A sore on the lining of the stomach or duodenum (beginning of the small intestine). Peptic ulcers are common — one in 10 Americans develops an ulcer at some time in his or her life. One cause of peptic ulcer is bacterial infection, but some ulcers are caused by long-term use of nonsteroidal anti-inflammatory agents (NSAIDs), like aspirin and ibuprofen. In a few cases, cancerous tumours in the stomach or pancreas can cause ulcers. Peptic ulcers are not caused by stress or eating spicy food.