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Drug allergy: Classification and clinical features

Werner J Pichler, MD
Section Editor
N Franklin Adkinson, Jr, MD
Deputy Editor
Anna M Feldweg, MD


A drug allergy, or an allergic drug reaction, is an adverse drug reaction that results from a specific immunologic response to a medication.

The classification and clinical features of drug allergy will be reviewed here, beginning with a categorization of the different types of adverse drug reactions. A detailed discussion of the pathogenesis of drug allergy and an approach to the diagnosis and management of drug allergies are found elsewhere. (See "Drug allergy: Pathogenesis" and "An approach to the patient with drug allergy".)


An adverse drug reaction is a general term referring to any untoward reaction to a medication. Adverse drug reactions may be broadly divided into two types, type A and type B (table 1):

Type A reactions — Type A reactions make up 85 to 90 percent of all adverse drug reactions. These can affect any individual, given sufficient dose and exposure, and are predictable from the known pharmacologic properties of a drug. Examples of type A reactions include diarrhea in response to antibiotics, gastritis in association with long-term use of nonsteroidal antiinflammatory drugs (NSAIDs), or aminoglycoside nephrotoxicity.

Type B reactions — Type B reactions make up 10 to 15 percent of adverse drug reactions. These are hypersensitivity reactions, mediated by immunologic or other types of mechanisms, which occur in a susceptible subgroup of patients, have signs and symptoms that are different from the pharmacologic actions of the drug, and usually cannot be predicted.

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Literature review current through: Nov 2017. | This topic last updated: Dec 03, 2015.
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