Duration of Histex (Diphenhydramine) Treatment
For allergic reactions following anaphylaxis, diphenhydramine should be continued every 6 hours for 2-3 days after discharge from the emergency department. 1
Post-Anaphylaxis Management
The NIAID-sponsored expert panel provides clear guidance on adjunctive antihistamine therapy following food-induced anaphylaxis:
- Diphenhydramine should be dosed every 6 hours for 2-3 days as part of the discharge treatment plan after anaphylactic reactions 1
- This regimen is combined with H2 antihistamines (ranitidine twice daily) and corticosteroids (prednisone daily) for the same 2-3 day period 1
- Alternative dosing with a non-sedating second-generation antihistamine may be substituted if sedation is problematic 1
Rationale for Duration
The 2-3 day treatment course aims to prevent biphasic or protracted allergic reactions, though the evidence supporting this specific duration is limited:
- Biphasic reactions can occur when inflammatory cells are recruited and long-acting mediators are released from mast cells 1
- The observation period after anaphylaxis should be 4-6 hours or longer based on severity, with continued antihistamine therapy extending beyond discharge 1
Treatment of Mild Allergic Reactions
For milder allergic reactions (flushing, urticaria, isolated mild angioedema):
- H1 antihistamines can be used as needed for symptom control without a fixed duration 1
- Ongoing observation is warranted to ensure no progression to anaphylaxis 1
- If symptoms progress or the patient has a history of severe reactions, epinephrine should be administered immediately 1
Important Caveats
- Antihistamines should never replace epinephrine for severe reactions—epinephrine remains first-line treatment 1, 2
- For patients requiring emergency transport, activated charcoal should not be given en route due to diphenhydramine's potential to cause loss of consciousness or seizures 3
- The sedating effects of first-generation antihistamines like diphenhydramine can impair concentration and performance 2