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Hereditary variations in the handling of a drug (pharmacogenetics) may result in adverse reactions in the skin. Such reactions could result from; (I) an inherited defect in enzymes responsible for drug metabolism (formation or detoxification of potentially toxic metabolites); (2) altered susceptibility of an endogenous metabolic pathway to inhibition by a drug. Increased alcoholdehydrogenase activity or decreased aldehyde dehydrogenase activity will predispose an individual to ethanol-induced flushing. Decreased uroporphyrinogen decarboxylase may result in porphyria cutanea tarda. Slow acetylators are more susceptible to developing drug induced lupus erythematosus. A hypersensitivity syndrome may result if a patient is unable to detoxify the toxic metabolites of a drug such as phenytoin. A pharmacogenetic defect should alert the clinician to the possibility of cross-re activity with other drugs or potential drug reactions in relatives of the patient. [ABSTRACT FROM AUTHOR]