Duodenal ulcer. Gross clinical specimen of part of a human duodenum (small intestine) with a peptic ulcer at centre. Ulcers are caused by excessive stomach acid (digestive juice). This acid, often combined with excess alcohol and caffeine, attacks the gastrointestinal tract, causing pain and discomfort. If the wall's protective mucus has become thinner, possibly due to mental stress or bacterial attack, the acid can eventually perforate it. Ulcers can be treated with antacid drugs that neutralise the stomach acid, but surgery may be required if the ulcer persists.