Metabolism is a biotransformation process, where endogenous and exogenous compounds are converted to more polar products to facilitate their elimination from the body. The process of metabolism is divided into 3 phases. Phase I metabolism involves functionalization reactions. Phase II drug metabolism is a conjugation reaction. Phase III refers to transporter-mediated elimination of drug and/or metabolites from body normally via liver, gut, kidney, or lung. This review presents basic information on drug-metabolizing enzymes and potential factors that might affect the metabolic capacities of the enzyme or alter drug response or drug-mediated toxicities.