Ondansetron ODT Dosing for an 85-Pound Child
For an 85-pound (38.6 kg) child with nausea or vomiting, administer ondansetron 8 mg orally as a single dose.
Weight-Based Calculation
- An 85-pound child weighs approximately 38.6 kg 1
- The standard pediatric dose is 0.15 mg/kg per dose 1
- Calculated dose: 38.6 kg × 0.15 mg/kg = 5.8 mg 1
- Round to the nearest available ODT formulation: 8 mg tablet 1
This weight places the child well above the threshold where adult dosing becomes appropriate, and 8 mg represents the standard single dose for children in this weight range.
Clinical Context Determines Administration
For Acute Gastroenteritis (Most Common Pediatric Scenario)
- Single dose only: Give one 8 mg ODT to facilitate oral rehydration 2, 1
- Age requirement: Only use in children >4 years of age 2, 3
- Purpose: Reduces immediate need for IV fluids and hospitalization 2, 4
- Critical limitation: Ondansetron does not replace fluid therapy—adequate rehydration remains the cornerstone of treatment 2, 1
- Do not prescribe multiple days of ondansetron for gastroenteritis 3
- Expected side effect: May increase stool volume/diarrhea 2, 1
The Infectious Diseases Society of America specifically recommends ondansetron for children >4 years with acute gastroenteritis associated with vomiting to facilitate oral rehydration, but emphasizes it is not a substitute for proper hydration 2.
For Chemotherapy-Induced Nausea (If Applicable)
- Dose: 8 mg ODT (or 0.15 mg/kg IV, maximum 16 mg) given 30 minutes before chemotherapy 1, 5, 6
- Repeat dosing: Every 8 hours as needed 1, 3
- Combination therapy required: For moderate-to-high emetogenic chemotherapy, ondansetron must be combined with dexamethasone; ondansetron alone is insufficient 1, 5, 6
- Maximum single dose: 16 mg (never exceed due to QT prolongation risk) 1, 5
Available ODT Formulations
- Ondansetron ODT is available in 4 mg and 8 mg tablets 1
- The ODT formulation dissolves instantly on the tongue without requiring water 7
- For this 38.6 kg child, the 8 mg ODT is the appropriate strength 1
Safety Considerations
Cardiac Monitoring
- Obtain ECG if the child has underlying cardiac disease, electrolyte abnormalities (hypokalemia, hypomagnesemia), or is taking other QT-prolonging medications 1, 5
- Ondansetron causes dose-dependent QT interval prolongation 1, 5
Maximum Dosing Limits
- Maximum single dose: 16 mg (any route) 1, 5
- Maximum daily dose: 32 mg in 24 hours 1, 5
- For this child receiving a single 8 mg dose for gastroenteritis, these limits are not a concern
Age Restrictions
- Do not use in children <4 years for gastroenteritis due to insufficient safety data 2, 3
- For chemotherapy, ondansetron can be used in younger children with appropriate weight-based dosing 1, 6
Common Pitfalls to Avoid
- Do not prescribe multiple doses for simple gastroenteritis—a single dose is sufficient and evidence-based 2, 3
- Do not use ondansetron as a substitute for rehydration—oral or IV fluids remain the primary treatment 2, 1
- Do not use ondansetron alone for chemotherapy—it must be combined with dexamethasone for moderate-to-high risk regimens 1, 5, 6
- Do not exceed 16 mg as a single dose due to cardiac safety concerns 1, 5
When to Consider Repeat Dosing
- Gastroenteritis: Do not repeat; single dose only 2, 3
- Chemotherapy: May repeat every 8 hours if needed, combined with dexamethasone 1, 3, 5
- Breakthrough nausea: If vomiting persists despite ondansetron, add a medication from a different class (metoclopramide, prochlorperazine) rather than simply increasing ondansetron frequency 1, 5
Evidence Quality Note
The recommendation for a single dose in gastroenteritis is supported by the 2017 Infectious Diseases Society of America guidelines (weak recommendation, moderate evidence) 2. Research studies confirm ondansetron reduces vomiting and hospitalization rates in pediatric gastroenteritis 4, 8, though it may increase diarrhea 2. The dose range of 0.13–0.26 mg/kg shows similar efficacy, supporting the use of 8 mg (0.15 mg/kg) for this 38.6 kg child 9.