Peptic ulcer. Illustration of a peptic ulcer in the wall of the human duodenum. The black tube is an endoscope, a viewing device used by doctors to study the inside of the gut. The tip of the endo- scope has just passed through the pyloric sphinc- ter, the muscular aperture that separates the stomach (right) from the duodenum (left). Peptic ulcers are painful open sores that can occur in the stomach or duodenum. They are caused by acidic conditions and the bacteria Helicobacter pylori. They sometimes lead to life-threatening complica- tions, such as internal bleeding or infection of the membrane surrounding the intestines (peritoni- tis). Treatment includes antacid and antibiotics.