idiosynkrasia

Idiosynkrasia, also spelled idiosyncrasy, is a term with two related meanings. In medicine and pharmacology, an idiosyncratic reaction is an uncommon, unpredictable response to a drug or chemical that occurs at a dose that would normally be tolerated and is not explained by known pharmacology. These reactions are not dose-dependent and are thought to arise from individual genetic, metabolic, or immune factors. Idiosyncratic reactions can be severe or even fatal and may require discontinuation of the drug, supportive treatment, or avoidance of the substance. The term is used to distinguish such reactions from ordinary, expected adverse effects and from allergic or hypersensitivity reactions, although immune mechanisms can be involved in some cases.

In a broader sense, idiosyncrasy refers to a peculiarity or distinctive characteristic of an individual or group. It describes habits, preferences, or behaviors that are unusual or unique to a person and not representative of a larger population.

Etymology: from Greek idios meaning "one's own" and synkrasia or krasis meaning "mixture" or "combination." The spelling idiosynkrasia is used in some sources as an alternate form of idiosyncrasy.

Examples: clinically, an idiosyncratic drug reaction may manifest as a skin eruption, liver injury, or hematologic abnormality in a patient who has tolerated similar doses of the drug previously; pharmacogenomic research seeks to explain such variability by identifying genetic risk factors.

See also: adverse drug reaction, pharmacogenomics, genetic polymorphism, hypersensitivity, allergy.