Stomach ulcer. Gastroscope (endoscope) view of a perforated gastric (peptic) ulcer of a 33 year-old male patient. Ulcers are caused by excessive stomach acid (digestive juice). This acid, often combined with excess alcohol and caffeine, attacks the stomach wall, causing pain and discomfort. If the wall's protective mucus becomes thinned, possibly due to mental stress or bacterial attack, the acid can perforate the stomach wall. A perforated ulcer can result in anaemia, bloody faeces (melaena) & vomiting blood (haematemesis).