Seasonal incidence of phenytoin allergy unrelated to plasma levels.

In a three-year prospective study of patients to whom phenytoin was administered, 26 (8.5%) of 306 patients manifested an erythematous morbiliform rash within three weeks of onset of therapy. Occurrence of the rash was not related to the mode of administration of treatment (loading v maintenance) or initial phenytoin levels. A striking seasonal incidence of the rash was noted: None of the 79 persons who received the initial dose during December to February had a reaction, whereas 13 (20.6%) of 63 persons treated during June to August manifested the rash. For March to May, the rate was 10% (8/88), and for September to November, 6.7% (5/76). Monthly rates were significantly different by chi 2 test.