Acute allergic reaction


Clinical Features

Raised urticaria
  • Presentation can be delayed

Differential Diagnosis

Acute allergic reaction



  1. H1 antagonist: Diphenhydramine 50mg IV/IM/PO
    • H1 antagonists with low sedating activity, such as fexofenadine, loratadine, cetirizine, are preferred over diphenhydramine and hydroxyzine when appropriate[1]
  2. H2 antagonist: Famotidine 40mg OR ranitidine 150mg IV/IM/PO
    • Improves urticaria but not angioedema at 2 hours[2]
  3. Consider corticosteroid: methylprednisolone 125mg IV/IM OR prednisone 60mg PO
    • Continue steroid burst if outpatient (40mg prednisone PO x 5 days)


  • Consider brief observation in the ED for improvement of symptoms
  • Send home with an anaphylaxis emergency plan and epinephrine autoinjector! (Epi-Pen)

See Also


  1. [Guideline] Zuberbier T, Asero R, Bindslev-Jensen C, Walter Canonica G, Church MK, Giménez-Arnau AM, et al. EAACI/GA(2)LEN/EDF/WAO guideline: management of urticaria. Allergy. 2009 Oct. 64(10):1427-43
  2. Lin, RY et al. Improved Outcomes in Patients With Acute Allergic Syndromes Who Are Treated With Combined H1 and H2 Antagonists. Annals of Emergency Medicine. 36:5 NOVEMBER 2000.