What is the first line treatment for an allergic reaction?

First-Line Treatment for Allergic Reactions

Epinephrine administered intramuscularly is the first-line treatment for anaphylaxis and severe allergic reactions. 1, 2

Treatment Algorithm Based on Severity

For Anaphylaxis (Severe Allergic Reaction)

First-Line Treatment:

• Epinephrine IM injection into the anterolateral thigh:
  • Adults and children ≥30 kg: 0.3-0.5 mg (0.3-0.5 mL of 1:1000 solution)
  • Children <30 kg: 0.01 mg/kg, maximum 0.3 mg
  • May repeat every 5-15 minutes as needed 1, 2

Adjunctive Treatments (after epinephrine):

• Position patient in recumbent position with lower extremities elevated if tolerated
• Supplemental oxygen if needed
• IV fluids for hypotension or incomplete response to epinephrine
• H1 antihistamine: Diphenhydramine 1-2 mg/kg (maximum 50 mg) IV or oral
• H2 antihistamine: Ranitidine 1-2 mg/kg (maximum 75-150 mg) oral or IV
• Bronchodilator: Albuterol via MDI or nebulizer for bronchospasm
• Corticosteroids: Prednisone 1 mg/kg (maximum 60-80 mg) oral or methylprednisolone IV 1

For Mild-to-Moderate Allergic Reactions (urticaria, pruritus without systemic symptoms)

• H1 antihistamines are appropriate first-line treatment 1
• Options include:
  • Diphenhydramine (first-generation): 1-2 mg/kg, maximum 50 mg
  • Second-generation (non-sedating) antihistamines like fexofenadine, cetirizine, or loratadine 3, 4, 5

Important Clinical Considerations

When to Use Epinephrine Immediately

• Any signs of respiratory compromise (wheezing, stridor)
• Hypotension
• History of severe previous reactions
• Involvement of more than one organ system
• Rapid progression of symptoms 1, 6

Common Pitfalls to Avoid

1. Delay in epinephrine administration: This is the most critical error in managing anaphylaxis. Research shows that delayed epinephrine administration is associated with increased mortality 6.

2. Relying solely on antihistamines for anaphylaxis: Antihistamines cannot reverse the life-threatening aspects of anaphylaxis such as airway obstruction, hypotension, and shock. They act more slowly (1-3 hours to reach maximum concentration) compared to epinephrine (<10 minutes) 6.

3. Improper route of epinephrine administration: The anterolateral thigh is the preferred injection site. Injections into buttocks, digits, hands, or feet can lead to tissue damage 2.

4. Underdosing epinephrine: Using the correct concentration (1:1000 for IM) and dose is essential 1, 2.

5. Failure to observe patients after treatment: Patients with anaphylaxis should be observed for 4-6 hours or longer due to the risk of biphasic reactions 1.

Special Considerations

• For patients with mild allergic reactions who have a history of severe reactions, consider epinephrine as first-line treatment 1, 6.

• Combination of H1 and H2 antihistamines may be more effective for urticaria than H1 antihistamines alone 7.

• Second-generation antihistamines have fewer sedative effects than first-generation antihistamines like diphenhydramine, making them preferable for ongoing management 4, 5.

• Patients with anaphylaxis should be prescribed epinephrine auto-injectors (2 doses) upon discharge and educated on proper use 1.

The evidence clearly demonstrates that while antihistamines are appropriate for mild allergic reactions, epinephrine is the definitive first-line treatment for anaphylaxis, with no acceptable substitute when addressing severe allergic reactions that could impact mortality and morbidity 6.