Causes Of Black Stools: Medical Red Flags To Watch
Black stools are most often caused by either something harmless, such as iron supplements or dark foods, or by digested blood from bleeding in the upper gastrointestinal tract, which can be a medical emergency if it is accompanied by weakness, dizziness, abdominal pain, vomiting blood, or a tarry smell. The medically important term for black, sticky stools from bleeding is melena, and it usually points to the esophagus, stomach, or first part of the small intestine.
What black stools mean
Black stool is not a diagnosis by itself; it is a sign that needs context. If the stool is simply dark after eating black licorice, blueberries, or taking iron or bismuth-containing medicine, the color change is usually temporary and not dangerous. If the stool is black, tarry, shiny, foul-smelling, or accompanied by symptoms like lightheadedness or vomiting blood, clinicians worry about upper GI bleeding.
In practice, the question is whether the dark color comes from a benign pigment or from blood that has been digested as it moves through the gut. Blood exposed to stomach acid and digestive enzymes turns darker, which is why bleeding higher up in the digestive tract often appears black by the time it is passed in stool. That distinction matters because harmless discoloration usually resolves on its own, while bleeding requires prompt evaluation.
Common harmless causes
Many cases of black stool are caused by diet, supplements, or medications rather than disease. The most common non-dangerous triggers include iron pills, bismuth subsalicylate products such as some upset-stomach medicines, activated charcoal, black licorice, blueberries, and other very dark foods or dyes. These causes can make stool appear very dark green to black without any actual blood present.
- Iron supplements. Iron can darken stool, especially when started recently or taken in higher doses.
- Bismuth medicines. Products containing bismuth can cause black stool as a harmless side effect.
- Activated charcoal. Charcoal can make stool look black after use for poisoning or stomach upset.
- Dark foods. Black licorice, blueberries, and some food colorings can mimic melena.
- Swallowed blood. A heavy nosebleed or bleeding after dental work can be swallowed and later appear as dark stool.
Serious medical causes
When black stool is caused by bleeding, the concern is usually a source in the upper digestive tract. Common serious causes include peptic ulcers, gastritis, esophagitis, Mallory-Weiss tears after forceful vomiting, esophageal or gastric varices related to liver disease, and cancers of the esophagus, stomach, or duodenum. Among acute upper GI bleeds, peptic ulcers are widely recognized as a leading cause.
Varices deserve special attention because they can bleed heavily and quickly. These enlarged veins form most often in people with cirrhosis or other liver disease, and a rupture can cause life-threatening blood loss. Other concerning causes include ischemia, vascular abnormalities, and less commonly tumors or severe inflammation of the stomach or small intestine.
| Cause | Typical clue | Urgency |
|---|---|---|
| Iron supplement | Black stool after starting iron, otherwise feels well | Usually low if no other symptoms |
| Bismuth medicine | Black stool after Pepto-Bismol-type product | Usually low if temporary |
| Upper GI bleed | Black, tarry, foul-smelling stool, weakness or dizziness | High, needs prompt evaluation |
| Peptic ulcer | Burning pain, nausea, possible blood loss | High if bleeding is suspected |
| Varices | Liver disease, vomiting blood, severe anemia | Emergency |
How doctors tell the difference
Clinicians usually start by asking about foods, medications, and supplements taken in the last several days. They then assess whether the stool is truly tarry, whether there is abdominal pain or vomiting, and whether there are signs of blood loss such as fatigue, pale skin, shortness of breath, dizziness, or a fast heartbeat. A stool test can confirm whether blood is present when the cause is unclear.
- Review recent foods, supplements, and medicines.
- Look for associated symptoms such as pain, vomiting, or weakness.
- Check whether the stool is tarry and foul-smelling rather than just dark.
- Test for blood if the cause is uncertain.
- Investigate the upper GI tract if bleeding is suspected.
If bleeding is suspected, doctors may order blood tests to check hemoglobin and iron status, and may perform endoscopy to locate the source. Endoscopy is especially important when there are signs of active bleeding, anemia, or a history suggesting ulcers, liver disease, or recent heavy vomiting.
When to seek care
Black stool that clearly follows iron, bismuth, charcoal, or dark foods may not require urgent care if it stops quickly and there are no other symptoms. But persistent black stool, or black stool plus dizziness, weakness, fainting, abdominal pain, chest pain, rapid pulse, or vomiting blood, should be treated as urgent.
Emergency evaluation is especially important if the stool looks tarry and sticky or if there is a history of ulcers, liver disease, heavy alcohol use, NSAID use, or prior GI bleeding. These factors increase the chance that the dark stool reflects active bleeding rather than a harmless color change.
What you can do now
If black stool appears once and you recently took iron or bismuth, note the timing and watch for return to normal color over the next day or two. If there is no obvious dietary or medication explanation, or if you feel unwell, medical assessment is the safer choice because black stool can be the first visible sign of hidden bleeding.
"Black or tarry stools with a foul smell are a sign of a problem in the upper digestive tract," according to MedlinePlus, which also notes that iron, bismuth, and certain dark foods can cause the same color change without bleeding.
Frequently asked questions
Practical takeaway
Black stool is usually either a benign side effect of food or medication, or a warning sign of digested blood from upper GI bleeding. The safest rule is simple: if there is no obvious harmless explanation, or if there are any symptoms of blood loss, the stool should be treated as medically important until proven otherwise.
What are the most common questions about When Stool Turns Black What Your Doctor Looks For?
Is black stool always blood?
No. Black stool can be caused by iron supplements, bismuth medicines, activated charcoal, or dark foods, and these are often harmless.
What does melena look like?
Melena is usually black, sticky, tarry, and foul-smelling because the blood has been digested while moving through the upper gastrointestinal tract.
Can stomach ulcers cause black stools?
Yes. Peptic ulcers are a common cause of upper GI bleeding, and that bleeding can produce black stool.
Should I go to the emergency room for black stool?
Go urgently if black stool is accompanied by vomiting blood, fainting, severe abdominal pain, weakness, dizziness, or a rapid heartbeat, because these can signal significant bleeding.
How long can iron cause black stool?
Iron can darken stool while you are taking it, and the color usually improves after stopping or changing the supplement, depending on the formulation and dose.