Diphenhydramine for Allergy Symptom Relief
For relief of allergy symptoms, diphenhydramine (Benadryl) should be dosed at 25-50 mg orally every 4-6 hours as needed, not exceeding 6 doses (300 mg) in 24 hours for adults and children over 12 years; children 6-12 years should receive 25 mg every 4-6 hours. 1
Dosage and Administration
- Adults and children over 12 years: 25-50 mg (10-20 mL of liquid formulation) every 4-6 hours, maximum 300 mg in 24 hours 1
- Children 6-12 years: 25 mg (10 mL) every 4-6 hours 1
- Children under 6 years: Not recommended 1
Mechanism and Clinical Effects
- Diphenhydramine is a first-generation H1-receptor antagonist with anticholinergic and sedative properties 2
- Onset of action occurs within several minutes with duration of effect up to 4-6 hours 2
- Primarily used for treating allergic symptoms including allergic rhinitis, urticaria, and as adjunctive therapy in anaphylaxis 3
Role in Anaphylaxis Management
- Diphenhydramine is considered second-line therapy in anaphylaxis at a dose of 1-2 mg/kg or 25-50 mg parenterally 2
- H1 antihistamines should never be administered alone in anaphylaxis treatment; epinephrine remains the first-line treatment 2
- In anaphylaxis management, a combination of diphenhydramine and ranitidine (H2 blocker) is superior to diphenhydramine alone 2
Adverse Effects and Precautions
- Common adverse effects include sedation, dizziness, blurred vision, dry mouth, urinary retention, and wheezing 2
- Hypnotic effects are increased when combined with alcohol, benzodiazepines, or opioid narcotics 2
- Caution is warranted in elderly patients due to increased risk of adverse effects 4
- Toxic dose threshold in children under 6 years is 7.5 mg/kg; in patients 6 years and older, toxicity can occur at 7.5 mg/kg or 300 mg (whichever is less) 5
Limitations and Alternatives
- Second-generation antihistamines (like fexofenadine, cetirizine) have similar efficacy with fewer sedative and anticholinergic effects 6, 4
- For insomnia, evidence suggests diphenhydramine 50 mg is not recommended as it falls below clinically significant improvement thresholds 2
- While diphenhydramine tends to work slightly faster than second-generation alternatives, the difference is not statistically significant, and the risk-to-benefit ratio favors newer agents 7
Special Considerations
- Diphenhydramine has a modest stimulatory effect on ventilation and may counteract opioid-induced hypoventilation 2
- In patients with acute allergic reactions, diphenhydramine is useful for relieving itching and urticaria but does not relieve stridor, shortness of breath, wheezing, GI symptoms, or shock 2
- For chronic or recurring allergic symptoms, second-generation antihistamines are preferred due to their favorable safety profile 6, 4