What can cause stools to be "tarry"?

Keith

When the stool appears black and tarry, it is usually a sign that bleeding is occurring somewhere in the intestinal tract. The medical term for this is melena. Melena usually stems from bleeding high in the digestive tract, most commonly from the stomach or upper intestine (duodenum). Less often, melena results from bleeding in the small intestine or upper parts of the colon.

Fresh blood is red. However, bacteria break down the blood's iron-rich red blood cells if they stay in the gastrointestinal tract more than 14 hours, and it is this iron that makes the stool appear black. Because of the amount of time required for red blood cells to degrade, it is much more likely that bleeding from the upper gastrointestinal tract will lead to black, tarry stools than bleeding in the lower regions.

It must be stressed that not all black stool represents melena. Iron supplements can also give the stool a blackish color. However, unlike degraded blood, they do not change the consistency of the stool, so the stool should not be tarry. Bismuth-containing compounds (such as Pepto-Bismol) will also lead to black stool that can be confused with melena. Since these medications are often used to ease diarrhea, the black stool that ensues will not usually appear formed, and may appear tarry.

Because gastrointestinal bleeding can be life-threatening, it is very important see a doctor if you have black, tarry stool that is not related to taking iron supplements or bismuth medications.