Gastritis is inflammation of the lining of the stomach. It may occur as a short episode or may be of a long duration. There may be no symptoms but, when symptoms are present, the most common is upper abdominal pain (see dyspepsia). Other possible symptoms include nausea and vomiting, bloating, loss of appetite and heartburn. Complications may include stomach bleeding, stomach ulcers, and stomach tumors. When due to autoimmune problems, low red blood cells due to not enough vitamin B12 may occur, a condition known as pernicious anemia.

Common causes include infection with Helicobacter pylori and use of nonsteroidal anti-inflammatory drugs (NSAIDs). Less common causes include alcohol, smoking, cocaine, severe illness, autoimmune problems, radiation therapy and Crohn's disease. Endoscopy, a type of X-ray known as an upper gastrointestinal series, blood tests, and stool tests may help with diagnosis. The symptoms of gastritis may be a presentation of a myocardial infarction. Other conditions with similar symptoms include inflammation of the pancreas, gallbladder problems, and peptic ulcer disease.

Prevention is by avoiding things that cause the disease. Treatment includes medications such as antacids, H2 blockers, or proton pump inhibitors. During an acute attack drinking viscous lidocaine may help. If gastritis is due to NSAIDs these may be stopped. If H. pylori is present it may be treated with a combination of antibiotics such as amoxicillin and clarithromycin. For those with pernicious anemia, vitamin B12 supplements are recommended either by mouth or by injection. People are usually advised to avoid foods that bother them.

Gastritis is believed to affect about half of people worldwide. In 2013 there were approximately 90 million new cases of the condition. As people get older the disease becomes more common. It, along with a similar condition in the first part of the intestines known as duodenitis, resulted in 50,000 deaths in 2015. H. pylori was first discovered in 1981 by Barry Marshall and Robin Warren.