Important caveat first: "Vomiting + abdominal pain" in a child is a presentation, not a diagnosis. Before reaching for an antiemetic, a clinician needs to rule out surgical/serious causes (appendicitis, intussusception, bowel obstruction, UTI/pyelonephritis, DKA) — red flags include bilious/bloody vomiting, localized RLQ tenderness/guarding, high fever, lethargy, or inability to tolerate any oral fluids. If any of these are present, the child needs urgent evaluation, not just symptomatic treatment. The advice below applies to the common scenario of vomiting from presumed acute gastroenteritis with mild-moderate dehydration, once serious causes have been reasonably excluded.

Ondansetron is the first-line antiemetic in pediatrics for vomiting associated with acute gastroenteritis — it's well studied in children and reduces vomiting episodes, need for IV fluids, and hospital admission (Canadian Paediatric Society position statement; Freedman et al., Roslund et al., Ramsook et al. trials).

A typical 6-year-old weighs roughly 18-22 kg, so the usual single dose is 4 mg orally, given once. Alternative weight-based formula: 0.15 mg/kg/dose (max single dose 8 mg) — this works out to about the same 4 mg for a 20 kg child.

Frequency: In most outpatient/ED protocols, a single dose is enough to stop the vomiting long enough to restart oral rehydration (start ORT 15-30 minutes after the dose). If repeated, it is generally given every 8 hours, not more often, and only for a short course (1-2 days) — this is symptomatic bridging therapy, not a drug meant for prolonged use.

Route: Oral (ODT or liquid) is preferred and works as well as IV for mild-moderate dehydration; IV ondansetron 0.15 mg/kg is used if the child needs IV access anyway or can't tolerate oral.

Cautions: Watch for QT prolongation (avoid in known long QT or with other QT-prolonging drugs), and it can cause mild diarrhea or headache. It does not treat the abdominal pain itself — that should resolve as the underlying cause (usually gastroenteritis) improves; give paracetamol if pain relief is needed and fever is present, but avoid NSAIDs until a surgical cause is excluded.

Alongside the drug: Oral rehydration solution (ORS) is the mainstay of management for gastroenteritis-related vomiting in children — ondansetron is an adjunct to help the child tolerate ORT, not a replacement for it (Tintinalli's Emergency Medicine; Canadian Paediatric Society).

Recent systematic reviews (2024-2025, PMIDs 39992376, 38220580) continue to support ondansetron's efficacy and safety for pediatric vomiting in procedural/perioperative and ED settings, consistent with the dosing above — no contradicting evidence found.

If you can tell me more about the clinical context (fever? diarrhea? duration? localized pain? able to keep fluids down?), I can narrow this down further — the safest next step for a child with new vomiting and abdominal pain is always a clinical exam to exclude a surgical abdomen before treating symptomatically at home.