Dark Brown Bowel Movement (Abnormal Brown) Causes

The normal color of bowel movement in humans varies from tan to dark brown. It is not unusual for stool color to vary within the shades of brown, even among successive bowel movements on the same day. There are various factors that influence the color of bowel movement. However, there are cases where bowel movement may be too dark brown in color. This can be a sign of various diseases.

Dark Brown to Black Stool

Dark brown may not always effectively desribe the darkness of the stool color. There are instances when the dark brown stool is almost black yet not black in color. This is sometimes described as a dark brown to black color. It is therefore important to differentiate tan to brown stool from an abnormally dark brown stool. While most people may not initially notice a difference, stool that has any significant change in color, consistency and odor should be investigated by a medical professional.

Read more on normal bowel movement color.

What makes feces brown?

It may be surprising to know that the brown color of stool has more to do with red blood cell breakdown in the body rather than the foods or beverages that are consumed. It is important to understand the process of red blood cell breakdown that influences stool color. Firstly, red blood cells are the oxygen-carriers in the blood but like an cell it has a limited lifespan. When these cells die or are too damaged, it is broken down in the body.

Secondly, the breakdown of red blood cells yield several products which have to be produced, mainly by the liver. One of these byproducts is known as bilirubin. It is passed out with bile into the small intestine. A combination of air and bacteria in the gut then act on this bilrubin to yield stercobilin and urobilin. Both contribute to the color of stool with stercbobilin usually associated with the brown color.

Most of this urobilin is reabsorbed from the gut and eventually passed out with urine. It causes urine to have the typical yellow color. Therefore when there is a problem with insufficient bile, stool can become very pale in color. However, undigested food and strong food colorants that are not broken down in the gut may also taint the stool with various different colors.

Read more on black bowel movement.

Causes of Very Dark Brown Stool

It is important to differentiate dark brown to black stool as being caused by blood in the stool or discoloration of the bowel movement due to strong dyes/colorants. Diagnostic investigation like a fecal occult stool test may have to be done to confirm the presence or absence of blood in the stool. The odd episode of very dark brown stool may not be a cause for concern but if it is recurrent or persistent then it must be investigated by a medical professional.

Gastrointestinal Bleeding

One of the common causes of darker than normal brown stool is due to the presence of blood in the stool. If there is any source of bleeding higher up the gut then it eventually degrades to give ths tool a dark, tarry appearance. This is known as melena. Some of the common causes of gastrointestinal bleeding and particularly upper GI bleeding includes:

  • Bleeding peptic ulcers (stomach or duoenum)
  • Mallory-Weiss tear
  • Gastrointestinal varices
  • Esophageal, stomach or small intestine cancer
  • Infectious gastroenteritis or enterocolitis, especially by microbes that cause hemorrhagic diarrhea.

Similarly any bleeding from the mouth or throat can also cause melena if the blood is swallowed and passes down the digestive tract. Bleeding from the gut is also more likely to occur in people with blood clotting disorders like hemophilia. When bleeding occurs lower down the gut like in the large intestine, then it may not discolor the stool a dark brown but is instead observed as ‘fresh’ red blood in the stool.

Read more on bleeding in the bowels.

Respiratory Tract Bleeding

The throat is a common passageway for both the digestive and respiratory tracts. Therefore bleeding anywhere from the nose down to the lungs may enter the throat and be swallowed. This can cause melena. Some of the possible causes that should therefore be considered includes:

  • Rhinitis and/or sinusitis (rhinosinusitis)
  • Pharyngitis and/or laryngitis.
  • Trachitis.
  • Bronchitis.
  • Pneumonia.
  • Respiratory cancers, particularly throat (pharynx), voice box (larynx), windpipe (trachea), bronchial and/or lung cancers.

Foods, Drugs and Supplements

Various consumed substances, including foods, drugs and supplements, can cause darker brown stools. In this instance the dark brown color is not due to bleeding in most cases. Rather it is a result of ingredients, such as colorants and dyes, that cause the stool to be discolored a darker brown color.

  • Beets
  • Blueberries
  • Charcoal, activated
  • Drugs that may cause/worsen gastrointestinal bleeding like aspirin and other NSAIDs.
  • Iron supplements
  • Licorice (black)
  • Medication containing bismuth
  • Vanadium supplements in large quantities.

Any food or beverage which is dark brown to black in color can also stain the stool. This is more likely to arise when these foods or beverages are consumed during diarrheal illnesses. The rapid transit through the gut prevents these dyes/colorants from being broken down in the gut thereby causing discoloration of the stool.


A host of different toxins can cause gastrointestinal bleeding or discolor the stool when it is not digested and absorbed higher up the gut. Lead and certain other metals can cause bleeing within the gut or remain in the stool where it alters the natural color of bowel movement. It can sometimes be difficult to differentiate between poisoning and some of the other causes of a very dark brown stool.

Any change in stool that arises suddenly, especially when it is accompanied by symptoms like nausea, vomiting and/or diarrhea could occur as a result of poisoning. This may be accidental or intentional. Corrosive substances that are consumed may also be responsible for gut bleeding. Often the poison is consumed with food or beverages. Therefore the symptoms of poisoning tends to occur or worsen after eating a meal tainted with the toxin.

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