Background:Acute-on-chronic liver failure (ACLF) is a fatal complication of cirrhosis. Hence, identification of risk factors for ACLF is crucial. Previous studies have linked proton pump inhibitor (PPI) treatment to complications of cirrhosis, however, a possible effect of PPI treatment on the risk of ACLF has not been investigated yet. Therefore, the present study aimed to characterize the impact of PPI treatment on ACLF development.Methods:A total of 642 patients hospitalized due to complications of cirrhosis were retrospectively identified, and PPI treatment during an observation period of 3 years following the hospitalization was reviewed. Subsequently, 74 patients with newly initiated PPI treatment at the time of hospitalization (PPI group) were 1:1 propensity score matched to 74 patients who received no PPI treatment (no-PPI group). Primary end point was the development of ACLF during the observation period, and secondary endpoints were mortality and upper gastrointestinal bleeding.Results:PPI and no-PPI groups had comparably severe chronic liver disease at baseline. Nevertheless, the cumulative incidence of ACLF in the presence of death as competing risk was markedly higher in the PPI group compared with the no-PPI group. ACLF-related deaths contributed significantly to a higher 3-year mortality in the PPI group. Uni and multivariable competing risk regression models confirmed that PPI treatment was an independent predictor of ACLF in the study collective (subdistribution HR: 1.892, 95% CI: 1.092–3.281,p= 0.023). The impact of PPI treatment on ACLF development was particularly strong in patients with a model for end-stage liver disease score >12. Upper gastrointestinal bleeding was slightly less frequent in the PPI group.Conclusions:The present results indicate that PPI treatment could be a risk factor for ACLF in patients with advanced cirrhosis.