Objective: Proton pump inhibitors (PPIs) are administered to patients with acid peptic disorders, such as gastroesophageal reflux disease. The efficiency of treatment by most PPIs largely depends on the cytochrome P450 2C19 (CYP2C19). The activity of this enzyme, genetically determined by the CYP2C19*2 (rs4244285) and CYP2C19*17 (rs12248560) allele variants, can be used to predict the efficacy of this enzyme and, thus, the rate of PPI metabolization. Nevertheless, the phenotype of a novel group of „ambivalent“ metabolizers with both the CYP2C19*2 and CYP2C19*17 variants is unknown. This work aims to evaluate the distribution of metabolizers, mainly focusing on the „ambivalent“ metabolizers in the Central European population and to determine their CYP2C19 metabolic activity. Methods: 545 patients were included in our study; of these, 58 used lansoprazole (Lanzul, 30 mg). Blood samples were drawn to 9 ml K3EDTA and genomic DNA was isolated. Genotypes of two variants CYP2C19*2 and *17 were determined using qPCR and haplogenotypes were evaluated. In patients using lansoprazole, blood samples were taken 3 hours after oral administration of the drug, plasma was transferred into 1.5 ml tubes immediately and stored at -80°C. A therapeutic drug monitoring approach was used to determine the patient‘s phenotype. Plasma concentrations of lansoprazole and its metabolite 5-hydroxylansoprazole were measured by liquid chromatography with absorbance or mass spectrometry detection. Results: Minor allele frequencies in the studied population were 14.7% for the CYP2C19*2 and 25.7% for the CYP2C19*17 variants. Individuals without any variant allele, known as extensive metabolizers (*1*1/*1*1; 36.2%), were the most common. Ultrarapid and rapid metabolizers (*1*17/*1*1 and *17*17/*1*1) were represented by 36.0%. Intermediate metabolizers were less common (*1*1/*1*2; 18.2%) and only 1.5% of participants were poor metabolizers (*1*1/*2*2). 8.3% of participants were shown to be „ambivalent“ metabolizers (*1*17/*1*2), a group whose phenotype was not known. Based on the metabolization rate, the phenotype of ambivalent metabolizers resembled that of extensive metabolizers evaluated in this study. There was no association between concentration of lansoprazole/5-hydroxylansoprazole and metabolic ratio/sex/smoking/alcohol consumption (p > 0.05). Conclusions: This study revealed that ambivalent metabolizers are relatively common in the Central European population and should be considered when discussing the rate of metabolization of PPIs. Their phenotype seems to be close to that of extensive metabolizers, which can play an important role in personalized pharmacotherapy by PPIs in these patients.