Not indicated: isolated single vomit, mild gastroenteritis tolerating ORT, or as substitute for ORT/deficit correction — antiemetic adjunct only (ESPGHAN 2014, NICE NG84, AAP).
Max 8 mg/dose · Max 24 mg/day · Duration: PRN · 0.15 mg/kg slow IV over 2–5 min — max 8 mg/dose q8h PRN; max 3 doses/24 h
• IV when **immediate vomiting after PO** antiemetic or child **NPO** for IV rehydration — same mg/kg as oral route (0.15 mg/kg, max 8 mg) (BNFC, AAP, ESPGHAN 2014).
• Administer **slow IV push** over **2–5 minutes** (or diluted per local protocol) — rapid injection increases risk of transient arrhythmia/QT effects (BNFC SmPC, FDA label, institutional policy).
• Give IV ondansetron **before** starting ORT once vomiting controlled enough to attempt oral fluids — do not substitute IV antiemetic for deficit replacement (ESPGHAN 2014, WHO Pocket Book).
• Single IV dose often sufficient in ED — reassess before repeat; if persistent vomiting despite 2 doses, reconsider diagnosis (bowel obstruction, appendicitis, DKA, intracranial pathology) (AAP, NICE NG84, CPS).
• Compatible with concurrent isotonic IV fluids for dehydration — coordinate timing so ORT can begin once nausea controlled (ESPGHAN 2014, NICE NG84).
Enter weight on Kids to calculate doses
Clinical notes
• Indications (AGE): persistent vomiting preventing ORT despite correct technique — **not** for isolated mild vomit or routine use in all gastroenteritis; ESPGHAN supports **single-dose** consideration in **moderate dehydration** with failed ORT (ESPGHAN/NASPGHAN 2014, AAP, NICE NG84, CPS).
• Give **before ORT attempt** when indicated — antiemetic improves oral fluid tolerance; continue ORT/deficit correction once vomiting controlled; ondansetron does not treat dehydration or infection (ESPGHAN 2014, AAP, WHO Pocket Book).
• Contraindications: **hypersensitivity** to ondansetron or 5-HT3 antagonists · **congenital long QT syndrome** — avoid or use only with cardiology input and ECG monitoring (BNFC, FDA label, CPS).
• QT prolongation: avoid concurrent **QT-prolonging drugs** (e.g. macrolides, fluoroquinolones, antipsychotics, methadone); correct **hypokalaemia** and **hypomagnesaemia** before dosing; caution in electrolyte losses from vomiting/diarrhoea (BNFC, FDA, AAP).
• Adverse effects and repeat limits: headache, constipation, dizziness, **sedation** (usually mild); max **2–3 doses/24 h** at 0.1–0.15 mg/kg (max 8 mg/dose) — reassess need for further antiemetic and alternative diagnoses if no improvement (ESPGHAN 2014, BNFC, CPS).
• ESPGHAN / NASPGHAN — Evidence-based guidelines for management of acute gastroenteritis (antiemetic use) (2014)
