What medications are used to treat allergic reactions?

Medications for Treating Allergic Reactions

Intramuscular epinephrine is the first-line and only definitive treatment for anaphylaxis and severe allergic reactions, with all other medications serving strictly as adjunctive therapy that should never delay or replace epinephrine administration. 1, 2

First-Line Treatment: Epinephrine

Epinephrine must be administered intramuscularly into the anterolateral thigh for all anaphylactic reactions. 1, 2

Dosing:

• Adults and children ≥30 kg: 0.3-0.5 mg IM (0.3-0.5 mL of 1:1,000 solution) 1, 2
• Children <30 kg: 0.01 mg/kg IM (maximum 0.3 mg per dose) 1, 2
• Repeat every 5-15 minutes as necessary 1, 2

Mechanism:

Epinephrine works through alpha-adrenergic receptors to reverse vasodilation and vascular permeability (treating hypotension and shock), and through beta-adrenergic receptors to relax bronchial smooth muscle (treating bronchospasm and respiratory distress). 2 It also alleviates pruritus, urticaria, angioedema, and gastrointestinal symptoms. 2

Critical Point:

There are no absolute contraindications to epinephrine in anaphylaxis—the risk of death from untreated anaphylaxis outweighs all other concerns, even in patients with cardiovascular disease, those on beta-blockers, or those taking MAO inhibitors. 1

Adjunctive Medications (Never Substitute for Epinephrine)

H1 Antihistamines

H1 antihistamines only relieve itching and urticaria—they do NOT treat stridor, shortness of breath, wheezing, gastrointestinal symptoms, or shock. 1

Options:

• Diphenhydramine: 1-2 mg/kg per dose (maximum 50 mg) IV or oral 1

  • Oral liquid is absorbed more rapidly than tablets 1
  • Causes significant sedation and cognitive impairment that may mask worsening symptoms 1

• Cetirizine: 10 mg oral (alternative second-generation option) 1

  • Preferred over first-generation agents due to rapid onset with minimal sedation 1, 3
  • Less cognitive impairment than diphenhydramine 3, 4

H2 Antihistamines

Minimal evidence supports H2 antihistamines in anaphylaxis, though they are commonly used concurrently with H1 antihistamines. 1

• Ranitidine: 1-2 mg/kg per dose (maximum 75-150 mg) oral or IV 1
• The combination of H1 and H2 antihistamines may provide additional benefit for urticaria specifically 5

Bronchodilators

Albuterol is adjunctive for bronchospasm but does NOT relieve airway edema (laryngeal edema) and must never substitute for epinephrine. 1

Dosing:

• Nebulized solution: Child 1.5 mL, Adult 3 mL every 20 minutes or continuously 1
• MDI with spacer: Child 4-8 puffs, Adult 8 puffs 1
• Nebulized therapy is preferred in emergency settings with respiratory distress 1

Corticosteroids

Corticosteroids have NO role in treating acute anaphylaxis due to their 4-6 hour onset of action, but are given empirically to potentially prevent biphasic reactions (which occur in up to 20% of cases). 1

Dosing:

• Prednisone: 1 mg/kg oral (maximum 60-80 mg) 1
• Methylprednisolone: 1 mg/kg IV (maximum 60-80 mg) 1
• Duration: Continue for 2-3 days only, as all biphasic reactions occur within 3 days 1

Supplemental Oxygen

Administer oxygen initially to all patients with anaphylaxis. 1

Intravenous Fluids

Large-volume IV fluids are essential for patients with orthostasis, hypotension, or incomplete response to epinephrine. 1

• Place patient in recumbent position with lower extremities elevated 1

Special Situations

Patients on Beta-Blockers

Glucagon must be administered for patients on beta-adrenergic antagonists who develop refractory hypotension or bradycardia despite epinephrine. 1

• Adult dose: 1-5 mg IV over 5 minutes, may repeat or follow with infusion of 5-15 μg/min 1
• Pediatric dose: 20-30 μg/kg (maximum 1 mg) 1
• Glucagon has inotropic and chronotropic effects not mediated through beta-receptors 1

Refractory Hypotension

Vasopressors (other than epinephrine) should be titrated for persistent hypotension despite epinephrine and IV fluids, with continuous blood pressure monitoring. 1

Bradycardia

Atropine should be administered IV for bradycardia. 1

Post-Discharge Adjunctive Treatment

After anaphylaxis, continue for 2-3 days: 1

• H1 antihistamine: Diphenhydramine every 6 hours (or non-sedating second-generation alternative) 1
• H2 antihistamine: Ranitidine twice daily 1
• Corticosteroid: Prednisone daily 1

Mild Allergic Reactions (Non-Anaphylactic)

For isolated flushing, urticaria, mild angioedema, or oral allergy syndrome without systemic symptoms, H1 and H2 antihistamines may be used alone with close monitoring. 1

Critical Caveat:

If there is any history of prior severe reaction, or if symptoms progress, administer epinephrine immediately—do not wait. 1 Ongoing observation is mandatory when antihistamines are used without epinephrine to ensure symptoms do not progress to anaphylaxis. 1