Aci indigestion is a type of indigestion involving an excess of hydrochloric acid in the stomach. Frequent occurrence of acid indigestion can lead to aggravation of the duodenum or to an aggravation of the lining of the stomach, both of which can lead to ulcers which can be life-threatening. However, it is important to note that some 85...95% of all stomach ulcers are caused by infection with the bacterium Helicobacter pylori, and not by excess acidity.