Antiallergic Medications: Recommended Agents and Dosing
For acute allergic reactions including anaphylaxis, epinephrine is the only first-line treatment and must be administered immediately, while antihistamines serve only as adjunctive therapy with no role in preventing life-threatening complications. 1, 2
Immediate Treatment for Anaphylaxis
First-Line: Epinephrine (CRITICAL)
Epinephrine is the only medication that prevents mortality in anaphylaxis and must never be substituted with antihistamines. 1, 2
Dosing:
- Adults and children ≥30 kg: 0.3-0.5 mg intramuscularly (1:1,000 solution) into the anterolateral thigh 1, 3
- Children <30 kg: 0.01 mg/kg IM (maximum 0.3 mg) into the anterolateral thigh 1, 3
- Auto-injector dosing: 0.15 mg for 10-25 kg; 0.3 mg for >25 kg 4
- Repeat every 5-15 minutes as needed if symptoms persist 4, 1
Critical pitfall: Epinephrine reaches peak plasma concentration in <10 minutes, while antihistamines take 1-3 hours—this time difference can be fatal. 2
Adjunctive Antihistamines (Second-Line Only)
H1-Antihistamines:
H2-Antihistamines (add for superior symptom control):
- Ranitidine: 1-2 mg/kg IV (maximum 75-150 mg) or 50 mg IV for adults 4, 1
- Famotidine: 20 mg IV if ranitidine unavailable 1
- The combination of H1 + H2 antagonists is superior to H1 alone for urticaria 1, 5
Corticosteroids (Adjunctive for Biphasic Reactions)
Corticosteroids provide NO acute benefit but may prevent late-phase reactions. 1
Dosing:
- Methylprednisolone: 1-2 mg/kg IV (maximum 60-80 mg) 4, 1
- Prednisone: 1 mg/kg oral (maximum 60-80 mg) 4, 1
- Hydrocortisone: 100 mg IV for adults; 50 mg for children 6 months-6 years; 25 mg for infants <6 months 1
Long-Term Management
Discharge Medications (2-3 Days Post-Reaction)
Prescribe the following regimen: 4
- H1-antihistamine: Diphenhydramine every 6 hours OR a non-sedating second-generation antihistamine (cetirizine 10 mg daily, fexofenadine 180 mg daily) 4, 6, 7
- H2-antihistamine: Ranitidine twice daily 4
- Corticosteroid: Prednisone daily 4
Essential discharge requirements: 4, 1
- Epinephrine auto-injector prescription (2 doses minimum)
- Written emergency action plan
- Allergen avoidance education
- Allergist referral
Non-Anaphylactic Allergic Reactions
Mild Urticaria or Allergic Rhinitis
For mild reactions without systemic symptoms, second-generation antihistamines are preferred over diphenhydramine due to superior safety profile. 7
Recommended agents:
- Cetirizine: 10 mg daily (fastest onset among newer antihistamines, minimal sedation) 8, 7
- Fexofenadine: 180 mg daily (no sedation or cognitive impairment, but slower onset) 9, 7
- Loratadine/Desloratadine: Less efficacious than cetirizine or fexofenadine 7
For acute urticaria specifically: Combination H1 + H2 antihistamines (diphenhydramine 50 mg + cimetidine 300 mg) provides superior relief compared to H1 alone. 5
Special Populations and Refractory Cases
Patients on Beta-Blockers
For patients unresponsive to epinephrine due to beta-blocker therapy: 1
- Glucagon: 1-5 mg IV over 5 minutes (20-30 mcg/kg for children, maximum 1 mg)
- Follow with infusion of 5-15 mcg/min if needed 4, 1
Persistent Hypotension
Escalate treatment systematically: 1, 10
- Epinephrine infusion: 5-15 mcg/min
- Norepinephrine: 0.05-0.5 mcg/kg/min
- Vasopressin: 1-2 IU bolus with infusion of 2 units/hour
Bronchospasm Unresponsive to Epinephrine
- MDI: 4-8 puffs (children), 8 puffs (adults)
- Nebulized: 1.5 mL (children), 3 mL (adults) every 20 minutes or continuously
Critical Pitfalls to Avoid
Never delay epinephrine administration—delayed injection is associated with fatality. 11, 2 Antihistamines do not prevent airway obstruction, hypotension, or shock, which are the mechanisms of death in anaphylaxis. 2
Do not discharge patients prematurely: Observe for minimum 6 hours as biphasic reactions can occur even after successful initial treatment. 1, 11
Do not use antihistamines as monotherapy for anaphylaxis: Despite being more commonly prescribed than epinephrine in practice, this represents dangerous undertreatment. 2