Black Stool – Blackish Bowel Movements and Diarrhea Causes

We are all accustomed to what constitutes the normal color of stool. It varies from a tan or light brown color to a dark brown color. There are instances where stool may be an abnormal color. Often this is related to foods and drinks with very strong colorants and it is usually not a problem. However, there are instances where a change in stool color persists and this could be a sign of some underlying disease especially with the bowels that needs medical treatment.

Why is stool black?

It is first important to understand why stool is the characteristic brown color that we all know. Stool is composed of undigested food, wastes, digestive enzymes, bile, water and bacteria. The brown color comes from bilirubin, a byproduct of red blood cell breakdown. The body is constantly breaking down old red blood cells and pigments like bilirubin are flushed out of the liver with bile. This bile then empties into the small intestine during digestion and eventually combines with the other substances to form stool.

Furthermore bacteria in the bowel breaks down this bilirubin in the stool to form the brown to dark brown color of stool. In people who have problems releasing bile or processing bilirubin, the stool appears very pale in color and sometimes almost white. Large amounts of bilirubin can make the stool much darker than normal but it is not usually black in color. Blood in the gut is the one of the main causes of black bowel movements, while foods and beverages as well as medication can also result in black stool.

Signs and Symptoms

Black stool is a sign of some underlying condition and is not a disease on its own. It may be accompanied by a host of other signs and symptoms depending on the cause. This includes:

  • Abdominal pain or craming
  • Diarrhea or constipation
  • Abnormally foul odor of the stool
  • Nausea and vomiting
  • Excessive gas – flatulence or belching
  • Pain in the rectum

However, black stools can occur on its own without any other symptoms.

Causes of Black Stool

There are various causes of black bowel movements and it is not always due to any disease. Identifying the exact cause of black stool is difficult without conducting further diagnostic investigations. It is important to seek medical advice when black stools are noticed. Immediate testing is necessary. Sometimes black stools may be an early symptoms of serious underlying diseases. Early diagnosis and treatment can therefore greatly impact on the prognosis.

Food and Beverages

This is one of the most common causes of sudden black bowel movement that lasts for a day or two.  It is due to strong pigments, colorants or dyes in these foods or beverages that are not digested or broken down in the gut. Usually these pigments are flushed out of the system within 48 hours. Possible edible substances that may cause black stool includes:

  • Black licorice
  • Blackberries or blueberries
  • Beets

In small amounts these foods and beverages may not cause any significant change in stool color. However, it becomes noticeable with large quantities.  Eating large amounts of raw meat or foods made of blood like black pudding may also be responsible as the breakdown of blood within the bowels can lead to black stools.

Gastrointestinal Bleeding

Bleeding within the gut is another common cause of black bowel movement. This is usually suspected when a person reports black stools which is medically referred to as melena. Bleeding from the rectum or anus usually causes bright red blood and the stool is typically not black in color. However, when bleeding occurs higher up in the gut like from the wall of the esophagus, stomach or small intestine then it tends to cause the typicaly black tarry stools known as melena. The severity of the bleed also plays a role. Slight bleeding may go unnoticed in the stool.

Possible causes of gastrointestinal bleeding includes:

  • Esophageal varices
  • Gastritis
  • Peptic ulcers
  • Cancer in the esophagus, stomach or small intestine
  • Diverticulitis
  • Inflammatory bowel disease (IBD)
  • Radiation therapy


A number of drugs can cause black stool for different reasons. Any substances with a strong black color that is not broken down or digested in the gut can taint the feces black. Some of these substances may react with the digestive enzymes and other substances in the bowel to turn black in color. Other substances may irritate the lining of the stomach and bowels. If there is underlying problems such as gastritis or ulcers it can lead to bleeding.

Some of the medication that may cause black stools include:

  • Aspirin and other NSAIDs
  • Bismuth subsalicylate
  • Charcoal supplements
  • Iron supplements

It is important never to stop medication if it is suspected that it may be the cause of black stools unless otherwise advised by a doctor or pharmacist.


Certain toxic substances can also cause black bowel movements, such as lead. These substances are usually not ingested intentionally. It may be occur with ingesting inedible substances like lead-based paints. Some herbal products have also been found to contain lead that could taint the stool black. Another poisonous substance that can cause black tarry stool is arsenic. Prolonged exposure to arsenic in even small amounts can lead to serious illness and even result in death. Rat poisons may also cause bleeding within the gut and thereby lead to black stools.

Black Diarrhea

Black diarrheal stool is often a cause for concern because it can be due to serious conditions. Certain infections can cause diarrhea and profuse gastrointestinal bleeding simultaneously. Usually this gives rise bloody diarrhea where the diarrheal stool is red and watery due to the presence of blood. One such example is cholera.

Since the movement through the bowels is so rapid in diarrhea, the blood usually does not have time to degrade and turn the stool black in color. However, there are instances where the bowel movement is suppressed like with the excessive use of antidiarrheal drugs. The bloody stool may then remain in the gut for a period of time and the blood in the stool can degrade and turn black.

Always seek immediate medical attention if black diarrhea arises.

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