What is the recommended pre kg (per kilogram) dose of diphenhydramine and its indications?

Diphenhydramine Dosing and Indications

The recommended parenteral dose of diphenhydramine is 1-2 mg/kg or 25-50 mg per dose, with anaphylaxis being its primary indication. 1

Dosing Guidelines

Pediatric Dosing

• Children under 6 years: 1-2 mg/kg per dose 1
• Maximum single dose: 50 mg 1
• Toxic threshold: ≥7.5 mg/kg requires emergency department referral 2, 3, 4

Adult Dosing

• Standard dose: 25-50 mg per dose 1
• Maximum single dose: 50 mg 1
• Toxic threshold: ≥300 mg or ≥7.5 mg/kg (whichever is less) requires emergency department referral 2

Primary Indications

1. Anaphylaxis management: Second-line adjunctive therapy after epinephrine 1

   • Used as part of the comprehensive management protocol
   • Never to be used alone in place of epinephrine

2. Allergic reactions: Treatment of type I hypersensitivity reactions 1

   • Urticaria
   • Pruritus
   • Angioedema

3. Adjunct for procedural sedation: 1

   • Can reduce requirements for other sedative medications
   • Provides modest sedative effect

Administration Routes

• Intravenous: Preferred in emergency situations for rapid effect 1
• Intramuscular: Alternative when IV access is unavailable 1
• Oral: Less appropriate for acute severe reactions due to delayed onset 1

Clinical Considerations

Efficacy Enhancement

• Combination with H2 antagonists (e.g., ranitidine 1 mg/kg) is superior to diphenhydramine alone for anaphylaxis management 1

Pharmacokinetics

• Onset of action: Several minutes (IV); longer for oral administration 1
• Duration of effect: 4-6 hours 1

Adverse Effects

• Sedation/drowsiness (most common) 1, 5
• Anticholinergic effects: dry mouth, blurred vision, urinary retention 1
• Hypotension (particularly with rapid IV administration) 1
• Dizziness 1

Important Cautions

1. Never use as monotherapy for anaphylaxis

   • Diphenhydramine is second-line therapy after epinephrine 1
   • Has much slower onset of action than epinephrine 1

2. Dosing safety thresholds

   • Doses ≥7.5 mg/kg are associated with significant risk of toxicity 2, 3, 4
   • For every log10 unit increase in mg/kg dose, odds of clinically significant toxicity increase 47-fold 4

3. Administration rate

   • Administer IV doses slowly to minimize risk of hypotension 1

4. Monitoring requirements

   • Monitor for sedation, particularly when combined with other CNS depressants 1
   • Observe for hypotension with parenteral administration 1

Follow-up Care After Anaphylaxis

• Continue diphenhydramine every 6 hours for 2-3 days after discharge 1
• Consider alternative dosing with non-sedating second-generation antihistamines 1
• Maintain combination with H2 antagonists for optimal effect 1