Gastritis is inflammation or irritation of the stomach lining. It can be caused by some medicines (such as aspirin or other anti-inflammatory medicines), overuse of alcohol, stomach acid, or bacterial infection.

An infection with the bacteria Helicobacter pylori (H. pylori) can cause gastritis. H. pylori is also the bacteria that causes ulcers.

One way to test for H. pylori is to take a tissue sample from the stomach. The doctor uses a thin, flexible, lighted viewing instrument (endoscope) to look down your throat and into your stomach. Looking through the endoscope, your doctor may also see irritation or inflammation in the lining of your stomach. This irritation is gastritis. And a tissue sample will tell the doctor if the cause is H. pylori.