Urticaria is a classic cutaneous manifestation of drug allergy considered like the second most frequent drug eruption after maculopapular exanthemas. Most of the time drugs are responsible of acute urticaria lasting less than 24 hours. The mechanisms of these acute urticarial reactions are multiple, mostly related to an IgE-induced reaction. Nevertheless, some drugs can induce immune complexes and activate the complement cascade (sickness disease). Others may directly release mast cells mediators or activate complement by non immunologic mechanisms in the absence of antibody. In every case, these drugs are unable to generate urticaria during more than 6 weeks, time allowed for calling a chronic urticaria. However drugs like nonsteroidal anti-inflammatory drugs and acetysalicylic acid can, by a pharmacologic mechanism, exacerbate or trigger chronic urticaria. Angiotensin-converting enzyme inhibitors, by a defect of degradation of bradykinin, may also induced angioedemas. In this context, if an allergologic investigation is useful in the exploration of acute urticaria, it seems useless for chronic urticaria.