such as diazepam (Valium), lorazepam (Ativan), oxazepam (Serax), and temazepam (Restoril). Other categories of drugs impacted by modified CYP2C19 include proton pump inhibitors, anticonvulsants, hypnotics, sedatives, antimalarial drugs, and antiretroviral drugs. On the basis of their ability to metabolize (S)-mephenytoin or other CYP2C19 substrates, individuals can be classified as ultrarapid metabolizers (UM), extensive metabolizers (EM) or poor metabolizers (PM). In the case of proton pump inhibitors, PMs exhibit a drug exposure that is 3 to 13 times higher than that of EMs. Loss-of-function alleles, CYP2C19*2 and CYP2C19*3 (and other, which are the subject of ongoing research) predict PMs, and the gain-of-function CYP2C19*17 allele predicts UMs. Although the amount of CYP2C19 enzyme produced by the *17 allele is greater than of the *1 allele, whether the carriers of the *17 allele experience any significant difference in response to drugs comparing to the wild-type, is a topic of ongoing research, studies show varying results. Some studies have found that the *17 variant's effect on the metabolism of omeprazole, pantoprazole, escitalopram, sertraline, voriconazole, tamoxifen and clopidogrel is modest, particularly compared to the impact of loss-of-function alleles (*2, *3), therefore, in case of these medications, the EM designation is sometimes applied instead of the UM one. For example, carriers of the *17 allele did not demonstrate different gastric pH comparing to *1 after taking the proton pump inhibitor omeprazole, a CYP2C19 substrate. Other studies concluded that the *17 allele seems to be the factor responsible for lower response to some drugs, even at higher doses, for example, to escitalopram for symptom remission in major depressive disorder patients. CYP2C19*17 carrier status is significantly associated with enhanced response to clopidogrel and an increased risk of bleeding; the highest risk was observed for CYP2C19*17 homozygous patients. A study found that escitalopram serum concentration was 42% lower in patients homozygous for CYP2C19*17. An important limitation of all these studies is the single-gene analysis, since most drugs that are metabolized by CYP2C19 are also metabolized by CYP2D6 and CYP3A4 enzymes. Besides that, other genes are involved in drug response, for example, escitalopram is transported by P-glycoprotein, encoded by the ABCB1 gene. In order for the studies on CYP2C19*17 to be conclusive, the differences in other genes that affect drug response have to be excluded. The prevalence of the CYP2C19*17 variant is less than 5% in Asian populations and is approximately four times higher in European and African populations. The