What is the appropriate ondansetron (Zofran) dose for a 5‑year‑old child weighing 43 lb (≈19.5 kg) with normal liver function and no contraindications?

Ondansetron Dosing for a 5-Year-Old Weighing 43 Pounds

For a 5-year-old child weighing 43 lb (19.5 kg), administer ondansetron 0.15 mg/kg per dose, which equals approximately 3 mg per dose, with a maximum single dose of 8 mg for oral administration. 1

Weight-Based Calculation

• The standard pediatric dose of ondansetron is 0.15 mg/kg per dose across all major guidelines. 1
• For a 43 lb (19.5 kg) child: 19.5 kg × 0.15 mg/kg = 2.9 mg per dose (round to 3 mg for practical administration). 1
• The maximum single oral dose is 8 mg, which this child does not approach. 1

Route and Frequency Selection

For Acute Gastroenteritis/Vomiting

• Oral route: 3 mg as a single dose, using oral disintegrating tablet (ODT) or oral solution. 1
• Intramuscular route: 3 mg (0.15 mg/kg) if the child cannot tolerate oral medication due to severe vomiting. 1
• Intravenous route: 3 mg (0.15 mg/kg) if IV access is already established for fluid resuscitation. 1
• Ondansetron can be repeated every 8 hours if needed, though a single dose is often sufficient for acute gastroenteritis. 1

For Chemotherapy-Induced Nausea (if applicable)

• Administer 0.15 mg/kg IV or IM (3 mg for this child) 30 minutes before chemotherapy, then repeat at 4 and 8 hours after the first dose. 1
• Combining ondansetron with dexamethasone significantly improves efficacy for highly emetogenic chemotherapy. 1

For Postoperative Nausea (if applicable)

• Administer 0.1–0.15 mg/kg IV (2–3 mg for this child) immediately before or after anesthesia induction. 2

Formulation and Administration

• Oral disintegrating tablet (ODT): Available in 4 mg and 8 mg strengths; give one 4 mg tablet (slightly higher than calculated 3 mg, but within safe range). 1
• Oral solution: Typically 4 mg/5 mL concentration; administer 3.75 mL to deliver 3 mg. 1
• Injectable formulation: 2 mg/mL concentration; draw up 1.5 mL for a 3 mg dose. 1
• Ondansetron can be given without regard to meals, though administration with food may reduce mild gastrointestinal side effects. 1

Critical Safety Considerations

• QT prolongation risk: Ondansetron prolongs the QT interval in a dose-dependent manner; the 0.15 mg/kg dose with an 8 mg maximum provides an appropriate safety margin. 1
• Avoid in congenital long QT syndrome: Do not administer ondansetron to children with known congenital long QT syndrome or significant electrolyte abnormalities (hypokalemia, hypomagnesemia). 1
• Age restriction for FPIES: While this 5-year-old is well above the minimum age, note that ondansetron should only be used in children ≥6 months of age for Food Protein-Induced Enterocolitis Syndrome. 1
• Monitor for headache: Headache is the most common adverse effect, occurring in approximately 10–15% of pediatric patients, and is typically mild and self-limited. 2, 3

Common Pitfalls to Avoid

• Do not exceed 8 mg per single oral dose in children, even if weight-based calculation suggests a higher dose. 1
• Do not use the 16 mg maximum cited for IV chemotherapy dosing in routine gastroenteritis; the 8 mg oral maximum is appropriate for this indication. 1
• Do not administer ondansetron to infants under 6 months unless the clinical situation is critical, due to limited safety data. 1
• Do not use household measuring spoons for oral solution; always use a calibrated oral syringe or dosing cup. 1