Many people think that a duodenal ulcer is caused by too much acid coming from the stomach. This is not true. Most people with a duodenal ulcer make a normal amount of acid. The problem is the way the lining of the duodenum copes with the acid. The duodenum makes chemicals and mucus which covers the surface and protects the tissues from the acid. An ulcer occurs if the acid breaks through this protection. The causes of duodenal ulcers include the following.

Infection by H. pylori is the cause in about 19 in 20 cases. This bacterium (bug) affects the lining of the duodenum in some way which allows the acid to cause inflammation and ulcers.
Anti-inflammatory medicines which many people take for arthritis, muscular pains, etc. For example: aspirin, ibuprofen, and diclofenac - but there are others. These medicines sometimes affect the lining of the duodenum and allow acid to cause an ulcer.
Other causes are rare. For example, the Zollinger-Ellison syndrome. In this rare condition, much more acid than usual is made by the stomach.
Other factors such as smoking, stress, and drinking heavily may possibly increase the risk of having a duodenal ulcer. However, these are not the underlying cause of duodenal ulcers.