Very dark or black stools have two broad categories of causes: harmless ones like foods, supplements, and certain medications, and serious ones involving bleeding somewhere in the upper digestive tract. The color alone doesn’t tell you which category you’re in, but the texture, smell, and accompanying symptoms usually do. True bleeding-related dark stool (called melena) is tarry, sticky, and has a distinctly foul odor that’s hard to miss. Dark stool from food or supplements looks more matte black or dark green and has a normal consistency.
Foods That Turn Stool Dark
Several common foods can temporarily darken your stool enough to cause alarm. Blueberries, blackberries, and dark cherries contain deep pigments that survive digestion largely intact. Beets can produce dark reddish-brown to near-black stool depending on how much you ate. Black licorice (the real kind, made with licorice extract) is another well-known culprit. Dark leafy greens like spinach and kale, especially in large quantities, can push stool toward very dark green or black.
These color changes are completely harmless and resolve within a day or two once the food clears your system. If your stool is dark but formed normally, and you recently ate any of these foods, that’s the most likely explanation.
Iron Supplements and Medications
Iron supplements are one of the most common non-food causes of dark stool. When iron reaches your gut, it reacts with digestive juices to form iron oxide, which creates a matte black or dark green pigment. This happens reliably in most people taking oral iron and is not a sign of bleeding or a reason to stop the supplement. The stool typically returns to normal color within a few days of stopping iron.
Pepto-Bismol and similar stomach remedies containing bismuth also cause striking black discoloration. The bismuth in the medication combines with small amounts of sulfur naturally present in your saliva and digestive tract, forming bismuth sulfide, a black substance. This can darken both your stool and your tongue. Like iron, it’s harmless and clears once you stop taking the medication.
Upper Digestive Tract Bleeding
When blood originates high in the digestive system, in the esophagus, stomach, or upper small intestine, it gets partially digested as it travels through the gut. Stomach acid and digestive enzymes break down the hemoglobin in blood, turning it from red to black. The result is melena: stool that’s not just dark but distinctly tarry, sticky, and unusually foul-smelling. It often looks like roofing tar or thick motor oil.
The most common sources of upper GI bleeding include stomach ulcers (often caused by prolonged use of anti-inflammatory painkillers or a bacterial infection called H. pylori), tears in the lining of the esophagus, and inflamed stomach lining. These can range from slow, low-grade bleeding that produces dark stool over days to rapid bleeding that becomes a medical emergency.
Liver Disease and Swollen Veins
Chronic liver disease creates a specific and dangerous pathway to dark stools. When scar tissue builds up in the liver (cirrhosis), it blocks normal blood flow and increases pressure in the large vein that feeds the liver. That pressure forces blood to reroute through smaller, thinner-walled veins, particularly in the lower esophagus. These veins balloon outward under the strain, forming what are called varices. When they rupture, they bleed heavily into the digestive tract, producing large volumes of dark, tarry stool, often accompanied by vomiting blood. This type of bleeding can be life-threatening and requires emergency treatment.
How to Tell the Difference
The key distinction is between dark stool that has a normal texture and dark stool that’s tarry, sticky, or liquid. If you recently took iron or bismuth, or ate a pile of blueberries, and your stool is solid and dark, you can reasonably attribute it to that source. The color change should resolve within one to three days of stopping the food or supplement.
Melena from bleeding looks and behaves differently. It clings to the toilet bowl, has a loose or tarry texture, and carries an unmistakable smell that’s far worse than typical stool. Even small amounts of blood, as little as a few tablespoons from the upper digestive tract, can turn stool black.
Symptoms That Signal an Emergency
Dark stool on its own warrants attention, but certain accompanying symptoms indicate significant blood loss that needs immediate care. These include vomiting blood or material that looks like dark coffee grounds, feeling dizzy, weak, or lightheaded, heart palpitations or shortness of breath, and several consecutive days of tarry black stool. Any of these combinations suggests active bleeding that may be worsening.
If you’re seeing persistently dark stool without an obvious dietary or medication explanation, and especially if it’s tarry rather than firm, getting it evaluated promptly matters. Doctors can test a stool sample for hidden blood in minutes, which quickly clarifies whether bleeding is involved. Early detection of upper GI bleeding dramatically improves outcomes compared to waiting until symptoms become severe.