A qualitatively abnormal or unusual response to a drug which is unique, or virtually so, to the individual who manifests the response. "Idiosyncratic Response" usually applies to a response which is not allergic in nature and cannot be produced with regularity in a substantial number of subjects in the population , and which is ordinarily not produced in a greater intensity in an individual, or in a greater fraction of the population, by the expedient of increase in the dose. In other words, were frequency or intensity of idiosyncratic response used as a measure of effect in constructing a dose-effect curve, a curve might indeed be constructed, but its slope would be found to be 0 (zero), indicating that effect was not significantly a function of dose. In practice, the mechanism of production of an idiosyncratic response is unknown; once the mechanism is known, the response can usually be classified in some other way.
Cf. Toxic effect , Side effect , Allergic response

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