Background: Proton pump inhibitors (PPI) are widely used to treat acid-related disorders of the upper gastrointestinal tract. However, large observational studies have raised concerns about PPI-associated adverse events. In recent years, data from next-generation sequencing studies suggested that PPIs affect the composition of the intestinal microbiota, while a balanced gut microbiome is essential for maintaining health.Aim: To review the available evidence from next-generation sequencing studies on the effect of PPIs on the intestinal microbiome and to discuss possible implications of PPI-induced dysbiosis in health and disease.Methods: A systematic review was conducted following the recommendations of the Preferred Reporting Items for Systematic reviews and Meta-Analyses statement. A PubMed query yielded 197 results. 19 publications met the prespecified eligibility criteria.Results: Twelve observational study cohorts with 708 PPI users and 11 interventional cohorts with 180 PPI users were included in the review. In most studies, PPI treatment did not affect microbiological richness and diversity, but was associated with distinct taxonomic alterations: In the upper gastrointestinal tract, PPI users showed overgrowth of orally derived bacteria, mostly Streptococcaceae (findings based on six independent cohorts with 126 PPI users). In faecal samples, PPIs increased multiple taxa from the orders Bacillales (eg, Staphylococcaceae), Lactobacillales (eg, Enterococcaceae, Lactobacillaceae, Streptococcaceae) and Actinomycetales (eg, Actinomycetaceae, Micrococcaceae), the families Pasteurellaceae and Enterobacteriaceae and the genus Veillonella. Taxa decreased by PPIs include Bifidobacteriaceae, Ruminococcaceae, Lachnospiraceae and Mollicutes (findings in faecal samples based on 19 independent cohorts with 790 PPI users). Conclusion: PPI use is associated with moderate alterations to upper and distal gut microbiota. The available data suggest that PPI-induced hypochlorhydria facilitates colonization of more distal parts of the digestive tract by upper gastrointestinal microbiota. MACKE Et Al 1 | INTRODUC TI ON: G A S TRI C ACID INHIB ITI ON AND ADVER S E EFFEC TS OF PPIS Gastric acid eliminates microorganisms, promotes the digestive process and facilitates the absorption of iron, calcium and vitamin B12. However, gastric acid also contributes to the development of mild and severe gastroduodenal pathologies. Proton pump inhibitor (PPIs) are the most effective therapy of acid-related diseases and are among the most commonly used drugs. 1 While generally considered a safe therapy, PPIs have been associated with numerous adverse effects in population-based observational studies conducted in recent years. These include cardiovascular and kidney disease, micronutrient deficiencies and osteoporosis, cognitive impairment and death. 2Most studies are retrospective and not hypothesis-driven in design, might be subject to significant confound and thus they cannot establish causality. 2,3 This is also the case for a recent study on ...