Interindividual variability in oral drug efficacy and toxicity is commonly observed in all therapeutic areas. Importantly, interindividual variability in drug uptake and metabolism can result in poor drug response, adverse drug reactions, or unfavorable drug-drug interaction. One of the common causes of individual variations in drug response is genetic variation of drug transporters and metabolizing enzymes. Pharmacogenetics are rapidly elucidating the inherited nature of these differences in drug disposition and effects, thereby providing a stronger scientific basis for optimizing drug therapy on the basis of each patient's genetic constitution. Knowledge of the genotype-phenotype correlation and frequency distribution of functional single nucleotide polymorphisms may be a valuable tool for individualizing drug therapy. This information can also be useful for explaining inter-individual and inter-ethnic variations in drug response and/or adverse effects. In this review, we focus on the interplay between efflux transporter (ATP-binding cassette, sub-family B (MDR/TAP), member 1/ABCB1) and cytochrome P450s according to genetic polymorphism.