Stool that is almost black with a thick consistency may be caused by bleeding in the upper digestive 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 (e.g. Pepto bismol)
- Activated charcoal
- Aspirin and NSAIDS (which can cause bleeding in the stomach)
- Dark foods such as black licorice and blueberries
Dark stool can also occur with constipation.
If you experience this type of stool, 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 digestive tract, such as the stomach, will look dark by the time it reaches exits the body as a bowel movement. Blood that is bright or dark red, on the other hand, is more likely to come from the large intestine or rectum.
Conditions that can cause blood in the stool include hemorrhoids, anal fissures, diverticulitis, colon cancer, and ulcerative colitis, among others.
Eating beets can also temporarily turn stools and urine 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.