Peptic ulcers are primarily caused by stress and lifestyle.

For much of the twentieth century, peptic ulcers were blamed mainly on excess stomach acid, stress, diet, smoking, and sometimes personality. Treatment therefore focused on antacids, bland diets, stress reduction, and in severe cases surgery.

In the early 1980s, pathologist Robin Warren repeatedly saw curved bacteria in inflamed stomach tissue. Barry Marshall joined him in studying the organism, later named Helicobacter pylori. The idea was surprising because the stomach was considered too acidic for bacteria to live there for long.

The evidence became hard to dismiss. H. pylori was strongly associated with chronic stomach inflammation and ulcer disease, and eliminating the infection with antibiotics greatly reduced the chance that ulcers would return.

A second major cause also became clear: nonsteroidal anti-inflammatory drugs, or NSAIDs, can weaken the protective lining of the stomach and duodenum and cause ulcers without H. pylori.

Stress and lifestyle can still affect symptoms and healing, and severe physical stress can contribute to ulcers in critically ill patients. But for most peptic ulcers, the main causes are now understood to be H. pylori infection and NSAID use.