Stool that is almost black, dark or tarry with a thick consistency may be an indication of bleeding in the upper part of the gastrointestinal tract. The most common medical conditions that cause dark, tar-like stool includes duodenal or gastric ulcer, esophageal varices, Mallory-Weiss tear (which can be linked with alcoholism) and gastritis.
Certain foods, supplements, and medications can temporarily turn stool black. These include:
- Bismuth compounds (e.g. Pepto-Bismol)
- Activated charcoal supplements
- Aspirin and NSAIDS (which can cause bleeding in the stomach)
- Dark foods such as black licorice, blueberries, Oreo cookies, blackberries, grape juice, or blueberries
Dark stool can also occur with constipation. Dark green stool from bile may look black in certain lighting.
If you experience this type of stool and it is not from food or supplements, you should see your doctor as soon as possible.
Bright Red Stool
When there is blood in stool, the color depends on where it is in the digestive tract. Blood from the upper part of the gastrointestinal tract, such as the stomach, will look dark by the time it exits the body as a bowel movement. Blood that is bright red, on the other hand, is more likely to come from bleeding in the lower intestinal tract, such as the large intestine or rectum due to arteriovenous malformations, hemorrhoids, anal fissures, ulcerative colitis, diverticulosis, or colon cancer.
Bright red stool is commonly caused by red food coloring (e.g. red or grape Kool-Aid or other drink mixes, gelatin, ice pops, red candy, red frosting, red licorice), beets, cranberries, or tomato juice or soup. Red medicines such as Amoxicillin may also turn stool red.
Blood in stool doesn't always appear bright red. Blood may be also present in stool but not visible, called "occult" blood. A test called the Fecal Occult Blood Test is used to detect hidden blood in stool.
Note: Speak with your doctor about any change or abnormality concerning bowel movements.