What medications are used to treat an allergic reaction?

Medications for Treating Allergic Reactions

Epinephrine is the first-line treatment for severe allergic reactions (anaphylaxis), while antihistamines, corticosteroids, and bronchodilators serve as adjunctive therapies based on symptom severity. 1

Treatment Algorithm Based on Severity

Severe Allergic Reactions (Anaphylaxis)

1. First-line treatment: Epinephrine

   • Administer intramuscularly in the anterolateral thigh 1, 2
   • Dosing:
     • Adults and children ≥30 kg: 0.3-0.5 mg (0.3-0.5 mL) 2
     • Children <30 kg: 0.01 mg/kg, maximum 0.3 mg 2
   • May repeat every 5-15 minutes if symptoms persist 1
   • No absolute contraindications to epinephrine use in anaphylaxis 1

2. Adjunctive treatments (to be given after epinephrine, not as substitutes):

   • Bronchodilators (albuterol) for bronchospasm 1
   • H1 antihistamines (diphenhydramine 25-50 mg) for itching and urticaria 1
   • H2 antihistamines (ranitidine) as supplementary treatment 1
   • Corticosteroids (prednisone) to prevent biphasic reactions 1
   • IV fluids for hypotension 1
   • Supplemental oxygen as needed 1

3. Post-discharge medications (2-3 days):

   • H1 antihistamine: diphenhydramine every 6 hours or non-sedating alternative 1
   • H2 antihistamine: ranitidine twice daily 1
   • Corticosteroid: prednisone daily 1

Mild-to-Moderate Allergic Reactions

• H1 antihistamines for mild symptoms (flushing, urticaria, mild angioedema) 1
  • First-generation (diphenhydramine) or second-generation (cetirizine) 1
• Ongoing observation is essential to monitor for progression to anaphylaxis 1
• Administer epinephrine immediately if symptoms progress or if patient has history of severe reactions 1

Mechanism of Action of Key Medications

1. Epinephrine:

   • Acts on alpha-adrenergic receptors to reduce vasodilation and vascular permeability 2
   • Acts on beta-adrenergic receptors to relieve bronchospasm and relax smooth muscle 2
   • Rapid onset and short duration of action when administered parenterally 2

2. Antihistamines:

   • H1 antihistamines relieve itching and urticaria but do not treat respiratory symptoms or shock 1
   • H2 antihistamines may provide additional benefit when used with H1 antihistamines 1

3. Corticosteroids:

   • Slow onset of action (hours) 1
   • May help prevent biphasic or protracted reactions 1
   • Not effective for immediate symptom relief 1

Important Considerations and Pitfalls

• Do not delay epinephrine administration in anaphylaxis; it is the only medication shown to reduce mortality 1, 3
• Do not inject epinephrine into buttocks, digits, hands, or feet to avoid tissue damage 2
• Antihistamines alone are insufficient for treating anaphylaxis and should never replace epinephrine 1, 4
• Monitor patients for at least 4-6 hours after anaphylaxis due to risk of biphasic reactions 1
• Patients at risk should carry two epinephrine auto-injectors at all times 3
• Special caution needed when administering epinephrine to patients with:
  • Cardiovascular disease
  • Taking monoamine oxidase inhibitors or tricyclic antidepressants
  • Taking stimulant medications
  • Certain pre-existing conditions (recent intracranial surgery, aortic aneurysm, uncontrolled hyperthyroidism) 1

Patient Education

• Provide training on proper use of epinephrine auto-injectors 1
• Educate on allergen avoidance and early recognition of symptoms 1
• Recommend medical identification jewelry indicating allergy 1
• Ensure patients understand the importance of seeking emergency care after using epinephrine 1

Epinephrine remains the cornerstone of anaphylaxis treatment, with all other medications serving supportive roles. The prompt administration of intramuscular epinephrine can be life-saving in severe allergic reactions, while antihistamines may be sufficient for milder symptoms with careful monitoring.