Individualization of pharmacotherapy is essential in order to optimize efficacy and minimize toxicity, especially for compounds with narrow therapeutic index. Cytochrome P450 phenotyping has been a valuable research tool and a way of assessing the genetic basis of metabolic capacity. Technique depends on estimating metabolic capacity based on conclusions drawn from another probe drug. Phenotyping allows estimation of the total influence of drug interactions, genetic polymorphisms, hepatic diseases and other factors altering pharmacokinetics. This requires the use of selective substrates for specific cytochrome enzymes. Recently some phenotyping methods are becoming widely used especially for the in vivo evaluation of multiple cytochrome enzymes by using probe cocktails. [Anadolu Psikiyatri Derg 2014; 15(4.000): 358-364]