As these CYP2C19 predictive phenotypes via CYP2C19 haplogenotypes (rs12248560/rs4244285) can be relatively easily determined using the current standard equipment of hospital laboratories, we prepared a set of recommendations for personalized PPI-based stress ulcer prophylaxis taking into account the patient's CYP2C19 predictive phenotype determined in this way. The gene discussed is CYP2C19; the disease is duodenal ulcer.