Gastric ulcers are ulceration of the lining of the stomach. As their name suggests, duodenal ulcers occur in the duodenum, which is the part of the small intestine immediately following the stomach in the gastrointestinal tract. Collectively, they are referred to as peptic ulcer disease. Their most common symptom is abdominal discomfort or pain. These symptoms typically come and go for several days or weeks, occur two to three hours after eating, and are relieved by eating. Symptoms may at their worst during the night, when the stomach is empty following a meal. Other symptoms include poor appetite, weight loss, bloating, nausea and vomiting. If the disease process is not treated, the ulcer may erode through the entire stomach wall, resulting in the potentially fatal outflow of its contents into the abdomen.

Until relatively recently, peptic ulcers were thought to be the result of stress, which was thought to increase acid secretion within the stomach and hence also the duodenum. More recently, this psychological explanation has been largely superseded by a biological model. This suggests that a bacterium known as Helicobacter pylori (H. pylori) is responsible for the disorder. Helicobacter pylori infection is thought to weaken the protective mucous coating of the stomach and duodenum, allowing acid to get through to the sensitive lining beneath. Both the acid and the bacteria irritate the lining and cause the ulcer.