Study of an anaphylactoid reaction to acetaminophen.

Generalized itching, urticaria and anaphylactic shock developed in a 9-year-old girl on two separate occasions after she ingested acetaminophen. She was admitted to our hospital for observation during oral challenge. Total eosinophil counts, total serum IgE, IgA, IgG, IgM, C3, and C4, specific IgE antibodies to six common allergens, and skin prick tests to purified acetaminophen and acetylsalicylic acid (aspirin) were unremarkable. No reaction occurred on open challenge with acetylsalicylic acid and mefenamic acid. However, urticaria and itching sensation occurred 45 min after ingesting 50 mg of purified acetaminophen. Dizziness, shivering, tachycardia and fainting also developed later. These symptoms resolved after treatment with a diphenhydramine injection and intravenous infusion of normal saline. There was a marked increase in the blood histamine level after challenge. In vitro histamine release before oral challenge was also abnormally as high as 50%. In summary, she had an immediate allergic reaction to acetaminophen but was tolerant to acetylsalicylic acid.