What is the recommended dose of intravenous (IV) antihistamines, such as diphenhydramine, for treating allergic reactions in adults?

IV Antihistamines for Allergic Reactions: Dosing Guidelines

For allergic reactions, the recommended dose of IV diphenhydramine is 25-50 mg for adults, administered at a rate not exceeding 25 mg/min. 1

Adult Dosing Guidelines

First-line Treatment

• Epinephrine is the first-line treatment for anaphylaxis, not antihistamines
  • IM epinephrine (0.01 mg/kg, max 0.5 mg) should never be delayed to administer antihistamines 2

Antihistamine Dosing (as adjunctive therapy)

• Diphenhydramine (H1 antihistamine):

  • Adults: 25-50 mg IV/IM 3, 1
  • Administer at a rate not exceeding 25 mg/min 1
  • Maximum daily dosage: 400 mg 1

• Ranitidine (H2 antihistamine):

  • Adults: 1 mg/kg (50 mg) IV 3, 2
  • Can be diluted in D5W to 20 mL and injected over 5 minutes 3
  • Maximum dose: 75-150 mg 3

Pediatric Dosing Guidelines

• Diphenhydramine:
  • 1-2 mg/kg per dose 3, 2
  • Maximum dose: 50 mg 3
  • 5 mg/kg/24 hr or 150 mg/m²/24 hr divided into four doses 1
  • Maximum daily dosage: 300 mg 1

Clinical Considerations

Role in Treatment Algorithm

1. Epinephrine first for anaphylaxis (IM in anterior-lateral thigh)
2. Antihistamines as adjunctive therapy for cutaneous symptoms
3. Consider H1 + H2 antihistamine combination for improved efficacy

Important Caveats

• Antihistamines should never delay epinephrine administration in anaphylaxis 2, 4
• Antihistamines primarily relieve cutaneous symptoms (urticaria, pruritus) but do not prevent or treat life-threatening respiratory or cardiovascular symptoms 4
• The combination of H1 (diphenhydramine) and H2 (ranitidine) antagonists is superior to diphenhydramine alone for urticaria 3, 5
• IV diphenhydramine has a faster onset of action (minutes) compared to oral antihistamines (1-3 hours) 4

Administration Considerations

• Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration 1
• Oral liquid diphenhydramine is more readily absorbed than tablets when oral administration is chosen 3
• For patients with "caine" allergies, diphenhydramine can also serve as a local anesthetic 6

Follow-up Care

• After acute management, consider transitioning to:
  • H1 antihistamine: diphenhydramine every 6 hours for 2-3 days or
  • Non-sedating second-generation antihistamine 3
  • H2 antihistamine: ranitidine twice daily for 2-3 days 3

Remember that while antihistamines are valuable for symptom management in allergic reactions, they should not replace epinephrine as the first-line treatment for anaphylaxis, as they do not address the life-threatening aspects of anaphylaxis and have a slower onset of action.