Gastritis is an inflammation, irritation, or erosion of the lining of the stomach. It can occur suddenly (acute) or gradually (chronic). What Causes Gastritis? Gastritis can be caused by irritation due to excessive alcohol use, chronic vomiting, stress, or the use of certain medicationssuch as aspirin or other anti-inflammatory drugs. It may also be caused by any of the following: Helicobacter pylori (H. pylori): A bacteria that lives in the mucous lining of the stomach; without treatment, the infection can lead to ulcers, and in some people, stomach cancer. Bile reflux: A backflow of bile into the stomach from the bile tract (that connects to the liver and gallbladder) Infections caused by bacteria and viruses If gastritis is left untreated, it can lead to a severe loss of blood and may increase the risk of developing stomach cancer.