What is the recommended dose of diphenhydramine (Intramuscular) (IM) for treating allergic reactions?

Diphenhydramine IM Dosing for Allergic Reactions

For allergic reactions, the recommended intramuscular (IM) dose of diphenhydramine is 25-50 mg for adults, with a maximum daily dose of 400 mg. 1

Adult Dosing

• The standard parenteral dose of diphenhydramine for adults is 25-50 mg per dose for allergic reactions 2
• For intramuscular administration specifically, adults should receive 10-50 mg deep IM, with a maximum daily dosage of 400 mg 1
• Diphenhydramine is considered a second-line therapy after epinephrine in anaphylaxis management and should never be administered alone 2

Pediatric Dosing

• For pediatric patients (excluding premature infants and neonates), the recommended dosage is 5 mg/kg/24 hr or 150 mg/m²/24 hr, divided into four doses administered deep intramuscularly 1
• Weight-based dosing for children is 1-2 mg/kg per dose, with a maximum single dose of 50 mg 2
• Maximum daily dosage for children is 300 mg 1

Administration Considerations

• Diphenhydramine should be administered deep intramuscularly for allergic reactions when the oral route is impractical 1
• Parenteral administration is preferred for faster onset of action in acute allergic reactions 2
• The onset of action for diphenhydramine is several minutes when given parenterally, with a duration of effect of 4-6 hours 2

Clinical Management Algorithm

1. First administer epinephrine for anaphylactic reactions (primary treatment) 3
2. Follow with diphenhydramine as adjunctive therapy:
   • Adults: 25-50 mg IM 3, 1
   • Children: 1-2 mg/kg IM (not exceeding 50 mg per dose) 2, 1
3. Consider adding an H2-antagonist (such as ranitidine) as the combination of diphenhydramine and ranitidine is superior to diphenhydramine alone for urticaria 3, 4
4. Administer corticosteroids (such as hydrocortisone 200 mg IV for adults) to prevent protracted or biphasic reactions 3

Important Considerations and Pitfalls

• Diphenhydramine should never replace epinephrine as the first-line treatment for anaphylaxis 2
• Be aware of sedative effects which may complicate discharge planning from emergency departments 5
• Common adverse effects include dizziness, blurred vision, dry mouth, hypotension, and urinary retention 2
• Hypnotic effects are increased when combined with alcohol, benzodiazepines, or opioid narcotics 2
• While newer non-sedating antihistamines may be considered for less severe allergic reactions, diphenhydramine's faster onset of action may be beneficial in acute situations 6, 5