An ulcer is an open sore, or lesion, usually found on the skin or mucous membrane areas of the body. An ulcer in the lining of the stomach or duodenum, where hydrochloric acid and the digestive enzyme called pepsin are present, is called a peptic ulcer. Peptic ulcers occur when the mucous lining of the stomach or duodenum is not sufficient to protect them against the corrosive action of stomach acid, pepsin, or other aggressive substances.

Ulcers affect about 5 million Americans each year, and more than 40,000 people annually have ulcer-related surgery. More often than not, ulcers occur as a result of an inflammation of the stomach lining called gastritis (when it is the duodenum that gets inflamed, the condition is called duodenitis). Each year, approximately 15,000 people in the US die of ulcer-related complications, the worst of which are an internal bleeding and a phenomenon called perforation. A perforation occurs when an ulcer eats a hole in the wall of the stomach or duodenum, releasing bacteria and partially digested food through the opening into the sterile abdominal cavity and causing peritonitis - an inflammation of the abdominal cavity and wall.

In the past, it was thought that lifestyle factors, such as alcohol abuse, stress, and smoking, cause peptic ulcers and gastritis. Later, it was determined that excessive amounts of stomach acids - hydrochloric acid and pepsin - contributed to ulcer formation. It was also thought that ulcers and gastritis may be caused by bacteria called Helicobacter pylori (H. pylori), although many researchers suggest that the presence of these bacteria in the majority of peptic ulcer patients does not necessarily mean that the bacteria cause ulcers, rather than just add to the severity of the problem. In fact, there is extensive and mounting evidence of the fact that all of the above-mentioned possible causes of ulcers and gastritis merely compound the real underlying cause: free radical damage and oxidative stress.