What Are the Causes of Black Stool?

Supplements or dark foods can sometimes cause black stools

Black licorice
Black licorice is a food that can cause black stools. skhoward/E+/Getty Images

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, that's reason to see your doctor.

Fast Facts: What You Need to Know About Black Stools

  • Most cases of black stools are from eating black foods or iron supplements.
  • The most common condition causing black stools is a bleeding ulcer.
  • Black stools caused by blood indicate a problem in the upper digestive tract.
  • Blood in the stool can be detected through a stool test.
  • Black stool along with pain, vomiting, or diarrhea is cause to see a doctor right away.
  • If blood in the stool is suspected, a doctor should contacted as soon as possible.

Black Stools Caused by Food or Supplements

A black stool could be caused by food, supplements, medication, or minerals. 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 black stools include:

  • Black licorice
  • Blueberries

If you're seeing black stools and can trace it back to a food you ate, that's OK. However, a doctor should be consulted immediately if black stools cannot be traced back to a food, an iron supplement, or Pepto-bismol.

Black Stools Caused By Blood: Melena

If there's no obvious reason for a black stool (such as food, a supplement, or a medication like Pepto-bismol), it could be time to look for blood in the stool.

A variety of medical reasons can cause black stools that are also tarry with a foul smell.

Blood that comes from higher up in the digestive tract (such as the esophagus or stomach) may turn stool black, which is called melena. As the blood passes through the body and interacts with enzymes in the digestive process, the blood changes from red to black. This makes it a bit more difficult to see in or on the stool than if there is red blood. Red blood in or on the stool (called hematochezia) is typically from lower in the digestive tract (such as the rectum or the colon). Blood from lower in the gastrointestinal system will be exposed to less of the digestive process and may keep its red color.

If the black stool is accompanied by other symptoms such as feeling faint or actually fainting, dizziness, pain, or vomiting (especially if there's blood in it or it looks like coffee grounds), contacting a doctor immediately is important, because it could be a medical emergency. For some people, there may already be a known risk factor for bleeding in the digestive tract. Talk to a doctor about the potential for bleeding and blood in the stool if any of these potential risk factors are present:

  • Liver disease
  • Cancer
  • ​Dieulafoy lesion (a rare condition of the stomach)
  • Erosive esophagitis (inflammation in the esophagus)
  • Erosive gastritis (inflammation in the stomach)
  • Intestinal ischemia (blood supply to the intestines is cut off)
  • Peptic ulcers
  • Tear in the esophagus (Mallory-Weiss tear)
  • Varices (abnormally large veins) in the esophagus or stomach

Diagnosing Blood in the Stool

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. The test that will likely be most imperative in determining the cause of the bleeding will be an esophagogastroduodenoscopy (EGD). Other tests that might be done could include x-rays, blood tests, colonoscopy, stool culture, and barium studies.

More Common Causes of Melena

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.

A Word From Verywell

In most cases, a black stool is from foods and isn't something to be concerned about. 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.

Sources:

Laine L, Jensen DM. "Management of patients with ulcer bleeding." Am J Gastroenterol. 2012; 107;345-360.

Lanza FL, Chan FK, Quigley EM; Practice Parameters Committee of the American College of Gastroenterology. "Guidelines for prevention of NSAID-related ulcer complications." Am J Gastroenterol. 2009 Mar;104:728-738.

National Institute of Diabetes and Digestive and Kidney Diseases. "Bleeding in the Digestive Tract." National Digestive Diseases Information Clearinghouse. 2017.

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