Dark or black stools usually result from something you ate, a supplement you’re taking, or an over-the-counter medication. In most cases, the cause is completely harmless. However, truly black, tarry stools with a foul smell can signal bleeding in the upper digestive tract, which needs prompt medical attention. The key is knowing how to tell the difference.
Common Harmless Causes
The most frequent reason for unexpectedly dark stools is something in your diet or medicine cabinet. Iron supplements are a top culprit: iron oxide reacts with digestive juices in your gut, producing a dark or black pigment that colors your stool. This typically shows up within a day or two of taking a dose and is entirely normal.
Bismuth-based medications like Pepto-Bismol cause the same kind of surprise. When bismuth meets small amounts of sulfur in your saliva and digestive system, the two combine to form bismuth sulfide, a black substance. As it works through your system, both your tongue and your stool can turn dark. This is harmless and clears up once you stop taking the medication.
Certain foods also darken stool noticeably. Black licorice, blueberries, blood sausage, and activated charcoal can all produce stools dark enough to cause alarm if you aren’t expecting it. If you recently ate any of these, that’s likely your explanation.
When Dark Stools Signal Bleeding
Black stools caused by bleeding in the upper digestive tract have a medical name: melena. This type of stool looks and feels distinctly different from the harmless kind. Blood that enters the stomach or upper intestine gets broken down by stomach acid and digestive enzymes as it travels through the gut. By the time it reaches the other end, it’s been chemically transformed into a glossy, tar-like black substance with a strong, unusually foul odor.
The conditions that cause this kind of bleeding include peptic ulcers (open sores in the stomach or the first part of the small intestine), severe inflammation of the stomach lining or esophagus, ruptured veins in the esophagus or stomach, a tear in the esophagus from violent vomiting, and cancers of the stomach, esophagus, or pancreas. Peptic ulcers are by far the most common cause.
How to Tell the Difference
A few features help you distinguish harmless dark stools from something more serious:
- Texture: Stools darkened by iron or food tend to be dry and coarse. Melena is sticky and tar-like, almost glossy.
- Smell: Iron and food-related dark stools smell normal. Blood that’s been digested has a distinctly foul odor that’s hard to mistake for anything else.
- Color quality: Iron supplements often produce a matte black or dark green color. Bleeding produces a shiny, jet-black appearance.
- Timing: If the color change lines up predictably with an iron dose, a Pepto-Bismol tablet, or a bowl of blueberries, you likely have your answer. Melena tends to persist across multiple bowel movements with no clear link to what you ate.
- Other symptoms: Dark stools from supplements might come with mild constipation or gas. Melena from internal bleeding often arrives alongside dizziness, weakness, abdominal pain, or vomiting blood.
Dark Stools in Newborns
If you’re a new parent, blackish-green stools in the first couple of days are completely expected. A newborn’s first bowel movements consist of meconium, a thick, sticky, tar-like substance that built up in the intestines before birth. Babies typically pass meconium within 24 to 48 hours after delivery, and the color gradually shifts to yellow or brown as feeding gets established. Dark stools in a newborn beyond those first few days, or stools that return to a dark color after already transitioning, are worth flagging with your pediatrician.
What Happens if You Get Tested
If there’s any uncertainty about whether blood is present, a fecal occult blood test checks for hidden blood that isn’t visible to the naked eye. There are two main types. The older version uses a chemical called guaiac and usually requires stool samples from two or three separate bowel movements. The newer version, called a fecal immunochemical test (FIT), uses an antibody to detect blood and is considered more accurate. FIT requires one to three samples depending on the brand.
A positive result on either test doesn’t diagnose a specific condition. It simply confirms that blood is present. The most common follow-up is a colonoscopy, where a thin camera is guided through the colon to identify the source of bleeding. Other options include a sigmoidoscopy, which examines just the lower colon, or a CT colonography (sometimes called a virtual colonoscopy) that uses imaging to create detailed pictures of the entire colon and rectum. A stool DNA test can also check for genetic changes that might point to polyps or colorectal cancer.
Red Flags That Need Urgent Attention
Most dark stools don’t require an emergency visit. But certain combinations of symptoms suggest significant blood loss and shouldn’t wait. Black, tarry stools paired with lightheadedness, feeling faint, rapid heartbeat, persistent abdominal pain, or vomiting blood (which can look like coffee grounds) point to active bleeding in the upper digestive tract. Pale skin, unusual fatigue, and shortness of breath are also signs that blood loss may be substantial. If you notice any of these alongside dark stools, seek care quickly rather than waiting to see if it resolves on its own.