What is the recommended dose of dimenhydrinate (antihistamine) for a 40 kg child to prevent vomiting during travel?

Dimenhydrinate Dosing for a 40 kg Child

For a 40 kg child, the recommended dose of dimenhydrinate for preventing travel-related vomiting is 50 mg administered intramuscularly or intravenously every 4 hours as needed, not to exceed 300 mg daily. 1

Weight-Based Dosing Calculation

• The FDA-approved pediatric dosing is 1.25 mg/kg of body weight administered four times daily 1
• For a 40 kg child: 1.25 mg/kg × 40 kg = 50 mg per dose 1
• This can be repeated every 4 hours as needed 1
• Maximum daily dose must not exceed 300 mg (which equals 6 doses of 50 mg) 1

Route of Administration

• Intramuscular route: Each 50 mg dose (1 mL) can be injected directly as needed 1
• Intravenous route: Each 50 mg dose (1 mL) must be diluted in 10 mL of 0.9% Sodium Chloride and injected over 2 minutes 1
• The injectable form is indicated when oral administration is impractical, such as during active vomiting 1

Duration of Action and Timing

• Each 50 mg dose provides approximately 4 hours of antiemetic control 1
• For travel sickness prevention, administer the first dose 30-60 minutes before departure (based on general medical practice)
• Subsequent doses can be given every 4 hours during travel as needed 1

Safety Considerations

Common pitfall: At 40 kg, this child is at the threshold where adult dosing begins to apply, but the weight-based calculation (1.25 mg/kg) and the maximum daily limit of 300 mg must still be strictly observed 1

Expected Side Effects

• Drowsiness is common and may actually be desirable during travel 1
• This sedative effect typically lasts for the duration of action (approximately 4 hours) 1

Toxicity Threshold

• Children 6 years and older who ingest at least 7.5 mg/kg or 300 mg (whichever is less) should be referred to an emergency department for potential toxicity 2
• For a 40 kg child, this toxicity threshold would be 300 mg in a single dose 2
• Since the recommended single dose is 50 mg, there is a reasonable safety margin 1, 2

Clinical Efficacy Evidence

• Dimenhydrinate at 0.5 mg/kg has been shown to significantly reduce vomiting episodes in pediatric populations (10% vs 38% in placebo) 3
• While primarily studied for postoperative and gastroenteritis-related vomiting, the mechanism of H1-receptor antagonism applies equally to motion sickness 3, 4
• The medication appears safe for pediatric use with minimal adverse effects when dosed appropriately 5, 3

Practical Administration Algorithm

1. Verify weight: Confirm the child weighs 40 kg
2. Calculate dose: 1.25 mg/kg × 40 kg = 50 mg 1
3. Choose route: IM (direct injection) or IV (diluted over 2 minutes) 1
4. Timing: First dose 30-60 minutes before travel begins
5. Repeat dosing: Every 4 hours as needed during travel 1
6. Daily maximum: Do not exceed 300 mg total in 24 hours 1