Overview of cutaneous drug reactions

Adverse drug reactions commonly affect the skin. Although cutaneous reactions are more likely to be caused by certain drug classes, almost any drug can cause a skin reaction—causation is often difficult to prove.

Adverse drug reactions can be:

immediate hypersensitivity reactions—these are rapid in onset (typically developing within minutes of or up to 2 hours after drug exposure). Also see Anaphylaxis, Urticaria and Angioedema.
delayed hypersensitivity reactions—these are more delayed in onset (typically developing hours or days after first or subsequent drug exposure).

Delayed hypersensitivity reactions will be the focus of this topic. See Assessment of cutaneous drug reactions for information on determining whether a reaction is severe or less severe.

For general information on types of antimicrobial hypersensitivity reactions (including immediate immune-mediated [IgE] and delayed immune-mediated [T-cell] hypersensitivity reactions), see here. Over 90% of reported penicillin allergies can be excluded by skin testing and oral provocation—for more information on how to assess and manage patients reporting hypersensitivity to penicillins, see here.

Note: Over 90% of reported penicillin allergies can be excluded by antibiotic allergy testing.

Pharmacogenetic studies are identifying an increasing number of genes that predispose patients to cutaneous drug reactions. Some human leukocyte antigen (HLA) phenotypes are associated with severe cutaneous drug reactions. Testing may be needed before starting therapy with commonly implicated drugs (eg carbamazepine, oxcarbazepine, allopurinol, abacavir). For advice on testing recommendations before starting therapy, refer to the product information of these drugs.