Black stool is not always bad, but it can be. The most common harmless causes are iron supplements, certain foods, and stomach-settling medications containing bismuth. The most concerning cause is bleeding in the upper digestive tract, which requires prompt medical attention. The key to telling the difference lies in what you recently consumed and what the stool actually looks and feels like.
Harmless Causes of Black Stool
Iron supplements are one of the most frequent reasons for unexpectedly dark or black stool. If you recently started taking iron, whether on its own or as part of a prenatal or multivitamin, this is likely your answer. The color change is a normal byproduct of how your body processes the extra iron and is completely harmless.
Bismuth subsalicylate, the active ingredient in Pepto-Bismol and similar over-the-counter stomach medications, also turns stool black. It can darken your tongue too. When bismuth reacts with trace amounts of sulfur in your digestive system, it forms a dark compound that passes through harmlessly. The effect disappears within a few days of stopping the medication.
Certain foods can also darken stool noticeably. Black licorice, blueberries, dark chocolate, and foods made with dark food coloring are common culprits. If your stool turned black shortly after eating something with intense pigment, that’s the likely explanation.
What Bleeding Looks Like in Your Stool
When bleeding happens in the upper part of the digestive tract (the esophagus, stomach, or the first section of the small intestine), blood is partially digested as it moves through the gut. By the time it exits, it has turned black. This type of stool has a specific medical name: melena.
Melena looks and feels distinctly different from stool that’s been darkened by food or supplements. It is tar-like in texture, noticeably sticky, and has a strong, unusually foul smell. People who have experienced it often remember the stickiness in particular. Stool darkened by iron or bismuth, by contrast, is simply dark in color. It keeps its normal consistency and doesn’t have that unmistakable tarry quality. If your stool is black but otherwise looks and feels normal, a harmless cause is far more likely.
Conditions That Cause Upper GI Bleeding
Several conditions can cause bleeding high enough in the digestive tract to produce black, tarry stool. The most common is a peptic ulcer, which is a sore on the lining of the stomach or the upper small intestine. Peptic ulcers are most often caused by a bacterial infection called H. pylori or by regular use of common pain relievers like aspirin, ibuprofen, and naproxen. Blood thinners also raise the risk.
Gastritis, an inflammation of the stomach lining, can produce shallow breaks that bleed. The causes overlap heavily with ulcers: H. pylori infection, pain relievers, blood thinners, and heavy alcohol use. In more serious cases, enlarged veins in the esophagus or stomach (called varices) can burst and bleed significantly. Varices are most often linked to liver cirrhosis.
The amount of bleeding varies. A small, slow bleed from a shallow ulcer may produce subtle changes in stool color over days or weeks. A larger bleed can cause dramatic tar-black stool along with other noticeable symptoms like weakness, lightheadedness, or vomiting blood.
Warning Signs That Need Immediate Attention
Black, tarry, sticky stool on its own warrants a call to your doctor. But certain accompanying symptoms signal a more urgent situation:
- Vomiting blood, which may look red or resemble dark coffee grounds
- Dizziness or lightheadedness, which can indicate significant blood loss
- Rapid heart rate or feeling faint, especially when standing up
- Abdominal pain, particularly in the upper stomach area
Severe bleeding that causes enough blood loss to drop your blood pressure can require hospitalization or surgery. This is uncommon, but it is the reason black tarry stool should never be written off without considering what else is going on.
How to Tell the Difference at Home
Before anything else, think about what you’ve consumed in the last day or two. If you’re taking iron supplements, bismuth-based stomach medication, or recently ate a large amount of darkly pigmented food, that is the most probable explanation. Try stopping the supplement or food for two to three days and see if the color returns to normal.
If none of those apply, pay attention to the texture. Normal stool that happens to be dark will hold its usual shape and consistency. Melena from bleeding is loose, tar-like, and sticky enough to smear. The smell is also a clue: digested blood has a sharp, distinctive odor that’s noticeably different from everyday stool.
What Happens if You Get It Checked
If your doctor suspects the black color could be from blood, the first step is usually a fecal occult blood test. This is a simple stool sample that detects hidden blood not visible to the naked eye. It can quickly rule bleeding in or out.
If blood is found, further testing pinpoints where it’s coming from. The most common follow-up is a colonoscopy, where a small camera is used to examine the inside of the colon. An upper endoscopy, which looks at the esophagus and stomach, is often used when an upper GI source is suspected. In some cases, a CT scan with contrast dye provides detailed images of the entire digestive tract without a scope. These tests identify the source so treatment can target the actual problem, whether that’s an ulcer, inflammation, or something else.
For the majority of people who search this question after noticing an unexpectedly dark toilet bowl, the answer turns out to be something they ate or a supplement they started. But when black stool is tarry, sticky, and can’t be explained by diet, it’s worth getting checked promptly rather than waiting to see if it resolves on its own.