Black diarrhea is almost always a sign that blood has been broken down somewhere in your upper digestive tract, usually the stomach or the first part of the small intestine. When blood spends enough time in the gut, digestive acids and bacteria turn it from red into a thick, black, tar-like substance. That is different from stool that simply looks dark because of something you ate or a supplement you took. The color matters, and so does the texture.
Why Your Diarrhea Is Black
The black color comes from digested blood. When bleeding happens high up in the digestive tract, the blood has a long way to travel before it leaves your body. Along the way, stomach acid and intestinal bacteria break down the hemoglobin in red blood cells. Iron in that hemoglobin gets oxidized, and the result is a dark pigment called hematin. That pigment is what gives the stool its black color.
This is why upper GI bleeding looks black while lower bleeding, such as from hemorrhoids or the colon, usually looks red or maroon. The higher the source, the darker the output. The medical term for this black, tarry stool is melena.
What Does Black Diarrhea Actually Look Like?
True melena has a specific appearance that separates it from ordinary dark stool. It is black, sticky, and looks like tar or asphalt. It often has a strong, foul odor that people describe as unusually bad. It tends to be shiny or greasy-looking.
Here is a detail many people miss. Melena is not always fully formed. It can be loose or watery, which is why black diarrhea is a real and important category. When bleeding is heavy and moves through the gut quickly, stool can be both black and liquid at the same time.
If your stool is dark brown, dark green, or simply a deeper shade than usual, that is often diet or supplements. If it is jet black, tarry, and smells strongly, that points more toward digested blood. Texture and smell are just as telling as color.
What Causes Black Diarrhea?
There are two broad buckets: things you swallow that turn stool black, and bleeding inside your digestive tract. Bleeding is the one that needs urgent attention.
Bleeding in the upper digestive tract
Several conditions can bleed into the stomach or upper small intestine:
- Peptic ulcers. Open sores in the stomach or duodenum can bleed. This is one of the most common causes of melena.
- Gastritis. Inflammation of the stomach lining can ooze blood, especially with heavy alcohol use or long-term NSAID use.
- Esophageal varices. Swollen veins in the lower esophagus, often linked to liver disease, can bleed heavily.
- Mallory-Weiss tears. Small tears at the junction of the esophagus and stomach, often after severe vomiting or retching.
- Gastrointestinal cancers. Tumors in the stomach or esophagus can bleed slowly or suddenly.
- Angiodysplasia. Fragile, abnormal blood vessels in the gut wall that can bleed.
Medications that increase bleeding risk
Some medicines raise the chance of GI bleeding. Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen, naproxen, and aspirin are well known for irritating the stomach lining and promoting ulcers. Blood thinners such as warfarin and direct oral anticoagulants make any existing bleed worse. If you take these and notice black stool, that combination deserves prompt medical attention.
Swallowed blood
Sometimes the blood does not start in the gut at all. A serious nosebleed that drains down the throat, or bleeding from dental work or mouth surgery, can be swallowed and then digested. The stool turns black for the same reason.
When Is Black Stool Not Blood?
A lot of black stool has nothing to do with bleeding. Common culprits include:
- Bismuth subsalicylate. This is the active ingredient in some upset-stomach medicines. It commonly turns stool black.
- Iron supplements. These are a frequent cause of dark or black stool.
- Licorice. Black licorice in large amounts can darken stool.
- Dark foods and drinks. Blueberries, blackberries, dark grape juice, and blood sausage can all darken stool.
- Activated charcoal. Sometimes taken for gas or poisoning treatment, it turns stool black.
Here is the key difference. These causes usually produce dark stool without the tarry texture and heavy odor of melena. If you recently started an iron pill or took a bismuth medicine and your stool is dark but otherwise normal in texture, that is often the explanation. If it is black, sticky, and foul-smelling, do not assume it is your supplement.
How Can You Tell the Difference?
You cannot always tell by looking alone, and that is an honest limitation. The table below shows general patterns, but it is not a diagnosis. Only a doctor can confirm the cause.
| Feature | Likely from blood (melena) | Likely from diet or medicine |
|---|---|---|
| Color | Jet black | Dark brown, dark green, or black |
| Texture | Sticky, tarry, greasy | Normal or slightly loose |
| Smell | Strong, foul | Usual |
| Timing | Sudden, no clear trigger | Follows a new food, pill, or supplement |
| Other symptoms | Dizziness, weakness, fast heartbeat | Usually none |
What Other Symptoms Matter?
Black diarrhea on its own is a signal. When it comes with other symptoms, the picture becomes more urgent.
Watch for dizziness, lightheadedness, weakness, fatigue, shortness of breath, a racing heartbeat, or pale skin. These can point to blood loss and anemia. Vomiting blood, or vomit that looks like coffee grounds, is a red flag. Pain in the upper belly, especially a burning or gnawing pain, can suggest an ulcer.
Any of these together with black stool means you should seek medical care promptly. Heavy GI bleeding can become serious quickly.
When Should You See a Doctor?
See a doctor right away if your stool is black and tarry, especially if you feel weak, dizzy, or short of breath. Go to an emergency room if you are vomiting blood, have severe abdominal pain, or feel faint.
If you are not sure whether your stool is truly black and tarry or just dark, it is still reasonable to get it checked. A doctor can order a stool test to look for hidden blood and a blood count to check for anemia. Being cautious here is not overreacting. Upper GI bleeding is treatable, and catching it early matters.
If you take NSAIDs or blood thinners and notice black stool, do not wait. Call your doctor or seek care the same day.
How Is Black Diarrhea Evaluated?
A doctor will start with your history. They will ask about recent foods, supplements, and medicines, and about any pain, vomiting, or prior ulcers. A physical exam checks for signs of blood loss like a fast heart rate or pale skin.
Tests often include a complete blood count to check for anemia and a stool test for occult blood. If bleeding is suspected, an upper endoscopy is the standard way to look directly at the esophagus, stomach, and duodenum to find the source. Depending on findings, other imaging or tests may follow.
Treatment depends entirely on the cause. That is why identifying the source comes first. If a medicine is the trigger, a doctor may adjust it. If an ulcer is found, treatment targets the ulcer. The point is that black diarrhea is a symptom, not a diagnosis, so the underlying cause drives what happens next.
What You Should Not Do
Do not assume black stool is harmless because you started an iron pill. Iron can darken stool, but it does not rule out bleeding happening at the same time. Do not try to treat possible GI bleeding at home. Do not take more NSAIDs for stomach pain, since they can worsen bleeding. And do not wait to see if it clears up on its own when you have other symptoms like dizziness or weakness.
Frequently Asked Questions
Is black diarrhea always a sign of bleeding?
No. Iron supplements, bismuth medicines, licorice, and dark foods can all turn stool black without any bleeding. However, jet black, tarry, foul-smelling stool is more concerning for digested blood and should be checked.
Can black diarrhea be caused by something I ate?
Yes. Foods like blueberries, blackberries, dark grape juice, and blood sausage can darken stool. These usually do not cause the sticky, tarry texture or strong odor seen with bleeding.
When should I go to the emergency room for black stool?
Go right away if you are vomiting blood, have severe belly pain, feel faint, or have a racing heartbeat along with black stool. These can signal significant blood loss.
Does iron cause black stool?
Yes, iron supplements commonly turn stool black or dark green. Because iron can also be taken alongside a real bleed, dark stool on iron is not proof that nothing else is wrong.
