A peptic ulcer is a sore in the lining of your stomach or duodenum *. The duodenum is the first part of your small intestine. A peptic ulcer in the stomach is called a gastric ulcer. One that is in the duodenum is called a duodenal ulcer. A peptic ulcer also may develop just above your stomach in the esophagus, the tube that connects the mouth to the stomach. But most peptic ulcers develop in the stomach or duodenum.