They usually begin 2 to 10 days after initiation of the drug, but can occur up to 14 days after a drug is stopped. The immunologic mechanism involved in this type of rash is not known. Morbilliform rash is frequently related to antimicrobial therapy. Sometimes a fever, leukemoid reaction, interstitial nephritis, or elevation of serum transaminase indicative of hepatocellular inflammation may accompany this type of rash. Treatment includes discontinuation of the suspected agent, topical or systemic antipruritic therapy, and a two week course of systemic corticosteroid therapy if the reaction is severe.
Most morbilliform rashes resolve with some degree of superficial skin desquamation.
Urticaria Urticaria, which may be associated with angioedema, can occur as a result of drug therapy and will present as evanescent, raised, red, pruritic, well demarcated lesions which disappear without residual scarring or hyperpigmentation. Urticaria due to type III reactions may be accompanied by serum sickness including fever, arthritis, nephritis, and lymphadenopathy. Certain agents such as opiates or radiocontrast media may have direct effects on mast cell membranes inducing degranulation with release of mast cell mediators such as histamine which result in urticaria. Treatment of urticarial drug reactions includes cessation of the offending drug, institution of systemic antihistamine therapy, and the use of corticosteroids if urticaria is severe and unresponsive to antihistamines or if signs of serum sickness are present. The lesions are red, raised, well demarcated plaques which will persist for several days to weeks.