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In a CYP2D6 genotyping study on Turkish psychiatric patients by Herken et al., poor metabolizer ratio has been reported as 1.45%, and ultra-rapid metabolizer 10.29%. The sum of the frequency of poor and rapid metabolizers in terms of CYP2D6 activity is about 12.7%. Hence, 12.7% of Turkish population is at risk for R treatment. In slow metabolizers, side effects of drugs occur more, while the drug ineffectiveness arises in fast metabolizers. In evaluation of CYP2D6 enzyme activity, genotyping, phenotyping by prop medication and TDM are methods that are used. Although genotyping for CYP2D6 enzyme is a very important data in terms of drug metabolism, it, alone, is not sufficient in terms of showing enzyme activity. Genotyping is recommended, if a substrate that has quite different metabolism is being used, if the drugs a have narrow therapeutic range (if it has the toxic effects due to genetic differences in metabolism), if the drug or metabolite has an unusual plasma concentration and if genetic factors thought to be responsible, if there is a chronic disorder that needs a lifelong treatment. Additionally, as the enzyme activity can be also affected by environmental factors in addition to genotyping factors, the most effective method for current enzyme activity of the individual is phenotyping. In a phenotyping study performed with dextromethorphan metabolites ratio, Gaedigk et al. reported in 60% of cases polymorphism commonly seen in CYP2D6 and in 40% of the cases rare polymorphisms. Hence, estimating phenotype through genotyping has been insufficient in explaining phenotype. Nowadays, phenotypic research indicating this enzyme’s activity is increasing. For the first time, in 1993, phenotypic investigation performed through deprisguin metabolic way has reported a similarity to R metabolism. In a study investigating the relationship between CYP2D6 enzyme activity and R hydroxylation, deprisguin probe was used and it was reported that phenotyping performed according to deprisguin metabolite and R/9-OHR rates are related and R plasma level might reşect enzyme activity. Today, most effective method for CYP2D6 enzyme phenotyping is dextromethorphan metabolism. According to this, 4 types of phenotyping—namely intermediate metabolizer (ARA), normal metabolizer (NM), fast metabolizer (HM), and slow metabolizer (YM)—was determined. In 1999, Huang et al. has investigated the relationship between DEX phenotyping and R plasma level in 6 cases for the first time, and 7-times decrease in the clearance of R in YM phenotype has been reported. On the other hand, TDM will provide information on pharmacokinetics by showing the drug plasma concentration. A strong correlation between CYP2D6 enzyme activity and R hydroxylation has been reported. According to this, risperidone / paliperidone ratio was adopted as CYP2D6 index. If risperidone/ paliperidone ratio is 01-02 normal range >1: YM or CYP2D6 inhibition has been reported. In another study, in HM cases with risperidone/ paliperidone less than or equal to 0.1, genotyping was performed and the ratio for HM was evaluated as highly sensitive but nonspecific.