What are the Causes of Black Tarry Stools?

Black tarry stools, technically known as melena, is a condition in which bowel movements have a deeper color and stickier texture than normal bowel movements. The brown color of healthy bowel movements is due to bile, a greenish yellow substance made in the liver that travels into the intestines during the digestion process and turns solid waste brown. If there is any bleeding in the intestines, the bile may react with it and produce a black color with a tacky texture. The intestinal bleeding that leads to abnormally dark stool can be caused by a variety of conditions that affect areas of the digestive tract.

The gastrointestinal (GI) tract is comprised of two portions: the upper GI tract and the lower GI tract. The upper GI tract is made up of the stomach, esophagus, and top of the small intestine, while the lower GI tract includes the bottom of the intestines, rectum, and anus. When any bleeding occurs in the upper GI tract, it mixes with the body’s digestive chemicals and turns black. Lower GI tract bleeding will generally result in red stool because the blood isn’t exposed to digestive juices.

One of the most common causes of upper GI tract bleeding, which can lead to black tarry stools, is a stomach ulcer, a condition in which sores or tears form in the tissue lining of the stomach as a result of bacteria, weakened immune system, or nonsteroidal anti-inflammatory drugs. Ulcers can sometimes start to bleed lightly if the sores spread into the blood vessels of the stomach lining. Although the bleeding is typically not heavy enough to be an immediate danger, it can cause severe pain in the region, especially after eating. It is often treated with medications that limit the amount of acids in the digestive juices to prevent future damage to the stomach lining.

Black tarry stools can also be due to conditions that affect the blood vessels inside the upper GI tract. If a person is born with vascular malformation, his or her blood vessels may not be formed correctly and cannot properly control blood flow. Vascular malformation may make a person more likely to have light bleeding in the upper GI tract. Common blood vessel abnormalities may include overly widened vessels that give too much blood at once, or tightened vessels that prevent blood from traveling quickly enough.

In less common instances, black tarry stools can actually be the result of a condition that is not blood-related. Eating foods with dark colors, such as blueberries and black licorice, can cause bowel movements to have a black appearance. Supplements or medications that contain metallic elements like bismuth or iron can also result in dark stools.