What is the appropriate ondansetron (Zofran) dosage for a 3‑year‑old child weighing 47.6 lb?

Ondansetron Dosing for a 3-Year-Old Weighing 47.6 lbs

For a 3-year-old child weighing 47.6 lbs (21.6 kg), administer ondansetron 0.15 mg/kg per dose, which equals approximately 3.2 mg per dose, rounded to a practical dose of 4 mg orally or intravenously. 1

Weight-Based Dosing Calculation

• The child weighs 47.6 lbs, which converts to 21.6 kg (47.6 ÷ 2.2 = 21.6 kg)
• The standard pediatric dose is 0.15 mg/kg per dose 2, 3
• This calculates to 3.2 mg per dose (21.6 kg × 0.15 mg/kg = 3.24 mg)
• Round to the nearest available tablet strength of 4 mg 1

Clinical Context-Specific Dosing

For Acute Gastroenteritis with Vomiting

• Administer 4 mg orally as a single dose 4, 5
• This dose can be given as an oral disintegrating tablet for ease of administration in a vomiting child 1, 5
• The dose range of 0.13–0.26 mg/kg has been shown equally effective, so 4 mg (0.185 mg/kg for this child) falls well within the therapeutic range 5

For Food Protein-Induced Enterocolitis Syndrome (FPIES)

• If the child is ≥6 months old with moderate-to-severe vomiting (>3 episodes), administer 0.15 mg/kg intramuscularly or intravenously, which equals 3.2 mg, with a maximum of 16 mg/dose 2
• For mild vomiting (1–2 episodes) in children ≥6 months, consider 0.15 mg/kg intramuscularly (3.2 mg) 2

For Chemotherapy-Induced Nausea and Vomiting

• Administer 0.15 mg/kg (3.2 mg, rounded to 4 mg) intravenously 30 minutes before chemotherapy 3
• This can be followed by repeat doses at 4 and 8 hours after the first dose 3
• Combining with dexamethasone significantly improves efficacy 3

Route of Administration

• Oral route is preferred when the child can tolerate it 1, 4, 5
• Intravenous or intramuscular routes are reserved for active vomiting or when oral administration is not feasible 2
• Oral disintegrating tablets (4 mg) are particularly useful in young children who may have difficulty swallowing standard tablets 1, 5

Frequency and Maximum Dosing

• For acute gastroenteritis, a single 4 mg dose is typically sufficient 4
• If repeat dosing is needed, doses can be given every 8 hours 2
• The maximum single dose is 16 mg 2
• Do not exceed 3 doses in 24 hours (total 12 mg/day maximum for this indication in pediatrics) 6

Critical Safety Considerations

• Ondansetron should be avoided if toxic megacolon is suspected in the setting of inflammatory diarrhea or diarrhea accompanied by fever 6
• Use caution in children with heart disease due to potential QT interval prolongation 2
• Monitor for diarrhea as a potential adverse effect, though this is generally mild 6, 3
• Headache, constipation, and drowsiness are the most common adverse events but are typically mild to moderate 3, 7

Common Pitfalls to Avoid

• Do not underdose by using a fixed 2 mg dose; weight-based dosing (0.15 mg/kg) is more appropriate and results in 4 mg for this child 3, 5
• Do not use ondansetron as monotherapy for chemotherapy-induced nausea; combine with dexamethasone for moderate-to-high emetogenic risk 3
• Do not administer if the child is under 6 months of age in the FPIES setting, as safety data are limited 2