Seeing black stool is unsettling, but it does not always mean something is wrong. Foods, supplements, and certain medicines can darken stool, while melena is a specific type of black stool caused by digested blood, usually from the upper gastrointestinal tract. The challenge is that the two can look similar, even though one may be harmless and the other can signal gastrointestinal bleeding.
Melena or Just Black Stool?
Black stool can result from harmless discoloration or bleeding. Because the two can look similar, appearance alone may not tell you which is happening.
Foods and supplements. Iron pills, bismuth medicines such as Pepto-Bismol, black licorice, blueberries, blood sausage, and activated charcoal can turn stool black without bleeding. Bismuth can temporarily darken stool through chemical changes in the digestive tract. Bismuth subsalicylate is a known cause of this discoloration.
Signs of melena. Melena is typically jet black, sticky or tar-like, and distinctly foul-smelling. It develops when blood is broken down as it passes through the digestive tract.
Why color alone is not enough. A known food or medicine trigger makes harmless discoloration more likely, but does not rule out bleeding. Bismuth can darken stool even when bleeding is happening at the same time.
If your stool is black and you cannot confidently explain why, seek prompt medical assessment rather than waiting to see what the next bowel movement looks like.
Why Blood Turns Stool Black
Melena usually points to bleeding in the esophagus, stomach, or duodenum, the first part of the small intestine. As blood moves through the digestive tract, it is broken down and can turn stool black. Around 50 mL of blood in the stomach may be enough to cause this change, although the amount varies. Clinical Methods describes how the location and speed of bleeding affect stool appearance.
Transit time matters. Blood entering the small intestine or cecum can also produce melena if it remains in the digestive tract long enough. A very rapid upper gastrointestinal bleed can instead appear as bright red or maroon blood passed through the rectum.
Black stool can also persist after bleeding has stopped. In classic studies, one to two liters of swallowed blood produced tarry stools for up to five days, with the first appearing four to twenty hours after ingestion. These findings do not provide a way to judge whether bleeding has stopped. Neither the number of black bowel movements nor how long the color lasts can reliably answer that question.
What Causes Melena?
Several conditions can cause bleeding in the upper digestive tract. Finding the source helps clinicians determine the right treatment.
Peptic Ulcers
Peptic ulcers are sores in the stomach or duodenum that can erode into a blood vessel and bleed. They are a leading cause of upper gastrointestinal bleeding. The two main causes of ulcers are Helicobacter pylori (H. pylori) infection and regular use of nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen and naproxen.
Gastritis, Esophagitis, and Duodenitis
These conditions involve inflammation of the stomach lining, esophagus, and duodenum, respectively. Inflamed tissue can become damaged and bleed.
Mallory-Weiss Tears
These tears develop where the esophagus meets the stomach, usually after forceful vomiting or retching. Many heal on their own, but some cause significant bleeding.
Varices
Varices are enlarged veins in the esophagus or stomach, often associated with cirrhosis and other serious liver conditions. A ruptured varix can cause severe bleeding and requires emergency treatment.
Abnormal Blood Vessels
Fragile or malformed blood vessels in the digestive tract can bleed with few other symptoms. In some cases, dark stool is the main visible sign.
Interrupted Blood Supply to the Intestine
When blood flow to part of the bowel is cut off, the intestinal lining can become damaged and bleed. This is uncommon but time-critical and often causes significant abdominal pain.
Tumors of the Upper Digestive Tract
Tumors in the esophagus, stomach, or duodenum can bleed and cause melena. Unexplained weight loss or loss of appetite alongside black, tarry stool warrants medical evaluation, although these symptoms have many possible causes.
Medications That Raise Bleeding Risk
Anticoagulants and antiplatelet medicines can increase the risk or severity of bleeding. NSAIDs and aspirin can also damage the stomach lining. Clinicians need to know about prescription medicines, over-the-counter drugs, and herbal products when evaluating possible bleeding.
If you take blood thinners, antiplatelet medicines, or regular NSAIDs and develop unexplained black stool, seek urgent medical assessment. Do not stop a prescribed medicine on your own.
What Happens at the Doctor?
Your clinician will assess whether bleeding is likely, how serious it may be, and where it could be coming from.
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Medical history. Expect questions about medicines, supplements, dark foods, previous ulcers, liver disease, vomiting, weight loss, and when the stool changed.
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Examination and tests. Your clinician may examine the stool and order blood tests. A stool blood test cannot reliably rule out active bleeding when black, tarry stool suggests melena.
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Blood tests. A complete blood count and related tests help assess blood loss. Early results may not fully reflect a recent bleed, so clinicians interpret them alongside your symptoms and examination.
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Upper endoscopy. A flexible camera examines the esophagus, stomach, and duodenum to locate the source. The clinician can often treat the bleeding during the same procedure.
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Timing. The American College of Gastroenterology guideline recommends endoscopy within 24 hours of presentation for patients admitted to hospital or kept under observation for upper gastrointestinal bleeding.
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Imaging, if needed. CT or CT angiography may help locate bleeding when endoscopy does not identify the source.
