Indigestion may be caused by stomach acid coming into contact with the sensitive, protective lining of the digestive system (mucosa). The stomach acid breaks down the lining, leading to irritation and inflammation, which can be painful. The majority of people with indigestion don't have inflammation in their digestive system. Therefore, their symptoms are thought to be caused by increased sensitivity of the mucosa (to acidity or stretching). In most cases indigestion is related to eating, although it can be triggered by other factors such as smoking, drinking, alcohol, pregnancy, stress or taking certain medications.