There are more than 80 specific cutaneous drug reaction patterns in the skin. Adverse cutaneous drug reactions are seen in 2%-3% of inpatients. This synopsis summarizes simple drug eruptions with minimal systemic involvement. Complex drug eruptions with systemic manifestations such as drug hypersensitivity syndrome, Stevens-Johnson syndrome, toxic epidermal necrolysis, and serum sickness-like reaction are discussed in greater detail elsewhere. Drug-induced eruptions should always be considered in the differential diagnosis of any patient on medication presenting with a sudden "rash," particularly in individuals who are on multiple medications or have recently started a new drug or drug preparation.

Drug eruptions are often of unknown etiology and mechanism but always constitute an adverse effect. They may be immunologic or nonimmunologic; not all drug eruptions imply allergy. Possible other causes include metabolic reaction, drug accumulation or overdosage, combined manifestation with a coexistent disease, or interactions with other medications. The most common morphologies seen are morbilliform (95%) and urticarial (5%). Pustular, bullous, and papulosquamous morphologies also occur but are less common. Drug reactions may cause pruritus without an obvious cutaneous manifestation. They occur more commonly in inpatients, the elderly, females, and the immunocompromised.

Drugs and classes of medications frequently reported to cause a simple exanthem include antibiotics (penicillins, cephalosporins, trimethoprim-sulfamethoxazole, quinolones, gentamicin), NSAIDs, angiotensin-converting enzyme (ACE) inhibitors, sulfonamides, anticonvulsants, allopurinol, thiazides, isoniazid, thalidomide, and nelfinavir.