An ulcer is essentially an open sore on the mucosal layer of the esophagus, stomach or small bowel. Also known as a peptic ulcer, duodenal ulcers cause the deterioration of the lining of the stomach or duodenum. The duodenum is the first section of the small intestine. The symptoms and complications will depend on the size of the ulcer. Smaller ulcers generally do not cause problems; however, larger ulcers can cause bleeding or perforation.