A sore that develops on the lining of the oesophagus, stomach or small intestine. Gastric and duodenal ulcers are both types of peptic ulcers. A duodenal ulcer is a sore that forms in the lining of the duodenum. A duodenal ulcer is usually caused by an infection with a germ (bacterium) called Helicobacter pylori (H. A duodenal ulcer is a peptic ulcer that develops in the first part of the small intestine (duodenum). Gastric and duodenal ulcers are types of peptic ulcer.