What is the therapeutic dose of diphenhydramine (antihistamine)?

Therapeutic Dosage of Diphenhydramine

The therapeutic dose of diphenhydramine is 25-50 mg per dose for adults, with a weight-based dosage of 1-2 mg/kg for children. 1

Adult Dosing

• Standard oral/parenteral dose: 25-50 mg per dose 1
• Maximum daily dose: Generally not to exceed 300 mg in 24 hours 2
• For sedation as an adjunct in procedures: 25-50 mg intravenously 1
• For allergic reactions: 25-50 mg parenterally 1
• For discharge therapy after allergic reactions: Diphenhydramine every 6 hours for 2-3 days 1

Pediatric Dosing

• Weight-based dosing: 1-2 mg/kg per dose 1
• Maximum single dose: 50 mg 1
• Toxic threshold: Children under 6 years who ingest at least 7.5 mg/kg should be referred to an emergency department 2

Administration Routes

• Oral: Available as 25 mg/10 mL solution 3
• Parenteral: Intravenous administration should be slow to minimize adverse effects 1
• For anaphylaxis: Parenteral administration preferred for faster onset of action 1

Clinical Considerations

Onset and Duration

• Onset of action: Several minutes when given intravenously 1
• Duration of effect: 4-6 hours 1
• Half-life varies significantly by age:
  • Children: 5.4 ± 1.8 hours 4
  • Young adults: 9.2 ± 2.5 hours 4
  • Elderly adults: 13.5 ± 4.2 hours 4

Clearance Rates

• Elderly adults: 11.7 ± 3.1 mL/min/kg 4
• Young adults: 23.3 ± 9.4 mL/min/kg 4
• Children: 49.2 ± 22.8 mL/min/kg 4

Common Adverse Effects

• Sedation (most common) 5
• Dizziness, blurred vision, dry mouth 1
• Hypotension (especially with rapid IV administration) 1
• Epigastric discomfort, urinary retention, wheezing 1

Important Cautions

• In anaphylaxis management, diphenhydramine is considered second-line therapy to epinephrine and should never be administered alone 1
• When used as an adjunct for sedation, diphenhydramine may allow for reduction in doses of other sedatives 1
• Hypnotic effects are increased when combined with alcohol, benzodiazepines, or opioid narcotics 1
• Dose adjustments may be needed in elderly patients due to prolonged half-life and reduced clearance 4
• For patients 6 years and older who ingest at least 7.5 mg/kg or 300 mg (whichever is less), emergency department referral is recommended 2

Special Populations

• Elderly: Consider lower doses due to prolonged elimination half-life (13.5 ± 4.2 hours vs. 9.2 ± 2.5 hours in young adults) 4
• Children: Higher clearance rates (49.2 ± 22.8 mL/min/kg) compared to adults 4
• Psychiatric patients with insomnia: Doses of 12.5-50 mg have shown efficacy 6