Allergic reaction to antipyrine, a marker of hepatic enzyme activity

Antipyrine is a pyrazole derivative used extensively as a marker for hepatic drug metabolizing enzyme activity. A subject participating in a clinical trial employing antipyrine experienced an acute allergic reaction to the drug which was characterized by diaphoresis, flushing, swelling of the throat, difficulty in breathing, vomiting, swelling of the upper lip, and a diffuse urticarial rash. Intravenous administration of diphenhydramine markedly improved the symptomatology. Clinical investigators as well as study participants should be alerted to the potential for antipyrine to cause an acute allergic reaction.