Ondansetron (Zofran) Dosing for 36-Month-Old, 18kg Child
For an 18kg, 3-year-old child, administer 5 mL of ondansetron 4mg/5mL oral solution per dose every 12 hours.
Dosing Calculation and Rationale
Weight-based dosing for pediatric ondansetron is 0.15 mg/kg per dose for chemotherapy-induced nausea/vomiting, which has been extensively studied and proven safe in children 1, 2.
For this 18kg child: 0.15 mg/kg × 18kg = 2.7 mg per dose, which rounds to approximately 3 mg per dose in clinical practice 1.
However, the available concentration is 4mg/5mL, making precise dosing challenging. The practical approach is:
- 5 mL delivers 4 mg (slightly higher than calculated 2.7-3 mg)
- 3.75 mL would deliver 3 mg (exact calculated dose)
The recommended approach is 5 mL (4 mg) per dose, as this falls well within the safe and effective range studied in pediatric populations 1, 3.
Safety Profile
Ondansetron has been extensively studied in children and demonstrates excellent safety, with adverse events rarely necessitating treatment withdrawal 1.
The most common side effects are mild: headache, constipation, and diarrhea in children receiving chemotherapy 1.
Pediatric patients have increased clearance compared to adults (half-life approximately 3.5 hours), supporting the every 12-hour dosing interval 2, 4.
Clinical Context
This dosing is based on chemotherapy-induced emesis studies, where ondansetron showed superior efficacy compared to metoclopramide and chlorpromazine 1, 5.
For postoperative nausea/vomiting, lower single doses (0.1 mg/kg) have been effective, but the 0.15 mg/kg dosing provides broader coverage for various etiologies 1.
No dose adjustment is needed for this age group, as children 3-6 years have normal hepatic metabolism 2.
Practical Administration
Administer 5 mL orally every 12 hours using an oral syringe for accurate measurement 3.
Oral administration is preferred when feasible, as it avoids the need for IV access and is well-tolerated in children 3.
Monitor for efficacy by assessing reduction in nausea/vomiting episodes, which should be evident within 1 hour of oral administration 2.