DESCRIPTION
Common. Many different clinical patterns. Can mimic various dermatoses.

HISTORY
Most patients are on multiple medications. Typical sequence begins with fever followed by a rash several hours later. Rash starts on face and torso, spreading to extremities. Reaction can occur after weeks or years without ill effect, but once sensitiza­tion occurs a reaction may occur within minutes to 24-48 h. Chemically related drugs may cross-react.

Different drug reaction patterns.

• Morbilliform: most common drug eruption, begins 7-10 days after starting drug, lasts 1-2 weeks.
• Urticarial: frequent causes—aspirin, penicillin, blood products; IgE-mediated—immediate reaction; immune complex-mediated (serum sickness) 4-21 days after drug ingestion.
• Internal-external reactions: topical sensitization to a drug results in rash after oral intake of drug.
• Erythema multiforme and toxic epidermal necrolysis.
• Exfoliative erythroderma.
• Fixed drug reactions: appear soon after exposure and occur at same site after exposure; drug-induced hyperpigmentation.
• Lichenoid drug reactions: latent period 3 weeks to 3 years.
• Photosensitive reactions: phototoxic (rash within 24 h of sun exposure), photoallergic (rash after 48 h of exposure).
• Small vessel necrotizing vasculitis.
• Chemotherapy-induced acral erythema: caused by cytosine arabinoside, fluorouracil, doxorubicin.
• Acute generalized exanthematous pustulosis: rash within 5 days of ingestion; resolves in 15 days; penicillin most common.

PHYSICAL FINDINGS

• Morbilliform: red macules and papules that become confluent; looks like a viral exanthem; starts on trunk, spreads to extremities; can involve mucous membranes, palms, and soles; often spares the face.
• Urticaria: hives, usually generalized.
• Internal-external: eczematous reaction, particularly in axillae and groin.
• Erythema multiforme and toxic epidermal necrolysis reactions: target lesions and widespread blistering and skin exfoliation; can involve mucous membranes.
• Exfoliative erythroderma: generalized redness and scaling.
• Fixed drug reactions: single or multiple round dusky red plaques.
• Drug-induced hyperpigmentation: brown to grey patches that can be in photodistributed areas (amiodarone), scars (minocycline), mucosal (zidovudine), flagellate pattern (bleomycin), melasma (oral contraceptives).
• Lichenoid drug reactions: multiple flat-topped, itchy, violaceous papules; may have oral involvement.
• Photosensitive: phototoxic—erythema and skin peeling confined to sun-exposed sites (can cause onycholysis); photoallergic—rash can spread to non-sun-exposed areas.
• Small vessel necrotizing vasculitis: palpable purpura (especially lower legs).
• Chemo­therapy acral erythema: tingling on the palms and soles followed in a few days by painful, symmetric, well-defined swelling and erythema.
• Acute generalized exanthematous pustulosis.

TREATMENT
Identify responsible drug and switch to chemically dis­similar drug. Symptomatic and supportive treatments.