What is the recommended dosage of Zofran (ondansetron) for a 16-year-old patient weighing 56.6 kg with vomiting?

Ondansetron Dosing for a 16-Year-Old with Vomiting

For a 16-year-old weighing 56.6 kg with vomiting, administer ondansetron 8 mg orally (or 0.15 mg/kg IV, which equals approximately 8 mg for this patient) as a single dose, with the option to repeat every 8 hours if vomiting persists. 1, 2

Weight-Based vs. Fixed Dosing

• For adolescents and adults, the standard dose is 8 mg orally or IV, which is appropriate for this 56.6 kg patient 1
• The weight-based dosing of 0.15 mg/kg IV (maximum 16 mg/dose) is equivalent to approximately 8.5 mg for this patient, making the standard 8 mg dose appropriate 1
• For patients 6 months and older, both fixed and weight-based dosing are acceptable, with this adolescent falling well within the adult dosing range 1

Route of Administration Selection

Oral administration is preferred if the patient can tolerate it:

• Oral ondansetron 8 mg (tablet, orally dissolving tablet, or oral soluble film) is the first-line choice for mild-to-moderate vomiting 1
• IV ondansetron 8 mg should be used if the patient has severe vomiting, lethargy, or cannot tolerate oral intake 1
• Orally dissolving tablets are particularly useful for patients with active nausea who may have difficulty swallowing standard tablets 1

Dosing Schedule Based on Severity

For mild vomiting (1-2 episodes, no lethargy):

• Give 8 mg orally as a single dose 1
• Attempt oral rehydration after administration 1
• Can repeat every 8 hours if vomiting recurs, up to maximum 24 mg/day 1

For moderate vomiting (>3 episodes with mild lethargy):

• Administer 8 mg IV or orally 1
• Monitor for 4-6 hours from onset of reaction 1
• Consider IV fluids if oral rehydration fails 1

For severe vomiting (>3 episodes with severe lethargy, hypotension):

• Give 8 mg IV (0.15 mg/kg) immediately 1
• Place IV line for fluid resuscitation with normal saline 20 mL/kg bolus 1
• Consider adding methylprednisolone 1 mg/kg IV (maximum 60-80 mg) for severe cases 1

Multi-Dose Regimens for Ongoing Symptoms

If vomiting persists beyond initial dose:

• Provide 8 mg orally every 8-12 hours for up to 48 hours after initial presentation 2, 3
• Recent high-quality evidence shows that multi-dose ondansetron (up to 6 doses over 48 hours) reduces moderate-to-severe gastroenteritis by 50% compared to placebo (adjusted OR 0.50,95% CI 0.40-0.60) 3
• The total number of vomiting episodes within 48 hours is reduced by approximately 24% with multi-dose therapy 3

Breakthrough Vomiting Management

If ondansetron alone is insufficient:

• Add an agent from a different drug class rather than increasing ondansetron dose 1, 4
• Consider metoclopramide 10-20 mg orally every 6-8 hours as a dopamine antagonist 1
• Alternative: prochlorperazine 5-10 mg orally or IV every 6 hours 1
• For anticipatory nausea, add lorazepam 0.5-2 mg orally every 4-6 hours as needed 1, 2

Important Safety Considerations

QT prolongation concerns:

• The 8 mg dose carries minimal QT prolongation risk compared to the previously used 32 mg IV dose, which is no longer recommended 2
• Standard doses (8 mg) are considered safe for adolescents without significant cardiac risk 4, 2

Common pitfalls to avoid:

• Do not exceed 16 mg as a single IV dose or 24 mg total daily dose in adolescents 1
• Assess for non-vomiting causes (electrolyte abnormalities, increased intracranial pressure) before escalating therapy 2
• Consider adding acid suppression if patient has difficulty distinguishing heartburn from nausea 2

Efficacy Data

• Single-dose ondansetron reduces vomiting within 8 hours (RR 0.33, NNT=2) in pediatric gastroenteritis 5
• Reduces need for IV hydration by 56% (RR 0.44) and hospitalization by 51% (RR 0.49) 6
• Mean decrease in nausea score is 4.0 points on a 10-point scale across all routes of administration 7