Acid Peptic Disease (APD) refers to a group of disorders caused by the damaging effects of gastric acid and pepsin on the mucosal lining of the stomach and duodenum. It commonly includes conditions such as peptic ulcer disease, gastroesophageal reflux disease (GERD), and gastritis.

The disease results from an imbalance between aggressive factors (gastric acid, pepsin, Helicobacter pylori, NSAIDs) and defensive mechanisms (mucus–bicarbonate barrier, prostaglandins, mucosal blood flow).