Digestive Disorders > Symptoms

Black Stool: What Causes Dark or Black Stool?

Elizabeth Oliver, PhD

Elizabeth Oliver, PhD

Black stool can be a sign of bleeding or other injuries in your gastrointestinal tract. Black stool is not alway a cause for concern. Dark colored foods or certain food supplements can also lead to dark or discolored bowel movements.

If you suspect you have blood in your stool, you should always contact your physician to rule out any serious medical conditions.

What causes black stool?

Blood in the stool

Bleeding in the upper digestive tract can cause black or tarry stools. Bleeding can occur in the esophagus, stomach, or duodenum. When the blood comes into contact with stomach acid or digestive fluids, the red blood cells break down, resulting in a dark tar-like appearance. This black stool is known as melena and often has an extremely bad smell.

In those who digest very slowly due to improper nutrition or metabolic disorders, blood can also come from the large intestine.

Depending on the underlying cause of the discolored stool you may also experience abdominal pain, diarrhea, nausea, or vomiting with blood. The stool may also have a consistency similar to coffee grounds. In these situations, it is very important that you contact your physician as soon as possible.

Causes of blood in the stool

Various gastrointestinal disorders can cause bleeding in the upper digestive tract, leading to tarry stool. Some of the more common include:

Cause of dark stool Frequency Description
Stomach ulcer (Gastric ulcer) 45% Stomach ulcers are one of the most common causes of tarry stool. They usually arise from inflammation of the gastric mucosa. They are typically a result of infection with a bacteria known as Helicobacter pylori. Long-term use of pain medications, also known as nonsteroidal anti-inflammatory drugs (NSAIDs), are another cause of stomach ulcers.
Ruptured esophageal varices (abnormally large veins) 25% Ruptured varices in the esophagus or stomach can lead to tarry stools and bloody vomiting. They most commonly occur as a result of high blood pressure caused by cirrhosis of the liver. The resulting heavy blood loss can be life-threatening. If ruptured varices are suspected, contact an emergency physician.
Mallory-Weiss syndrome (tear in the esophagus) 15% A tear in the mucous membrane that joins the esophagus and the stomach. Usually caused by severe vomiting in those with alcoholism or bulimia.
Mucosal damage (Erosions) 5% Damage to the mucosa of the esophagus, stomach or duodenum can cause bleeding resulting in tarry stools.
Stomach cancer 5% Gastric cancer develops from the gastric mucosa. During later stages, black bowel movements may occur.
Hemolytic anemia ? A type of anemia that causes red blood cells to break down (hemolysis) turning the stool black. This can be caused by underlying disorders such as sickle cell anemia, or as a result of infections or a side effect of medication.
Esophagitis ? Inflammation of the esophagus, which can result in symptoms such as heartburn and difficulty swallowing, as well as vomiting, diarrhea, and black bowel movements.
Nosebleeds ? Ingested blood from heavy nosebleeds can turn the stool dark or black.

Less common causes of black stool include duodenal cancer, vascular abnormalities, and biliary ulcers.

Red bloody stools are typically a result of bleeding in the lower digestive system, such as from the rectum or the colon. Blood from lower in the gastrointestinal system will be exposed to less of the digestive process, so it tends to keep its red color.

Foods, medications, and supplements

Certain black or red foods can cause the stool to appear bloody or dark without the presence of blood. Some of these foods include:

  • licorice
  • beetroot
  • red wine
  • blueberries
  • black pudding

Some medications can also lead to black-colored stools. Examples of these include:

  • iron supplements
  • bismuth-based medications
  • charcoal tablets

If you are unable to trace your dark stool back to a food or food supplement that you have recently eaten you should consult your physician for advice.

How is the cause of black stool diagnosed?

In most cases, stool color alone cannot be used to make a diagnosis. Your physician will also collect information about your medical history and perform a physical examination. A stool sample can be used to confirm the presence of blood in your stool.

If blood in the stool is detected, further procedures may be carried out such as a gastroscopy or colonoscopy in order to determine the exact location of the bleeding.

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How is black stool treated?

Treatment options for black stool depend on the underlying cause.

In the case of a gastric ulcer, your doctor may prescribe acid-reducing medications, or antibiotics if Helicobacter pylori infection is diagnosed.

Vein abnormalities may require surgical repair if the bleeding doesn’t stop on its own.

Why does my newborn have black bowel movements?

Black stool is perfectly normal in newborn babies. It is called meconium, and contains mucus, skin cells, and amniotic fluid. Black stool shouldnt last more than a couple of days.

Is black stool normal when I am pregnant?

Black stools during pregnancy are usually no cause for concern. During pregnancy, your doctor will often prescribe you iron supplements. The iron oxidizes in the digestive tract, causing the stool to turn dark green to black. If you are not taking any iron supplements or are unsure it is always best to consult your physician.

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Koelz, H.R. and Arn, M., 2006. Neue Epidemiologie der akuten gastrointestinalen Blutung. Der Chirurg, 77(2), pp.103-110. Online: https://link.springer.com/article/10.1007/s00104-005-1142-7

Adler, G., Beglinger, C., Manns, M.P., Müller-Lissner, S. and Schmiegel, W. eds., 2013. Klinische Gastroenterologie und Stoffwechsel. Springer-Verlag. Online: https://www.springer.com/de/book/9783642629778

Goralnick, E. and Meguerdichian, D.A., 2014. Gastrointestinal bleeding. Rosen’s emergency medicine concepts and clinical practice, ed, 8, pp.248-253. Online: https://pdfs.semanticscholar.org/25a1/11b152d05ecd5594990c5104499aedebcd43.pdf

Elizabeth Oliver, PhD

Elizabeth Oliver, PhD

Elizabeth Oliver is a researcher and freelance writer with a passion for health sciences. She completed her degree in pharmacology at the University of Edinburgh and PhD in reproductive health at Imperial College London. She currently works as a researcher at the Karolinska Institutet Stockholm.

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