What medications are used to treat allergic reactions?

Medications for Treating Allergic Reactions

Epinephrine is the only first-line medication for treating anaphylaxis and severe allergic reactions, administered intramuscularly at 0.3-0.5 mg for adults (0.01 mg/kg for children, max 0.5 mg), with all other medications serving strictly as adjunctive therapy. 1, 2

First-Line Treatment: Epinephrine

Epinephrine must be given immediately for any severe allergic reaction involving respiratory symptoms, diffuse hives, hypotension, or airway swelling. 1, 2

Dosing

• Adults and children ≥30 kg: 0.3-0.5 mg (0.3-0.5 mL of 1:1,000 solution) intramuscularly into the anterolateral thigh 1, 2
• Children <30 kg: 0.01 mg/kg (max 0.3 mg) intramuscularly into the anterolateral thigh 1
• Repeat every 5-15 minutes as needed if symptoms persist or recur 1

Critical Pitfall

The most common and dangerous error is substituting antihistamines for epinephrine as primary treatment, which significantly increases risk of progression to life-threatening reactions. 3 There are no absolute contraindications to epinephrine in anaphylaxis—the risk of death from untreated anaphylaxis outweighs all other concerns. 1

Adjunctive Medications (Never Substitute for Epinephrine)

H1 Antihistamines

Diphenhydramine is the standard H1 antihistamine for acute allergic reactions, but it only relieves itching and urticaria—not respiratory symptoms, hypotension, or shock. 1, 3

• Dosing: 1-2 mg/kg per dose (maximum 50 mg) IV or oral 1
• Oral liquid is absorbed more rapidly than tablets 1
• Alternative: Second-generation antihistamines (cetirizine 10 mg) may be used and cause less sedation 1, 4
• Discharge regimen: Continue diphenhydramine every 6 hours for 2-3 days 1

Important caveat: First-generation antihistamines like diphenhydramine cause significant sedation and cognitive impairment, which can mask worsening anaphylaxis symptoms. 1, 4 For mild allergic reactions not requiring epinephrine, second-generation antihistamines (fexofenadine, loratadine, cetirizine) are preferred. 4, 5

H2 Antihistamines

Ranitidine (or famotidine) provides additional benefit when combined with H1 antihistamines, though evidence for H2 blockers alone is minimal. 1, 3

• Dosing: 1-2 mg/kg per dose (maximum 75-150 mg) oral or IV 1
• The combination of H1 + H2 antihistamines is more effective than either alone for urticaria 3, 6
• Discharge regimen: Continue twice daily for 2-3 days 1

Corticosteroids

Prednisone or methylprednisolone may help prevent biphasic reactions (occurring in up to 20% of cases), though evidence is limited. 1

• Prednisone: 1 mg/kg oral (maximum 60-80 mg) 1
• Methylprednisolone: 1 mg/kg IV (maximum 60-80 mg) 1
• Onset is 4-6 hours—useless for acute symptoms 1
• Discharge regimen: Continue daily for 2-3 days 1

Bronchodilators

Albuterol treats bronchospasm but does not address the underlying allergic cascade. 1

• Nebulized: 1.5 mL (child) or 3 mL (adult) every 20 minutes or continuously 1
• MDI: 4-8 puffs (child) or 8 puffs (adult) 1

Special Situations

Refractory Hypotension

• Glucagon for patients on beta-blockers who may not respond to epinephrine: 1-5 mg IV for adults (20-30 μg/kg for children) 1, 3
• Vasopressors (other than epinephrine) titrated to effect 1
• Large-volume IV fluids for persistent hypotension 1

Bradycardia

• Atropine titrated to effect 1

Treatment Algorithm by Severity

Mild Reactions (localized urticaria, mild pruritus, no respiratory/cardiovascular symptoms)

• H1 antihistamine (preferably second-generation like cetirizine or fexofenadine) 1, 4
• Monitor closely for progression—if any worsening occurs, give epinephrine immediately 1

Moderate to Severe Reactions (diffuse urticaria, angioedema, respiratory symptoms, hypotension)

1. Epinephrine IM immediately 1, 2
2. Place patient supine with legs elevated 1
3. Supplemental oxygen 1
4. H1 antihistamine (diphenhydramine) 1
5. H2 antihistamine (ranitidine) 1
6. Corticosteroid (prednisone or methylprednisolone) 1
7. Albuterol if bronchospasm present 1
8. IV fluids if hypotensive 1

Discharge Management

• Prescribe epinephrine auto-injector (2 doses) with training 1
• Continue H1 antihistamine every 6 hours for 2-3 days 1
• Continue H2 antihistamine twice daily for 2-3 days 1
• Continue corticosteroid daily for 2-3 days 1
• Refer to allergist 1

Critical Observation Period

Observe all patients with anaphylaxis for 4-6 hours minimum (longer for severe reactions) due to risk of biphasic reactions occurring within 3 days. 1