Black blood in stool doesn’t look like blood at all. Instead of the red you might expect, it resembles thick, dark tar. The stool is jet black, sticky, and has a distinctly foul smell that’s noticeably worse than a normal bowel movement. This appearance is so characteristic that doctors have a specific name for it: melena.
How to Recognize It
Melena looks like a shiny, black, tar-like substance. The texture is sticky or tacky, almost like roofing tar, and it tends to cling to the toilet bowl. The color is uniformly dark, not just streaked or speckled. If you wiped it on a white surface, it would leave a dark, almost iridescent smear.
The smell is the other major giveaway. Melena has a particularly strong, offensive odor that’s hard to ignore. That smell comes from blood being broken down by acid and digestive enzymes as it moves through your gut. If your stool is black but doesn’t have that unusually foul odor or sticky consistency, something other than blood is likely the cause.
Why Blood Turns Black
Blood only looks black in stool when the bleeding starts high up in the digestive tract, typically in the esophagus, stomach, or the first part of the small intestine. When blood is exposed to stomach acid and digestive enzymes, the iron in hemoglobin (the oxygen-carrying molecule in red blood cells) gets chemically altered. This process turns bright red blood into a dark, tarry substance over the several hours it takes to travel through the rest of your intestines.
This is why the location of bleeding matters so much. Blood from the lower intestines or rectum doesn’t have time to be digested, so it stays red or maroon. Black, tarry stool specifically signals that something is bleeding further up.
What Causes It
The most common cause of upper digestive tract bleeding is peptic ulcer disease, accounting for roughly 32% to 36% of cases. These are open sores in the lining of the stomach or upper small intestine, often caused by a bacterial infection or long-term use of anti-inflammatory painkillers like ibuprofen or aspirin.
Other frequent causes include inflammation of the esophagus (about 24% of cases), inflammation of the stomach lining (18% to 22%), inflammation of the upper small intestine (13%), and swollen veins in the esophagus related to liver disease (11%). Less commonly, tumors or tears in the lining of the esophagus from severe vomiting can also cause this type of bleeding.
Black Stool That Isn’t Blood
Not every black stool means you’re bleeding internally. Several common substances can turn your stool dark black without any blood being involved:
- Iron supplements are one of the most frequent culprits
- Bismuth subsalicylate (the active ingredient in Pepto-Bismol)
- Activated charcoal
- Certain foods like black licorice, blueberries, and blood sausage
The key difference is texture and smell. Stool that’s dark from iron pills or Pepto-Bismol is typically firm and dark but not sticky or tar-like, and it won’t have that distinctively awful odor. If you recently started an iron supplement or took an antacid containing bismuth, that’s very likely your explanation. Stop the supplement for a couple of days and see if the color returns to normal.
How It Differs From Red Blood in Stool
Bright red blood on toilet paper, on the surface of stool, or in the toilet water is a completely different situation. Red blood typically comes from the lower digestive tract: the colon, rectum, or anus. Hemorrhoids and small anal fissures are the most common sources, and the blood is usually a small amount that coats the stool rather than mixing into it.
Melena, by contrast, changes the entire stool. The whole bowel movement is uniformly black and tarry because the blood has been thoroughly mixed and digested during its transit. Maroon-colored stool falls somewhere in between, sometimes indicating a bleed in the middle of the digestive tract where blood is only partially digested.
Warning Signs of a Serious Bleed
A single episode of black, tarry stool warrants medical attention, but certain accompanying symptoms suggest a more urgent situation. Feeling lightheaded or dizzy when you stand up, a racing heartbeat, unusual fatigue, or pale skin can all indicate that you’ve lost enough blood to affect your circulation. Vomiting material that looks like coffee grounds is another sign of active upper digestive tract bleeding, and it often occurs alongside melena.
Even without those dramatic symptoms, persistent melena means blood is leaving your body faster than it should be. The underlying cause needs to be identified, which typically involves a scope examination where a camera is passed down the throat to visualize the esophagus, stomach, and upper intestine directly. Most causes are treatable once found.