Dark or black stools usually result from something you ate, drank, or took as a supplement. But in some cases, they signal bleeding in the upper digestive tract, which needs prompt medical attention. The key distinction is whether the color change has a clear dietary explanation or whether it comes with other symptoms like dizziness, vomiting, or a tar-like consistency.
Why Stool Is Normally Brown
Your liver releases bile salts into the digestive tract, and as bile breaks down during digestion, it gives stool its characteristic brown color. The shade varies naturally depending on what you’ve eaten and how quickly food moves through your system. Faster transit times can produce lighter or greenish stools, while slower movement tends to darken them. Any significant departure from your usual brown range is worth paying attention to, though it’s not always a reason for concern.
Common Harmless Causes
The most frequent reason for dark stools is something you put in your body on purpose. Iron supplements are one of the top culprits. They commonly turn stool dark green to black, and this is a well-known, expected side effect that doesn’t indicate a problem.
Bismuth subsalicylate, the active ingredient in Pepto-Bismol and similar products, is another common cause. When bismuth meets the small amounts of sulfur naturally present in your saliva and digestive tract, it forms bismuth sulfide, a black compound. As it moves through your system and mixes with food waste, it turns stool gray or black. This can last a day or two after you stop taking the medication.
Certain foods also darken stool noticeably:
- Black licorice
- Blueberries and other dark purple fruits
- Dark chocolate in large amounts
- Beets (can also cause dark reddish stool mistaken for blood)
If you can trace the timing of your dark stool back to one of these foods, supplements, or medications, the color change is almost certainly harmless and will resolve on its own once you stop consuming the cause.
When Dark Stool Means Bleeding
Black, tarry stool with a distinct sticky texture and foul smell is called melena in medical terms, and it typically indicates bleeding in the upper digestive tract: the esophagus, stomach, or the first section of the small intestine. Blood that originates high up in the gut gets partially digested as it travels downward, which is what gives it that dark, tar-like appearance rather than a bright red color.
Common causes of upper GI bleeding include stomach ulcers (often from long-term use of anti-inflammatory painkillers or bacterial infection), tears in the esophagus, and inflamed stomach lining. Less commonly, it can signal something more serious like a bleeding blood vessel or a tumor. Colon cancer can also produce very dark stools when the bleeding originates from the right side of the colon, higher up in the large intestine. In advanced stages, blood in the stool becomes more common and stools can appear very dark.
Bleeding from lower in the colon or rectum, by contrast, usually shows up as bright red or maroon-colored blood. But there’s overlap. A massive bleed in the upper GI tract can sometimes produce red blood, and a slow bleed from the right colon can look like melena.
How to Tell the Difference
The texture and smell matter more than color alone. Melena from internal bleeding is sticky, almost like roofing tar, and has a distinctly strong, unpleasant odor that’s different from normal stool. Dark stool from iron supplements or bismuth, on the other hand, tends to have a more normal consistency.
Context is your best guide. Ask yourself whether you’ve recently started iron supplements, taken Pepto-Bismol, or eaten dark-colored foods. If the answer is yes and you feel otherwise fine, the dietary explanation is likely correct. If you can’t identify an obvious cause, or if the dark stool persists for more than a couple of days after eliminating potential food and supplement causes, that warrants a closer look.
If your doctor wants to check for hidden blood in your stool, they may use a test that detects trace amounts of blood invisible to the eye. It’s worth knowing that certain foods can interfere with this test’s accuracy. Broccoli, cauliflower, cantaloupe, carrots, and tomatoes contain natural compounds that can trigger false positive results. Your doctor will typically ask you to avoid these foods for a few days before testing.
Symptoms That Need Immediate Attention
Dark stool on its own, without other symptoms, rarely requires an emergency visit. But certain combinations of symptoms suggest significant blood loss and need urgent evaluation:
- Dizziness or lightheadedness when standing, which can indicate a drop in blood pressure from blood loss
- Vomiting blood or material that looks like coffee grounds
- Rapid heartbeat or feeling faint
- Persistent dark, tarry stools without a clear dietary cause
Severe blood loss from GI bleeding can cause a dangerous drop in blood pressure and may require hospitalization. If you’re experiencing dark stools alongside any of these warning signs, that’s a situation where getting evaluated quickly matters.
Dark Stools in Newborns
If you’re a new parent, dark stool in the first day or two of life is completely normal. A newborn’s first bowel movements consist of meconium, a thick, dark green-to-black, sticky substance that built up in the intestines before birth. Babies typically pass meconium within 24 to 48 hours after delivery. Once they begin drinking breast milk or formula, their digestive system pushes the remaining meconium out and stool transitions to the lighter yellow, green, or brown colors typical of newborn poop. Healthcare providers monitor this transition before discharge to make sure everything is moving along as expected.
What to Do Next
Start by reviewing what you’ve eaten or taken in the past 24 to 48 hours. If iron supplements, bismuth-containing medications, or dark-colored foods are in the picture, stop them temporarily and see if your stool returns to its normal color within a day or two. Keep an eye on how you feel overall. A single episode of dark stool without other symptoms, especially with a clear dietary explanation, is rarely anything serious.
If the dark color persists beyond two to three days after eliminating potential causes, or if it has that characteristic tarry, sticky quality, getting it evaluated is a reasonable next step. For anyone over 45, or with a family history of colorectal cancer, unexplained changes in stool color are worth mentioning at your next appointment even if they resolve, since stool changes can occasionally be an early sign of something that benefits from early detection.