What is the appropriate dose of ondansetron (antiemetic medication) for a pediatric patient weighing 19.5kg?

Ondansetron Dosing for a 19.5kg Pediatric Patient

For a child weighing 19.5kg, administer ondansetron 2.9mg (calculated as 0.15 mg/kg), with a practical dose of 3mg IV/IM or oral equivalent, depending on the clinical indication. 1, 2

Weight-Based Calculation

• Standard pediatric dosing is 0.15 mg/kg per dose with a maximum single dose of 16mg 1, 2
• For 19.5kg: 0.15 mg/kg × 19.5kg = 2.925mg per dose 1
• Round to 3mg for practical administration 1

Route-Specific Dosing

Intravenous/Intramuscular Administration

• 3mg IV or IM (0.15 mg/kg) 1, 2
• The same weight-based calculation applies to both IV and IM routes 1

Oral Administration

• 3mg oral (using liquid formulation or appropriate tablet strength) 1
• Ondansetron oral suspension is available at 6mg/mL concentration, so 3mg = 0.5mL 3
• Can be administered without regard to meals, though food may improve GI tolerability 3

Clinical Context-Specific Protocols

Chemotherapy-Induced Nausea/Vomiting

High-emetic-risk chemotherapy:

• 3mg IV 30 minutes before chemotherapy, then repeat at 4 and 8 hours after the first dose 2, 4
• Combine with dexamethasone for significantly improved efficacy 1, 4, 5
• Consider adding aprepitant for optimal three-drug antiemetic control 4

Moderate-emetic-risk chemotherapy:

• 3mg IV or oral before chemotherapy 1, 4
• Combine with dexamethasone (ondansetron alone is less effective) 1, 5

Low-emetic-risk chemotherapy:

• 3mg as monotherapy 4

Postoperative Nausea/Vomiting

• 3mg IV as a single dose administered before or at the end of surgery 5
• Prophylactic administration is more effective than waiting for symptoms to develop 5

Radiation-Induced Nausea/Vomiting

• 3mg oral or IV once daily before radiation therapy 2
• Continue daily on treatment days 3

Acute Gastroenteritis (if age-appropriate)

• 3mg oral as a single dose for children >4 years with persistent vomiting 1
• This should not replace appropriate fluid and electrolyte therapy 1

Critical Safety Considerations

QT Interval Prolongation

• Exercise special caution in children with underlying heart disease 1, 2
• The 0.15 mg/kg dose with 16mg maximum provides an appropriate safety margin for QT prolongation risk 2
• Ondansetron prolongs QT interval in a dose-dependent manner 2

Age Restrictions

• Ondansetron has been safely studied in children as young as 6 months 1, 4
• At 19.5kg, this patient is likely well above 6 months of age, so no age-related contraindications apply 1

Common Adverse Effects

• Headache, constipation, and diarrhea are most frequently reported 5, 6
• These are typically mild to moderate and rarely require treatment discontinuation 5
• No extrapyramidal reactions (major advantage over metoclopramide) 1, 7

Pharmacokinetic Considerations

• Peak plasma concentration occurs 0.5-2 hours after oral administration 8
• Elimination half-life is approximately 3.5 hours (shorter in children than adults) 8, 7
• Administer at least 30 minutes before chemotherapy to ensure adequate plasma levels 8
• Pediatric patients have increased clearance compared to adults, supporting weight-based dosing 8

Combination Therapy Recommendations

When combining with dexamethasone (strongly recommended for chemotherapy):

• Ondansetron 3mg IV/oral plus dexamethasone (dose based on body surface area or weight) 1, 4, 5
• This combination is significantly more effective than ondansetron alone 1, 5, 9

Common Pitfalls to Avoid

• Do not use adult fixed-dose regimens (8mg) in pediatric patients; always calculate weight-based dosing 1, 2
• Do not substitute metoclopramide as first-line therapy; ondansetron has superior efficacy and safety profile 1, 5
• Do not omit dexamethasone in chemotherapy settings; monotherapy is substantially less effective 1, 5, 9
• Do not exceed 16mg maximum single dose regardless of calculated weight-based dose 1, 2