A peptic ulcer is an open sore in the lining of the stomach or duodenum (first part of the small intestine). Although less common, it can occur in the lower esophagus (in the event of acid reflux) or even extend as far as the jejunum or ileum of the small intestine (in cases of a Meckel diverticulum). Duodenal ulcers are more common than stomach ulcers and in most cases there is a solitary (single) ulcer.
A peptic ulcer appears as a round to oval punched-out sore in the lining. The bases of the ulcer is smooth and underlying tissue or even blood vessels may be visible upon an upper GI endoscopy. Most peptic ulcers are small shallow ulcers measuring less than 3mm (millimeters) in diameter. Deeper ulcers tend to be large, often over 6mm in diameter.