n. the study of genetic factors that influence the response of individuals to different drugs and to different dosages of drugs. Inherited variations in enzymes or other metabolic components can affect the efficacy of a drug or cause adverse reactions to normal doses. For example, some 40% to 70% of Caucasians have an enzyme variant that causes them to metabolize the antituberculosis drug isoniazid very slowly. They require only a fraction of the standard dose. Also, there are genetic variants of various cytochrome P450 enzymes that can predict rapid or poor drug metabolism. See also pharmacogenomics.