What is the appropriate treatment for allergic reactions, considering medication options?

Treatment of Allergic Reactions

Epinephrine administered intramuscularly in the anterolateral thigh is the first-line and only definitive treatment for anaphylaxis and severe allergic reactions, with antihistamines and corticosteroids serving strictly as adjunctive therapy that should never delay or replace epinephrine. 1, 2, 3

Immediate Management: Epinephrine First

Administer epinephrine IM immediately when anaphylaxis is suspected—do not wait for complete symptom development. 1, 2, 3

Epinephrine Dosing

• Adults and children ≥30 kg: 0.3-0.5 mg (0.3-0.5 mL of 1:1,000 solution) IM 1, 3
• Children <30 kg: 0.01 mg/kg IM, maximum 0.3 mg 1, 3
• Auto-injector dosing: 0.15 mg for children <25 kg; 0.3 mg for ≥25 kg 1
• Injection site: Anterolateral thigh (vastus lateralis muscle) is mandatory—never inject into buttocks, digits, hands, or feet 1, 3
• Repeat dosing: Every 5-15 minutes as needed if symptoms persist or recur 1, 3

Route Selection

• IM epinephrine is superior to subcutaneous because it achieves faster plasma and tissue concentrations 1
• IV epinephrine (1:10,000 solution) is reserved only for patients unresponsive to IM epinephrine and fluid resuscitation, or those in shock who cannot adequately perfuse muscle tissue 1

Adjunctive Pharmacotherapy (Never First-Line)

These medications are administered after or concurrent with epinephrine, never as substitutes. 1, 2

H1-Antihistamines

Diphenhydramine 25-50 mg IV or oral is the traditional choice, but has significant limitations. 1, 4, 5

• Pediatric dosing: 1-2 mg/kg (maximum 50 mg) 4, 2
• Critical limitation: H1-antihistamines only relieve pruritus and urticaria—they do NOT treat stridor, bronchospasm, hypotension, or other life-threatening manifestations 1, 2
• Onset disadvantage: Diphenhydramine works significantly slower than epinephrine and cannot reverse anaphylaxis 4, 2

Second-generation antihistamines (cetirizine 10 mg, loratadine 10 mg) are preferred alternatives when sedation must be avoided, as they have faster onset than other second-generation agents and lack the cognitive impairment of diphenhydramine. 1, 2

H2-Antihistamines

Consider adding an H2-blocker (ranitidine or famotidine 1-2 mg/kg IV, maximum 75-150 mg) in combination with H1-antihistamines for urticaria, as the combination is more effective than H1-antihistamines alone. 1, 4, 6

• Evidence shows combined H1/H2 blockade provides superior relief of urticaria compared to H1-antihistamines alone (92% vs 46% relief) 6
• H2-antihistamines alone have minimal evidence for anaphylaxis treatment 1

Corticosteroids

Administer corticosteroids to prevent biphasic or protracted reactions, not for immediate symptom relief. 1, 4, 2

• Dosing: Hydrocortisone 200 mg IV or prednisone 1 mg/kg oral (maximum 50-60 mg) 1, 4
• Timing: Corticosteroids do not provide immediate benefit and should never delay epinephrine 1, 2
• Duration: Continue for 2-3 days post-discharge 4

Bronchodilators

Albuterol nebulizer is adjunctive for bronchospasm unresponsive to epinephrine. 1

• Albuterol does NOT relieve laryngeal edema and cannot substitute for epinephrine 1
• Use nebulized therapy over MDI when respiratory distress is severe 1

Management of Mild-to-Moderate Reactions (Without Anaphylaxis)

For isolated urticaria, pruritus, or mild angioedema without respiratory, cardiovascular, or GI involvement: 2, 7

• Second-generation H1-antihistamine (cetirizine 10 mg or loratadine 10 mg) is preferred over diphenhydramine due to lack of sedation 2
• Monitor for 30 minutes minimum for progression to anaphylaxis 1, 4
• Have epinephrine immediately available even for mild reactions 2

Critical Pitfalls to Avoid

Never Delay Epinephrine

The single most dangerous error is delaying epinephrine while administering antihistamines or corticosteroids first. 1, 4, 2

• Antihistamines take significantly longer to work and cannot reverse life-threatening symptoms 4, 2
• Any delay in epinephrine administration increases mortality risk 4

Avoid First-Generation Antihistamines in Infusion Reactions

Do not use diphenhydramine for IV iron or chemotherapy infusion reactions, as it can paradoxically worsen hypotension, tachycardia, and shock. 1

• Second-generation antihistamines (cetirizine 10 mg IV/oral or loratadine 10 mg oral) are safer alternatives 1

Recognize Inadequate IM Injection

Ensure adequate needle length to reach muscle, especially in obese patients where subcutaneous fat may prevent proper IM delivery. 1

• Subcutaneous epinephrine provides some benefit but is significantly less effective than IM 1

Post-Treatment Monitoring and Discharge

• Observe for minimum 4-6 hours after anaphylaxis due to risk of biphasic reactions 7
• Prescribe two epinephrine auto-injectors for any patient with anaphylaxis 2
• Discharge regimen: H1-antihistamine every 6 hours for 2-3 days, H2-antihistamine twice daily for 2-3 days, and corticosteroid daily for 2-3 days 4
• Refer to allergist for comprehensive evaluation and immunotherapy consideration 2