Dark stool in adults is usually caused by something you ate, drank, or took as a supplement. Iron pills, bismuth-based medications like Pepto-Bismol, black licorice, blueberries, and blood sausage can all turn stool noticeably dark or even black. But when diet and supplements don’t explain the change, dark stool can signal bleeding in the upper digestive tract, which needs medical attention.
The key distinction is between harmless color changes and a condition called melena, which is jet black, tarry, sticky stool caused by digested blood. Understanding the difference helps you figure out whether you’re dealing with last night’s blueberry smoothie or something more serious.
Foods and Supplements That Darken Stool
Several everyday items can turn stool dark brown to black without any cause for concern. The most common culprits are iron supplements, which are widely prescribed for anemia and frequently cause stool to become very dark green or black. Activated charcoal supplements do the same. Among foods, blueberries, black licorice, and blood sausage are well-known triggers. Dark leafy greens like spinach, when eaten in large amounts, can also deepen stool color significantly.
Bismuth-based medications deserve special mention. When you take Pepto-Bismol or similar products, the bismuth reacts with trace amounts of sulfur in your saliva and digestive system to form bismuth sulfide, a black-colored compound. This can turn both your tongue and your stool black. The effect is temporary and harmless, usually clearing within a few days after you stop taking the medication.
The simplest way to test whether food or supplements are responsible: stop consuming the suspected item for two to three days. If your stool returns to its normal brown color, you have your answer.
When Dark Stool Means Bleeding
Dark stool caused by bleeding in the upper digestive tract (the esophagus, stomach, or the first section of the small intestine) looks and smells distinctly different from diet-related color changes. It’s jet black, has a tarry or sticky consistency, and carries a particularly strong, offensive odor. That smell comes from blood being broken down and digested as it travels through the GI tract.
Peptic ulcers are the most common cause of acute upper GI bleeding. These are open sores in the stomach lining or the upper small intestine, often caused by a bacterial infection or long-term use of anti-inflammatory pain relievers like ibuprofen or aspirin. The bleeding can be slow enough that you don’t feel much pain, which is why the stool change sometimes appears as the first noticeable symptom. Silent ulcers, tumors, and ruptured blood vessels can all bleed without producing any abdominal discomfort.
Medical Conditions Behind Upper GI Bleeding
Beyond ulcers, several conditions can cause enough bleeding to produce dark, tarry stool:
- Gastritis: inflammation of the stomach lining, which can erode tissue and cause slow or sudden bleeding. Alcohol use, certain medications, and infections are common triggers.
- Esophageal tears: violent or prolonged vomiting can tear the lining of the esophagus (sometimes called a Mallory-Weiss tear), producing bleeding that darkens stool.
- Enlarged veins in the esophagus or stomach: liver disease, particularly cirrhosis, causes blood to back up into veins around the esophagus and stomach. These swollen veins can rupture and bleed heavily.
- Abnormal blood vessels: malformed or overgrown blood vessels in the stomach, esophagus, or upper small intestine can leak blood intermittently.
- Cancer: tumors in the esophagus, stomach, or upper small intestine can bleed as they grow, sometimes producing dark stool as an early sign.
- Loss of blood supply to the intestines: when blood flow to a section of the intestine is blocked, the damaged tissue can bleed into the digestive tract.
Pain location sometimes points to the source. Abdominal pain suggests a stomach condition like an ulcer or gastritis. Chest pain may point to an esophageal problem. Upper abdominal or back pain can indicate a pancreatic issue. But the absence of pain doesn’t rule out serious causes.
How Doctors Determine the Cause
If diet and supplements don’t explain your dark stool, the first step is usually a fecal occult blood test, which checks for hidden blood. There are two main types. One uses a chemical called guaiac to detect blood, while the other uses an antibody that specifically attaches to human hemoglobin. The antibody-based version tends to be more accurate because certain foods, including red meat and some raw fruits and vegetables, can trigger false positives on the chemical test.
If hidden blood is confirmed, further testing typically involves an upper endoscopy, where a camera is guided through the mouth into the esophagus, stomach, and upper small intestine to locate the bleeding source. This procedure allows doctors to both identify and often treat the problem in the same session.
Symptoms That Signal an Emergency
Dark stool on its own, especially after eating known trigger foods or taking iron supplements, is rarely an emergency. But certain accompanying symptoms change the situation entirely. Vomiting blood or material that looks like coffee grounds, feeling dizzy, weak, or lightheaded, experiencing a rapid heart rate, or having shortness of breath all indicate significant blood loss that needs immediate attention.
Severe GI bleeding can lead to shock, which is life-threatening. Signs of shock include confusion or loss of consciousness, a fast heart rate, pale skin, cold hands and feet, and heavy sweating. Several consecutive days of black, tarry stool, even without these other symptoms, also warrants urgent evaluation. The volume of blood needed to produce melena can be surprisingly small, as little as a few tablespoons, but persistent dark stool suggests ongoing bleeding that can quickly become dangerous if left unaddressed.