Dark diarrhea is most often caused by something you ate, a supplement you’re taking, or a medication like Pepto-Bismol. But when dark stool is also tarry, sticky, and unusually foul-smelling, it can signal bleeding in the upper digestive tract, which needs prompt medical attention. The key distinction is whether the color has an obvious, harmless explanation or whether it appeared unexpectedly alongside other symptoms.
Foods and Drinks That Darken Stool
Several common foods can turn stool noticeably darker, sometimes dramatically so. Black licorice, blueberries, and blood sausage are well-known culprits. Dark leafy greens, dark chocolate, and foods with dark artificial coloring can also shift stool toward a deep green or near-black shade. Beets and red-colored foods sometimes produce a dark reddish stool that people mistake for blood.
If you recently ate any of these foods, the color change is harmless and typically resolves within a day or two after you stop eating them. The stool may look alarming, but it will have a normal texture and smell, which is the easiest way to tell it apart from something more serious.
Medications and Supplements
Iron supplements are one of the most common causes of very dark or black stool. The unabsorbed iron oxidizes in the gut, producing that characteristic color. This is expected and not dangerous, though it can be startling if nobody warned you about it. The darkening typically continues for as long as you take the supplement and clears within a few days after stopping.
Bismuth subsalicylate, the active ingredient in Pepto-Bismol and similar products, also turns stool black. This happens because bismuth reacts with trace amounts of sulfur produced by bacteria in your gut, forming a dark compound called bismuth sulfide. The reaction is harmless. Activated charcoal, sometimes taken for gas or bloating, produces the same visual effect for a simpler reason: the charcoal itself is black and passes through largely intact.
When Dark Stool Means Bleeding
The medical term for black, tarry stool caused by bleeding is melena. It looks and feels distinctly different from stool darkened by food or medication. Classic melena is jet black with a tarry, sticky consistency, almost like roofing tar. It also has a particularly strong, offensive odor that is noticeably worse than normal. That smell comes from blood being broken down and partially digested as it moves through the intestines.
Melena indicates bleeding somewhere in the upper digestive tract, meaning the esophagus, stomach, or the first part of the small intestine. Blood turns black during digestion, so the farther the bleeding site is from the rectum, the darker the stool appears. If you’re seeing this kind of stool and can’t attribute it to food, iron, or bismuth, it needs medical evaluation quickly, especially if you also feel lightheaded, unusually tired, or faint.
Common Causes of Upper GI Bleeding
Peptic ulcers are one of the most frequent sources. These are open sores on the inner lining of the stomach or the upper small intestine. They can bleed slowly over time, leading to gradual blood loss and anemia, or they can bleed enough to produce visible melena. Other symptoms of a bleeding ulcer include vomiting blood (which may look red or dark, like coffee grounds), dizziness, and fatigue.
Gastritis, or inflammation of the stomach lining, can also cause slow bleeding that darkens stool. Heavy alcohol use, prolonged use of anti-inflammatory painkillers, and bacterial infections are common triggers for gastritis.
Liver disease adds another layer of risk. When the liver becomes scarred (cirrhosis), blood has trouble flowing through it, causing pressure to build in the veins that connect the digestive organs to the liver. That pressure forces blood into smaller, thinner-walled veins in the esophagus and stomach, causing them to swell into fragile structures called varices. When varices rupture or leak, the bleeding can be slow and ongoing, or sudden and severe. Blood collecting in the stomach eventually passes through the intestines and appears as dark or black stool.
How Doctors Identify the Cause
The first step is usually a fecal occult blood test, which checks a stool sample for hidden blood that isn’t visible to the naked eye. The fecal immunochemical test is the preferred version because it’s more sensitive and doesn’t require any dietary changes beforehand. An older version, the guaiac-based test, typically requires collecting samples from three separate bowel movements on different days. Some guaiac-based versions are available over the counter as flushable pads that change color in the toilet if blood is present.
These tests only confirm whether blood is there. They can’t pinpoint where it’s coming from. If the test is positive, an endoscopy (a thin camera passed through the mouth into the stomach) is the standard next step to find and sometimes treat the bleeding source directly.
Dark Stool in Newborns
Parents of newborns sometimes worry about very dark stool, but in the first day or two of life, it’s completely normal. A newborn’s first stools, called meconium, are thick, sticky, and dark greenish-black. Meconium should pass within the first 24 to 48 hours. After that, as the baby begins feeding, stools transition to a yellowish-green color and eventually settle into shades of yellow, green, or brown depending on whether the baby is breastfed or formula-fed.
If an infant continues to have black stools after the meconium phase, or if black stool reappears later, that warrants a call to the pediatrician. Grey, white, red, or persistent black stools in infants are not normal and should be evaluated.
How to Tell Harmless From Serious
The texture and smell matter more than the color alone. Stool darkened by food, iron, or bismuth looks dark but has a relatively normal consistency and odor. You can usually trace it back to something specific you ate or took. Melena, by contrast, is sticky, tarry, and has an unmistakable foul smell that stands out even compared to typical stool.
Context also helps. If you started iron supplements three days ago and your stool turned black, the explanation is straightforward. If dark, tarry diarrhea appeared out of nowhere, especially with lightheadedness, weakness, a racing heartbeat, or vomiting that looks like coffee grounds, the concern shifts to active bleeding. The combination of dark stool with any of those symptoms points to significant blood loss and needs urgent attention.
For food and medication-related causes, stool color generally returns to normal within one to three days after you stop the offending item. If the dark color persists beyond that window without an obvious dietary explanation, it’s worth getting checked.