The gastric enzyme hydrogen potassium ATPase, also known H⁺/K⁺-ATPase, is located in parietal cells of gastric mucosa. The enzyme plays an important role in gastric hydrochloric acid production. Proton pump inhibitors are a group of medicine indicated in the treatment of gastric and duodenal ulcers, eradication therapy associated with Helicobacter pylori infections and reflux oesophagitis as well as Zollinger Ellison syndrome. These drugs are characterized by only a few and not severe adverse effects, which might result in overuse. In this paper we consider possible consequences of long-term proton pump inhibitor therapy.