Causes of Black Stool

Supplements or Dark Foods Can Sometimes Cause

Bowl of blueberries
A bowl of blueberries is a healthy snack, but they can also turn your stool black, causing "false melena.". Image © Brad Harrison

There are many medical reasons that stools could appear black or tarry with a foul smell, but sometimes dark stools are caused by eating certain foods or taking iron supplements. However, black stools can also be a result of bleeding, and the darker color indicates that the blood could be coming from a point in the upper gastrointestinal tract. When stools are black or tarry due to blood being present, it is called "melena." "False melena" refers to stools that are dark or black, but actually do not contain any blood.


Black stool isn't always due to a big problem. There are many reasons stool could appear black, and iron supplements or even Oreos could be to blame. This is especially true for people who have had ostomy surgery or a colectomy because food is not being digested as fully as it is in people who have not had any abdominal surgery. However, if you have had gastrointestinal bleeding in the past, there is a foul smell, or the problem goes on for longer than a few days, see your doctor right away.

False Melena

A black stool caused by food, supplements, medication, or minerals is known as "false melena." It is called "false" because the black color is not actually due to bleeding. Iron supplements, taken alone or as part of a multivitamin for iron-deficiency anemia, may cause black stools or even green stools. Foods that are dark blue, black or green may also cause black stools. Substances that are often found to cause false melena include:

A physician should be consulted immediately if black stools cannot be attributed to a benign cause, such as an iron supplement or a food.

Diagnosing Melena

The black color alone is not enough to determine that it is in fact blood that is being passed in the stool.

Therefore, a doctor will need to confirm whether there actually is blood in the stool. This can be done in a doctor's office through a rectal exam. Or, it can be done at home with a kit that is used to collect a small stool sample which is then sent to a lab for evaluation.

Blood in the stool that is dark could be caused by several different conditions, including a bleeding ulcer, gastritis, esophageal varices, or a tear in the esophagus from violent vomiting (Mallory-Weiss tear). The tarry appearance of the stool is from the blood having contact with the body’s digestive juices.

After melena is diagnosed, a physician may order other diagnostic tests to determine the cause and the exact location of the bleeding. These tests could include x-rays, blood tests, colonoscopy, gastroscopy, stool culture, and barium studies.

Causes of Melena

Bleeding ulcer: An ulcer is a type of sore on the lining of the stomach, which can cause bleeding and result in melena. Contrary to popular belief, stomach ulcers are not usually caused by stress or spicy food (although these can aggravate an already existing ulcer).

In fact, they are typically caused by an infection with a bacterium called Helicobacter pylori (H. pylori). Antibiotics are normally prescribed to eliminate the infection, and sometimes an acid reducer.

Another cause of stomach ulcers is the long-term use of pain medications known as NSAIDs (nonsteroidal anti-inflammatory drugs). NSAIDs can irritate the stomach by weakening the lining's ability to resist acid made in the stomach. For this same reason, NSAIDs have an adverse effect on Crohn's disease and ulcerative colitis. NSAIDs include common over-the-counter drugs such as ibuprofen, naproxen sodium, and aspirin. Some NSAIDs are prescribed by physicians. Stomach ulcers caused by NSAIDs usually heal after the offending drug is discontinued.

Gastritis: Gastritis is the inflammation of the stomach lining. This inflammation can be caused by too much alcohol or food, eating spicy foods, smoking, infection with bacteria, or by the prolonged use of NSAIDs. Gastritis can also develop after surgery or trauma, or it may be associated with already existing medical conditions.

Esophageal varices: Esophageal varices are dilated veins in the wall of the lower esophagus or upper stomach. When these veins rupture, they may cause bleeding, which can cause blood to appear in the stool or in vomit. Esophageal varices are a serious complication resulting from portal hypertension (high blood pressure) brought on by cirrhosis of the liver.

Mallory-Weiss tear: This is a tear in the mucous membrane that joins the esophagus and the stomach. If this tear bleeds, it can result in melena. This condition is fairly rare (only occurring in 4 of 100,000 people), and may be caused by violent vomiting, coughing, or epileptic convulsions.

What To Do If Your Stool Is Black

If you have black stools that you can not attribute to a food or to iron supplements, see your doctor as soon as possible. A black stool once in a while that always occurs after eating foods that are dark in color is typically nothing to worry about. If the black color is accompanied by other symptoms such as vomiting, diarrhea, or acute pain, seek medical attention immediately.


National Institute of Diabetes and Digestive and Kidney Diseases. "Bleeding in the Digestive Tract." National Digestive Diseases Information Clearinghouse. 

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