Tarry stool is jet black, sticky, and has a consistency similar to roofing tar or thick molasses. It clings to the toilet bowl and is difficult to flush. Beyond its appearance, it produces a distinctly strong, foul odor that’s noticeably different from normal stool. This combination of color, texture, and smell is the hallmark of what doctors call melena, and it signals bleeding somewhere in the upper digestive tract.
Color, Texture, and Smell
The classic tarry stool is a deep, glossy black. It’s not simply dark brown or muddy looking. The surface has an almost shiny, tar-like quality, and the texture is notably sticky. If you were to touch it (or try to wipe it), it would smear and leave dark residue behind, much like actual tar.
That said, the appearance can vary depending on how much bleeding has occurred. A small amount of bleeding may produce stool that looks more dark brown than true black. If the bleeding is also causing diarrhea, the stool can be looser and wetter rather than thick and formed. But even in these cases, the color remains abnormally dark.
The smell is often the most distinguishing feature. Tarry stool has a uniquely offensive odor that people describe as unmistakable once they’ve encountered it. This happens because blood is broken down by digestive enzymes and gut bacteria as it travels through the intestines. The longer the blood has been in the digestive system, the darker and more pungent the stool becomes. This smell is a key difference between true tarry stool and stool that’s been darkened by something harmless like food or medication.
Why It Looks Like Tar
Tarry stool gets its distinctive appearance from digested blood. When bleeding occurs high in the digestive tract, in the esophagus, stomach, or the first part of the small intestine, the blood passes through the entire length of the gut before it exits. During that journey, digestive enzymes and intestinal bacteria break down the hemoglobin in red blood cells, converting it into dark compounds. This process is what transforms bright red blood into the black, sticky substance you see in the toilet.
The location of bleeding matters. Blood from higher up has a longer transit time, which is why it comes out black and tarry. Bleeding from lower in the digestive tract, like the colon or rectum, doesn’t have time to be fully digested, so it tends to appear as bright red or maroon-colored blood mixed into the stool. That’s a different condition called hematochezia, and it points to a different set of causes.
Foods and Medications That Mimic Tarry Stool
Not every black stool means bleeding. Several common substances can darken your stool enough to cause alarm. Iron supplements are one of the most frequent culprits. Pepto-Bismol and other bismuth-containing medications can also turn stool black. Activated charcoal does the same. On the food side, black licorice, blueberries, and blood sausage can all produce dark or black stool.
The critical difference is texture and smell. Stool darkened by food or medication will typically have a normal consistency and a normal odor. It won’t be sticky or tar-like, and it won’t have that distinctive foul smell. If your stool is black but you recently took iron pills or ate a bowl of blueberries, the cause is likely harmless. If it’s black, sticky, and unusually foul-smelling, that pattern points to digested blood.
Common Causes of Tarry Stool
Tarry stool is a sign of upper gastrointestinal bleeding, and several conditions can be responsible. Peptic ulcers, which are open sores in the stomach or upper small intestine, account for roughly 32% to 36% of cases. Inflammation of the esophagus (esophagitis) causes about 24%. Stomach inflammation (gastritis) is behind 18% to 22% of cases, and inflammation of the upper small intestine (duodenitis) accounts for about 13%.
Esophageal varices, which are swollen veins in the esophagus often related to liver disease, cause around 11% of upper GI bleeding. Mallory-Weiss tears, small rips in the lining where the esophagus meets the stomach (often from severe vomiting), account for 5% to 15%. Less common causes include abnormal blood vessels in the GI lining and tumors.
Some of these causes are minor and resolve on their own. Others involve significant blood loss. The appearance of the stool alone can’t tell you which category you fall into, which is why tarry stool always warrants medical evaluation.
Warning Signs That Need Urgent Attention
Tarry stool on its own is concerning enough to bring up with a doctor. But certain accompanying symptoms suggest more serious or rapid blood loss. Feeling lightheaded or dizzy when standing, a racing heartbeat, unusual fatigue, pale skin, or fainting are all signs that bleeding may be significant. Vomiting blood, whether bright red or dark and coffee-ground-like, alongside tarry stool indicates active upper GI bleeding.
Even without these dramatic symptoms, ongoing tarry stools can lead to iron-deficiency anemia over time. Sometimes the bleeding is slow enough that it doesn’t cause obvious symptoms right away, but it gradually depletes your body’s iron stores. Unexplained fatigue or shortness of breath during normal activities, combined with dark stools, can point to this kind of chronic low-level blood loss.
How the Source of Bleeding Is Found
When someone presents with tarry stool, the primary tool for finding the source is an upper endoscopy, a thin, flexible camera passed through the mouth into the esophagus, stomach, and upper small intestine. This is typically done within 24 hours and allows doctors to both see the bleeding site and treat it during the same procedure.
If the bleeding source isn’t found in the upper tract, a colonoscopy may follow. For cases where bleeding is very subtle or chronic, a fecal occult blood test can detect microscopic amounts of blood in the stool that aren’t visible to the naked eye. This test is sometimes how slow, hidden bleeding is first discovered, often flagged alongside unexplained iron-deficiency anemia on routine bloodwork.
The advantage of endoscopy is that it’s both diagnostic and therapeutic. If a bleeding ulcer or abnormal vessel is found, the doctor can often stop the bleeding right then using heat, clips, or other techniques applied through the scope. Most people go home the same day or the next, depending on the severity of the bleed.