A peptic ulcer is a spot where the lining of the stomach and the tissues beneath--and sometimes part of the stomach muscle itself--have been eroded, leaving an open wound inside the stomach. The surrounding tissue is usually swollen and irritated. An ulcer results when the lining of the stomach fails to provide adequate protection against the effect of digestive acids, and the acids in effect start to digest the stomach itself. Ulcers can occur anywhere along the gastrointestinal tract, but are most common in the stomach (gastic ulcers) and duodenum (duodenal ulcers). They affect approximately 10% of the U.S. population.