Black Bowel Movement (Stool), Causes of Black Diarrhea

Normal stool (bowel movement) color is tan to brown. Sometimes it can be slightly lighter or darker depending on foods and beverages that were recently consumed as well as medication, supplements and even the speed of movement through the gut. However, there are instances when bowel movement color can be considered to be abnormal. Black stool is therefore one such case of abnormal bowel movement color.


The causes of black bowel movement can be divided into dietary, physiologic and pathologic causes. Dietary refers to food and beverages that may alter the color of the stool. Physiologic includes the causes of black bowel movement that are not considered to be a disease but rather a disturbance in the normal processes within the body that contributes to bowel movement color. Pathologic causes refers to diseases that leads to black bowel movement.

It is important to note that black bowel movement like any abnormality of stool color is a clinical sign of some underlying problem and not a condition on its own. When black stool is noticed, it is also important to take note of the shape, form and texture of stool as well as any change in bowel habit. These details may be helpful in diagnosing the underlying cause.

Fatty Stool Diarrhea


Consuming large amounts of foods or beverages containing the following ingredients can alter bowel movement color. This may be evident within a few hours to up to 72 hours after consuming these substances. It is a temporary disturbance in stool color and resolves once the substance in question is passed out of the digestive tract.

  • Beets
  • Blueberries or blackberries in large amount
  • Licorice
  • Raw or undercooked red meat, or other foods with substantial amount of animal blood, like black pudding.

Some beverages when consumed in large quantities will also color the stool black. This may be seen with overindulgence of alcoholic drinks containing black dyes. The stool thereafter can appear dark to black in color.


Any blood passing through the stomach will be colored black, due to oxidation of hemoglobin by stomach acid. Some stomach acid may appear in the duodenum, so any bleeding arising from the duodenum as well will result in black, tarry and foul smelling stool. This type of stool is referred to as melena. Possible causes of black stool from bleeding include:

  • Stomach or duodenal inflammation (gastritis), erosion, or ulcer due to alcohol, smoking, strong spices, or infection with H. pylori bacteria.
  • Stomach or duodenal cancer.
  • Esophageal varices due to liver cirrhosis (often in alcoholics), or portal hypertension.
  • Mallory-Weiss tear (a tear in the esophagus from violent vomiting).
  • Duodenal ulcer, or duodenal inflammation (severe celiac disease) (stomach acid may appear in duodenum and change the blood color from red to black).
  • Small intestine: Meckel’s diverticulum (an inborn defect, where a part of stomach mucosa appears in the small intestine pouch, secretes acid and often bleeds)
  • Nose or paranasal sinuses: high blood pressure, tumor, injury.
  • Mouth: tooth extraction, injury.
  • Throat: severe inflammation, bleeding tonsils, throat cancer.
  • Lung, bronchi or trachea: cancer, tuberculosis, rarely heavy pneumonia.
  • Spontaneous bleeding into the gut due to thrombocytopenia, hemophilia, metastases or sepsis.

There are cases when black bowel movement may be due to bleeding in the lower digestive tract. If the blood remains in the gut for long periods, as may be the case in constipation, then it can become degraded and color the stool black. However, if there is profuse bleeding or the bleeding occurs in the lower bowels then it may appear as bloody stool.


Various supplements, OTC and prescription drugs can cause black bowel movement for several reasons. Firstly, some of the substances within the medication may be a strong black color which can therefore discolor the stool to make it appear black.

Diarrhea Rehydration

Another possible cause is that these substances may lead to bleeding within the gut. However, these drugs may not cause the bleeding but rather lead to conditions or aggravate pre-existing conditions which may then result in gastrointestinal bleeding.

These substances include:

  • Activated charcoal
  • Iron supplements color stool black or green.
  • Pepto-Bismol (bismuth subsalicylate, an anti-diarrheal drug).
  • Any medication that can cause stomach bleeding: aspirin, ibuprofen, naproxen, and other non-steroid antirheumatic drugs (NSAIDs).
  • Vanadium products, often used by bodybuilders.

Never stop any prescription drug is black bowel movement appears after using it. Instead consult with a doctor immediately once the black stool is noted. The black discoloration of stool can be innocuous and stopping the drug in question to avert the black bowel movement may be dangerous and even life threatening.


Toxic substances can also lead to black stool when ingested. This may occur when ingesting a poison just once or with repeated exposure to the poisonous substance.

  • Lead
  • Any substance that causes bleeding in the upper digestive tract or discolors the stool.


Black pepper-sized specks in stool may be due to clumps of bacteria, commonly appearing in small intestinal bacterial overgrowth (SIBO). However, this is not always easily visible and most people may not notice it as inspecting the stool is not a common practice.


Although uncommon, sometimes black stool may be due to a reaction with toilet water. Chemicals in stool may react with chemicals in the toilet water and cause a black appearance. It depends on the substances that are in the toilet water and how it interacts with the constituents of stool.

Another possible, albeit uncommon, cause of black bowel movement is when there is a black discharge or black bleeding from the genitalia. This is more likely to occur with vaginal bleeding, whether it is menstrual bleeding or intermenstrual bleeding. Black vaginal bleeding is more likely to occur when the blood is delayed from being expelled and may degrade before it exist. This blood could possibly taint stool in the toilet bowl.

Black Diarrhea

There are various causes of diarrhea and when it occurs in the backdrop of one or more of the causes mentioned above then black diarrhea may occur. Black diarrhea may be due to:

  • Medications/supplements: anti-diarrheal drug Pepto-Bismol, iron pills
  • Bleeding from the stomach or upper small intestine during diarrhea.

WARNING: Black bowel movement must be investigated by a medical professional. This may require laboratory investigations like a stool analysis.


  1. Black tarry stools.

Last updated on September 15, 2018.

About Jan Modric (209 Articles)
Health writer

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