Gastritis itself doesn’t always change how your stool looks. Many people with mild stomach lining inflammation have completely normal bowel movements. But when gastritis becomes severe enough to cause bleeding, the most telling sign is black, tarry stool, a condition called melena. In some cases, you may also notice red or maroon blood mixed into your stool.
Black, Tarry Stool and What Causes It
The signature stool change from gastritis is a black, sticky, tar-like consistency. This happens when blood from the inflamed or eroded stomach lining travels through the entire digestive tract before being passed. As blood moves through the intestines, bacteria break it down over several hours, turning it dark black. It takes roughly 50 milliliters of blood in the stomach (about 3 tablespoons) to produce visibly black stool. The first black stool typically appears 4 to 20 hours after the bleeding starts.
This type of stool has a distinct look and smell. It’s not just dark brown; it genuinely resembles tar, both in color and texture. It tends to be loose, shiny, and very sticky. The smell is notably foul, different from a normal bowel movement. If you’ve taken Pepto-Bismol or iron supplements recently, those can also darken your stool, so it’s worth ruling those out before assuming the worst.
Red or Maroon Blood in Stool
Less commonly, gastritis can produce red or maroon-colored blood in your stool. This typically signals heavier, faster bleeding where the blood moves through the intestines too quickly to be fully broken down. The National Institute of Diabetes and Digestive and Kidney Diseases lists both black/tarry stool and red or maroon blood as signs that the stomach lining is actively bleeding. Either appearance warrants immediate medical attention.
Why Erosive Gastritis Is Different
Not all gastritis causes visible stool changes. The type that does is erosive gastritis, where the inflammation is severe enough to wear away the protective stomach lining, creating small open sores or ulcers. These raw spots can bleed, and that blood is what ultimately shows up in your stool.
Non-erosive gastritis, the milder and more common form, inflames the lining without breaking it open. Your stool will likely look normal in this case, even though you might still have stomach pain, nausea, bloating, or a burning feeling in your upper abdomen. The absence of visible stool changes doesn’t mean gastritis isn’t present; it just means the lining hasn’t started bleeding.
Common Causes That Affect Stool
Two of the most frequent triggers for erosive gastritis are regular use of anti-inflammatory painkillers (like ibuprofen and aspirin) and heavy alcohol consumption. Both damage the stomach’s protective barrier, and using them together compounds the risk. NSAIDs suppress the chemicals your stomach uses to maintain its lining, while alcohol directly irritates the exposed tissue.
Alcohol also disrupts the muscle movements throughout your digestive tract, which can produce diarrhea alongside gastritis symptoms. It slows stomach emptying while speeding transit through the intestines, leading to looser, more frequent stools. In people who drink heavily, this combination of diarrhea and potential stomach bleeding means stool changes can be a mix: loose or watery with a dark or unusual color.
H. pylori, a bacterial infection that’s the most common cause of chronic gastritis worldwide, can also eventually erode the stomach lining enough to cause bleeding. The Mayo Clinic lists bloody or black, tarry stools as a key warning sign of H. pylori infection that has progressed to this stage.
Hidden Blood You Can’t See
Gastritis sometimes causes slow, low-level bleeding that isn’t enough to visibly change your stool’s appearance. Your poop may look completely normal in color and consistency, yet still contain small amounts of blood. This is called occult (hidden) bleeding, and it can only be detected through lab testing.
Doctors use stool tests to check for this. The most common are guaiac-based tests, which detect a component of blood called heme. These tests can pick up blood from the stomach, though they aren’t perfect. Newer antibody-based stool tests are designed to catch blood from the lower digestive tract and won’t reliably detect stomach bleeding at all. If your doctor suspects gastritis-related bleeding, the type of stool test matters. Over time, even small amounts of hidden blood loss can lead to iron-deficiency anemia, causing fatigue, shortness of breath, and lightheadedness.
Other Stool Changes With Gastritis
Beyond color, gastritis can occasionally affect stool consistency. Some people experience looser stools or mild diarrhea, particularly when the underlying cause is alcohol use or an infection. Others notice no change at all. Gastritis is primarily a stomach condition, so its main symptoms tend to be upper abdominal pain, nausea, bloating, loss of appetite, and a burning or full feeling after eating. Stool changes are generally a later sign that the condition has progressed.
What These Stool Changes Mean
Normal stool during gastritis is common and simply means the lining isn’t bleeding. Mildly looser stools can be a secondary effect, especially with alcohol-related gastritis, and aren’t immediately dangerous on their own.
Black, tarry stool or visible red/maroon blood is a different situation entirely. These indicate active bleeding in the digestive tract and need prompt medical evaluation. Other symptoms that often accompany this stage include stomach cramps, vomit that looks like coffee grounds, dizziness, and unusual fatigue. If your stool suddenly turns black and sticky with no obvious explanation like iron supplements or Pepto-Bismol, that’s a signal to get help quickly rather than wait to see if it resolves.