The cytochrome P450 (CYP) is a group of metabolic enzymes, from which the 2D6 and CYP2C19 polymorphisms are specifically related to the metabolism of psychiatric drugs. The prevalence of CYP2D6 and CYP2C19 polymorphisms differs among ethnicities. Depending on the number of functional alleles, individuals are classified as Poor Metabolizer (PM), Intermediate Metabolizer (IM), Extensive Metabolizer (EM) or Ultra Rapid Metabolizer (UM).