What is the second-line treatment for an allergic reaction?

Second-Line Treatment for Allergic Reactions

The second-line treatment for allergic reactions includes H1 antihistamines (such as diphenhydramine), H2 antihistamines (such as ranitidine), and corticosteroids (such as prednisone), which should be administered after epinephrine in cases of anaphylaxis. 1

H1 Antihistamines

• Diphenhydramine is the most commonly used H1 antihistamine for allergic reactions, administered at 1-2 mg/kg per dose with a maximum dose of 50 mg IV or oral 1
• For ongoing management after discharge, diphenhydramine should be continued every 6 hours for 2-3 days 1
• Alternative dosing with non-sedating second-generation antihistamines may be used instead of diphenhydramine 1
• H1 antihistamines should never be used alone in the treatment of anaphylaxis, as they have a much slower onset of action than epinephrine 1

H2 Antihistamines

• Ranitidine is the preferred H2 antihistamine, administered at 1-2 mg/kg per dose with a maximum dose of 75-150 mg oral or IV 1
• For ongoing management after discharge, ranitidine should be continued twice daily for 2-3 days 1
• The combination of H1 and H2 antihistamines works better than either one alone 1
• H2 antihistamines should not be used without H1 antihistamines in the treatment of anaphylaxis due to their slower onset of action 1

Corticosteroids

• Prednisone is recommended at 1 mg/kg with a maximum dose of 60-80 mg orally 1
• For ongoing management after discharge, prednisone should be continued daily for 2-3 days 1
• Methylprednisolone can be used as an alternative at 1 mg/kg with a maximum dose of 60-80 mg IV 1
• Corticosteroids are primarily used to prevent recurrent or protracted anaphylaxis, although there is no clear evidence to support such therapy 1

Important Considerations

• These second-line treatments should only be administered after epinephrine in cases of anaphylaxis 1
• Recent evidence from the Cross-Canada Anaphylaxis Registry suggests that prehospital antihistamine use may have beneficial effects, with patients who received antihistamines being less likely to have uncontrolled reactions or require hospital admission 2
• However, patients who received prehospital corticosteroids were more likely to require IV fluids and hospital admission, suggesting their role in acute management should be reconsidered 2

Cautions and Special Situations

• Be aware that allergic reactions to corticosteroids themselves can occur, particularly in asthmatic and renal transplant patients 3
• High doses of corticosteroids (≥500 mg) should be given over 30-60 minutes with patient observation for at least the same time period 3
• For patients on beta-blockers who may have reduced response to epinephrine, glucagon should be available as an additional treatment option 1
• In patients with refractory hypotension, glucagon can be administered at 20-30 μg/kg for children or 1-5 mg for adults 1

Follow-up Care

• After an allergic reaction, especially anaphylaxis, patients should have a follow-up appointment with their primary care physician 1
• Consider referral to an allergist/immunologist for further evaluation, particularly after food-induced anaphylactic reactions 1
• Patients should receive education on allergen avoidance and an anaphylaxis emergency action plan 1
• For patients at risk of recurrent anaphylaxis, prescribe an epinephrine auto-injector (2 doses) with proper training on its use 1