Multiple H1‐antihistamine‐induced urticaria

H 1 -antihistamines are widely used in the treatment of various allergic diseases. Particularly, a cornerstone of the management of chronic idiopathic urticaria is treatment with H 1 -antihistamines. However, a few cases of Hi-antihistamine-induced urticaria have been reported. A 34-year-old woman presented with a 4-month history of recurrent urticaria, which was prominently exacerbated by the administration of H 1 -antihistamines. The patient consented to a provocation test of fexofenadine among drugs including cetirizine and hydroxyzine, which were suspected of inducing severe symptoms in episodes. One hour after challenge with 12 mg fexofenadine (one-fifth of the therapeutic dose), a urticarial reaction rapidly developed on nearly the entire body with remarkably increased levels of plasma histamine (190 nmol/L) and plasma leukotriene B4 (150 pg/mL). In challenge tests with other antihistamines, generalized urticaria occurred 5 and 1 h after intake of 10 mg loratadine and 10 mg bepotastine, respectively, whereas challenges with chlorpheniramine, mequitazine and azelastine were all negative. Skin prick tests with H 1 -antihistamines used in the challenges were all negative, indicating that the urticarial reactions after challenges with the causative drugs might not be immunoglobulin E-mediated. Among the causative drugs in our case, cetirizine and hydroxyzine are the piperazine derivatives, whereas fexofenadine, bepotastine, ebastine and loratadine are the piperidine derivatives. The chemical structures of both derivatives are very similar. Therefore, in this case, H 1 -antihistamine-induced urticaria may have been due to cross-reactivity between metabolites of these drugs, but not to drugs before metabolization. Hypersensitivity to H 1 -antihistamines should be considered when urticarial lesions worsen after H 1 -antihistamine treatment.