For most patients, gastritis symptoms are not severe and it is not a serious condition. The most common symptoms are pain or discomfort in the upper abdomen and indigestion. Acute gastritis generally causes sudden symptoms, whereas patients with chronic gastritis often do not exhibit any symptoms.

The pain involved is frequently a gnawing or burning ache, usually located in the upper abdomen. However, it may extend up the chest through the esophagus. The severity of the pain varies widely. Very mild pain or discomfort may occur in acute stress gastritis. Similar or sharper pain occurs most commonly in erosive gastritis, atrophic gastritis and Ménétrier's disease.

indigestion (nonulcer dyspepsia) is another common symptom, but not as common as pain or discomfort. Indigestion is most common in erosive gastritis, atrophic gastritis, plasma cell gastritis and Ménétrier’s disease. Indigestion may encompass several symptoms, including:

• Nausea and vomiting
• Loss of appetite and weight
• Diarrhea
• Belching or bloating
• Feeling of fullness after eating small amounts
• Heartburn
• Hiccups

Gastritis is a major cause of upper gastrointestinal bleeding, though this is rarely serious unless accompanied by ulcers. Bleeding in the stomach may cause hematemesis (vomiting blood) or melena (tarry black stool). It occurs most frequently in erosive gastritis and Ménétrier’s disease. Stress-induced gastritis has a particularly high risk of bleeding, which may be particularly extensive or even life-threatening. Bleeding may begin within just a few days in acute stress gastritis. In other forms, it tends to develop more gradually.

Fluid retention and edema (swelling) may occur due to the loss of proteins from inflamed stomach tissue, especially in Ménétrier’s disease. Anemia (low levels of red blood cells) may also occur. This may be due to bleeding or low levels or absence of vitamin B12, which is important in the production of red blood cells. Deficiency in vitamin B12 due to autoimmune attack (in which antibodies attack the body's own tissues) on the stomach is known as pernicious anemia and may occur as a result of decreased production of a gastric protein that is necessary for B12 absorption. Pernicious anemia is usually associated with atrophic gastritis. Symptoms of both pernicious anemia and anemia due to bleeding include fatigue, weakness and light-headedness.

People experiencing the symptoms of gastritis for more than two or three consecutive days should consult with a physician. Individuals who notice blood in their vomit or stools should contact their physician immediately.