The characteristic pharmacological profile associated with a specific drug is due to the physiological and biochemical makeup of cells targeted by specific drug receptors, but extrinsic and intrinsic influences that impinge on targeted cells can cause a remarkable degree of variation in response. Variations are best considered in the light of the drug's pharmacokinetics and pharmacodynamics, mechanisms that define the disposition and actions of drugs in individuals. Mechanisms that involve interactions between the xenobiotic and specific enzymes or receptors targeted by the xenobiotic are usually pertinent to variations of interest. Identification of the pharmacological mechanism responsible for an unusual variation in drug response can be a formidable task, but performance of a pharmacokinetic analysis in susceptible persons can often yield preliminary insights into the relative importance of the pharmacokinetic and pharmacodynamic phases of the response.

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