What is the correct ondansetron (Zofran) dose, route, and dosing interval for a 10‑year‑old child weighing 69 lb (≈31 kg)?

Ondansetron Dosing for a 10-Year-Old Child Weighing 69 lb (31 kg)

Administer ondansetron 0.15 mg/kg per dose, which equals 4.65 mg (can be rounded to 4.5 mg for practical administration using available formulations), with a maximum single dose of 16 mg. 1, 2

Weight-Based Calculation

• The child weighs 69 lb = 31.3 kg 1
• Standard pediatric dose: 0.15 mg/kg per dose 1, 2, 3
• Calculated dose: 31.3 kg × 0.15 mg/kg = 4.7 mg per dose 1
• Doses must be calculated exactly according to weight; rounding beyond the precise 0.15 mg/kg calculation is discouraged to avoid under- or overdosing 1

Route of Administration

The route depends on clinical context and severity of symptoms:

Oral Administration

• Preferred for mild to moderate nausea/vomiting when the child can tolerate oral intake 1, 4
• Ondansetron oral suspension (6 mg/mL concentration): for a 4.5–4.7 mg dose, administer approximately 0.75–0.78 mL 2
• Can be given without regard to meals, though co-administration with food may improve gastrointestinal tolerability 1
• Peak plasma concentration occurs 0.5–2 hours after oral ingestion 5

Intravenous/Intramuscular Administration

• Use for moderate to severe vomiting or when oral route is not feasible 1, 2
• Same dose: 0.15 mg/kg (4.7 mg) IV or IM 1, 2, 3
• IV administration should be given over 2–5 minutes for postoperative nausea or over 15 minutes for chemotherapy-induced nausea 3

Dosing Interval

• Single dose is often sufficient for acute vomiting episodes 1, 4
• If repeat dosing is needed: every 8 hours as necessary 1
• Maximum: 2–3 doses in 24 hours for most indications 1
• For chemotherapy-induced nausea: administer 30 minutes before chemotherapy, then repeat at 4 and 8 hours after the first dose 1, 3

Context-Specific Considerations

Acute Gastroenteritis

• Single oral dose of 4.5–4.7 mg reduces risk of recurrent vomiting, need for IV fluids, and hospital admissions 6
• The American Academy of Pediatrics recommends ondansetron for children >4 years with acute gastroenteritis and vomiting 4
• Do not use as a substitute for appropriate fluid and electrolyte therapy; oral rehydration solution remains first-line treatment 2, 4

Chemotherapy-Induced Nausea/Vomiting

• For high-emetic-risk chemotherapy: combine ondansetron with dexamethasone and aprepitant for optimal efficacy 1, 2
• For moderate-emetic-risk chemotherapy: combine ondansetron with dexamethasone 1, 2
• For low-emetic-risk chemotherapy: ondansetron monotherapy is sufficient 1, 2
• Adding dexamethasone significantly improves antiemetic efficacy compared to ondansetron alone 1, 2, 7

Postoperative Nausea/Vomiting

• Single dose of 4 mg IV (or weight-based 0.15 mg/kg) given immediately before induction of anesthesia 3
• In pediatric surgical patients, ondansetron 0.1–0.15 mg/kg was superior to droperidol or metoclopramide 7

Critical Safety Warnings

QT Interval Prolongation

• Ondansetron can prolong the QT interval in a dose-dependent manner 1, 2
• Exercise special caution in children with congenital long QT syndrome, heart disease, or electrolyte abnormalities (hypokalemia, hypomagnesemia) 1, 2
• Avoid concurrent use with other QT-prolonging medications (certain antibiotics, antiarrhythmics) 2
• The pediatric dose of 0.15 mg/kg with a 16 mg maximum provides an appropriate safety margin 1

Common Side Effects

• Headache is the most frequently reported adverse event 7, 8
• Ondansetron may increase diarrhea frequency as a side effect, but this does not worsen clinical outcomes 4
• Constipation may occur, particularly with repeated dosing 7

Contraindications and Precautions

• Avoid if the child has bloody diarrhea or high fever suggesting inflammatory/invasive diarrhea 4
• Do not use in infants <6 months of age due to limited safety data 1, 2
• In overdose (particularly in infants), severe toxicity including seizures, obtundation, and serotonin syndrome has been reported 9

Practical Administration Tips

• Oral disintegrating tablets (ODT) are available and may be preferred for children who have difficulty swallowing 1
• For oral suspension: use an accurate measuring device (oral syringe) rather than household spoons 1
• Administer at least 30 minutes before chemotherapy if used for chemotherapy-induced nausea 3, 5
• Ondansetron is significantly more effective than metoclopramide and has a more favorable tolerability profile, particularly lower risk of extrapyramidal reactions 2, 7

Efficacy Data

• In pediatric patients receiving chemotherapy, 58–59% had complete response (no emetic episodes) on Day 1 with ondansetron 3
• For acute gastroenteritis, single-dose ondansetron produces a 41% higher chance of vomiting cessation within 8 hours compared to placebo 2
• Ondansetron reduces oral rehydration therapy failure by 57% and IV hydration needs by 56% 2