What is the recommended dose of diphenhydramine (antihistamine) for an 11-year-old boy weighing 56kg with generalized urticaria (hives)?

Diphenhydramine Dosing for an 11-year-old with Generalized Hives

For an 11-year-old boy weighing 56kg with generalized urticaria (hives), the recommended dose of diphenhydramine is 1-2 mg/kg per dose, with a maximum of 50 mg per dose. 1, 2

Dosing Calculation

• For a 56kg child:
  • At 1 mg/kg: 56 mg (exceeds maximum single dose)
  • At 2 mg/kg: 112 mg (exceeds maximum single dose)
  • Therefore, administer the maximum dose of 50 mg 2

Administration Options

• Oral route is preferred for non-emergency urticaria 1
  • Oral liquid formulation is more readily absorbed than tablets 2
  • Duration of effect is 4-6 hours 1
• For severe reactions:
  • Parenteral (IM/IV) administration provides faster onset of action 1
  • Maximum dose remains 50 mg 2

Important Clinical Considerations

• Diphenhydramine is considered adjunctive therapy for urticaria 2
• For simple urticaria without anaphylaxis, diphenhydramine alone may be sufficient 1
• For anaphylaxis, epinephrine is the first-line treatment; diphenhydramine should never replace epinephrine 1, 2
• Common adverse effects include:
  • Drowsiness and sedation (most common) 1, 3
  • Dry mouth, blurred vision 1
  • Potential for cognitive and psychomotor impairment 2

Treatment Optimization

• Consider adding an H2-antagonist (such as ranitidine) for better symptom control 1, 4
  • Combination therapy of H1 and H2 antagonists is superior to diphenhydramine alone for urticaria 1, 4
  • Ranitidine dosing: 1-2 mg/kg per dose (maximum 75-150 mg) 2
• For persistent symptoms, consider:
  • Second-generation (less sedating) antihistamines for ongoing management 2
  • Cetirizine may be an effective alternative with less sedation 5

Follow-up Recommendations

• Observe for symptom improvement within 1-2 hours 3
• Duration of treatment typically 2-3 days for acute urticaria 2
• If symptoms worsen or signs of anaphylaxis develop (difficulty breathing, hypotension, etc.), seek immediate medical attention 2, 1

Cautions

• Avoid exceeding the maximum dose of 50 mg, regardless of weight 2
• Monitor for excessive sedation, especially if combined with other sedating medications 1, 6
• Cardiovascular effects are rare but can occur, especially with IV administration 7