What is the appropriate dose of diphenhydramine (antihistamine) for a 12-year-old child weighing 36kg experiencing vomiting?

Diphenhydramine Dosing for a 12-Year-Old with Vomiting

Diphenhydramine is NOT recommended as a first-line antiemetic for vomiting in a 12-year-old child, but if used, the appropriate dose would be 12.5-25 mg orally every 4-6 hours as needed. 1

Why Diphenhydramine Should Not Be First-Line for Vomiting

• Diphenhydramine is primarily indicated for dystonic reactions from other antiemetics (like metoclopramide or prochlorperazine), not as a primary antiemetic agent. 1
• The evidence supports ondansetron as the preferred antiemetic for pediatric vomiting, with a dose of 0.2 mg/kg oral (maximum 4 mg) for children unable to take fluids due to persistent vomiting. 2, 3
• Diphenhydramine has significant anticholinergic side effects including oversedation, confusion, and impaired psychomotor performance that make it less favorable than alternatives. 1, 4, 5

If Diphenhydramine Must Be Used

For a 36 kg child, the appropriate dose is 12.5-25 mg orally every 4-6 hours during the vomiting episode. 1

Dosing Specifics:

• The FDA-approved dosing for children 6 to under 12 years is 10 mL (25 mg) every 4-6 hours, not to exceed 6 doses in 24 hours. 6
• Since this child is 12 years old, they fall into the "adults and children over 12 years" category, which allows 10-20 mL (25-50 mg) every 4-6 hours. 6
• Start with the lower dose of 12.5-25 mg given the child's weight and age at the transition point. 1, 6

Critical Safety Considerations

• Monitor for anticholinergic effects: confusion, excessive sedation, dry mouth, blurred vision, urinary retention, and tachycardia. 4, 7
• Avoid rapid administration if given intravenously, as this increases risk of seizures and cardiovascular toxicity. 4
• The toxic dose threshold is 7.5 mg/kg (270 mg for this 36 kg child), which would require emergency department referral. 8
• Do not use for sleep or as routine antiemetic - newer alternatives are safer and more effective. 4, 5

Preferred Alternative Approach

For gastroenteritis-related vomiting, ondansetron 0.2 mg/kg oral (7.2 mg for this child, round to 8 mg) is superior to diphenhydramine. 2, 3

• Ondansetron reduces vomiting, facilitates oral rehydration therapy, and has fewer adverse effects than older antiemetics. 3
• For low emetogenic risk situations, other options include metoclopramide or prochlorperazine (though these carry dystonic reaction risk). 1

When to Escalate Care

• Bilious or bloody vomiting, altered mental status, severe dehydration, or symptoms persisting beyond 4 hours warrant emergency department evaluation regardless of medication choice. 2, 9