Inhibitors of some cytochrome P450s (CYPs) are used to design target-specific drugs. CYPs belonging to families 1-4 play important roles in drug metabolism and therapeutics. Some isozymes of CYPs also activate pro-carcinogens into their carcinogenic forms. Approximately 40-50% human CYP-dependent drug metabolism is carried out by polymorphic CYPs resulting in therapeutic failure and adverse reactions. Phenotyping of CYPs involved in the metabolism of a drug is important to understand the potential of clinical interactions and to predict the possible individual variations due to genetic polymorphisms of certain CYPs. (C) 2009 Elsevier B.V. All rights reserved.