Quick Answer
Mild reactions usually resolve within a few hours after antihistamines. Moderate reactions treated with steroids typically improve within 24 hours and fully resolve within 3 to 5 days. A small percentage of severe reactions have a delayed second wave (biphasic reaction) up to 12 hours after the initial event — that's why we always send patients home with written return precautions and, when appropriate, an epinephrine auto-injector.
This question comes up frequently for patients researching allergic reactions & asthma attacks at Good Health NC in Knightdale, NC.
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