What is the recommended dosage of Zofran (ondansetron) for a 6-year-old female of average weight?

Ondansetron Dosing for a 6-Year-Old Female

For a 6-year-old child of average weight (approximately 20 kg), administer ondansetron 0.15 mg/kg intravenously (3 mg) or 4 mg orally, with doses repeated every 8 hours as needed for nausea and vomiting. 1, 2

Weight-Based Dosing Calculation

• Average weight for a 6-year-old female: approximately 20 kg
• Standard pediatric IV dosing: 0.15 mg/kg per dose 3, 4
  • For a 20 kg child: 0.15 mg/kg × 20 kg = 3 mg IV per dose
• Standard pediatric oral dosing: 4 mg orally per dose 5, 3
• Dosing interval: Every 8 hours as needed 1, 2

Context-Specific Dosing Recommendations

For Chemotherapy-Induced Nausea/Vomiting

• Moderate emetogenic chemotherapy: 0.15 mg/kg IV (3 mg for 20 kg child) given 30 minutes before chemotherapy, then every 8 hours for 1-2 days post-treatment 6, 3
• Highly emetogenic chemotherapy: Same IV dosing, but should be combined with dexamethasone for enhanced efficacy 6, 3
• Alternative oral dosing: 4 mg orally every 8 hours, starting before chemotherapy 3

For Acute Gastroenteritis

• Oral dosing preferred: 4 mg orally as a single dose, with option to repeat every 8 hours 5
• In the landmark gastroenteritis trial, children aged 6 months to 12 years received oral ondansetron with significant reduction in vomiting episodes and IV fluid requirements 5

For Postoperative Nausea/Vomiting

• Single prophylactic dose: 0.1-0.15 mg/kg IV (2-3 mg for 20 kg child) given before or at the end of surgery 3
• This dosing has proven superior to placebo, droperidol, and metoclopramide in pediatric surgical patients 3

Important Clinical Considerations

Maximum Dosing Limits

• Maximum single IV dose: Do not exceed 16 mg per dose in any patient due to cardiac safety concerns 2
• Maximum daily dose: Should not exceed 32 mg total per 24 hours 1
• For this 6-year-old, the calculated 3 mg IV dose is well below safety thresholds 3

Formulation Options

• Oral dissolving tablets (ODT): Available in 4 mg strength, ideal for children who have difficulty swallowing 6, 1
• Oral solution: Can be compounded if needed 3
• IV formulation: 0.15 mg/kg (3 mg for 20 kg child) administered over 2-5 minutes 3, 4

Management of Breakthrough Nausea

• If nausea persists despite ondansetron: Add (do not replace) a medication with a different mechanism of action 1
  • Consider adding metoclopramide 0.1-0.2 mg/kg every 6-8 hours 1
  • Consider adding dexamethasone 0.15 mg/kg for enhanced antiemetic effect 6, 3
• Avoid simply re-dosing ondansetron too soon: The half-life is 3.5 hours, so therapeutic levels persist for several hours after administration 4, 7

Common Pitfalls and Caveats

Adverse Effects to Monitor

• Headache: Most common adverse effect in pediatric patients 3, 7
• Constipation: Can occur and may paradoxically worsen nausea if severe 1
• Increased diarrhea: Noted specifically in gastroenteritis patients receiving ondansetron 5
• Sedation and drowsiness: Generally mild but should be monitored 3, 4

When Ondansetron May Be Insufficient

• For highly emetogenic chemotherapy: Ondansetron monotherapy is inadequate; must combine with dexamethasone and consider NK1 receptor antagonists 6, 2
• For delayed nausea: Efficacy beyond 24-48 hours is less well-established; consider adding scheduled dopamine antagonists 1, 4

Dosing Adjustments

• Pediatric patients have increased clearance compared to adults, supporting the higher weight-based dosing (0.15 mg/kg vs. fixed adult doses) 4, 7
• No renal dose adjustment needed: Less than 10% is excreted unchanged in urine 4, 7
• Hepatic metabolism: Use caution in severe liver disease, though specific pediatric guidance is limited 7

Administration Timing

• For chemotherapy: Give 30 minutes before chemotherapy administration 6, 3
• For surgery: Give before induction or at end of surgery 3
• For gastroenteritis: Give immediately, then wait 15 minutes before attempting oral rehydration 5