Dark stool 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 even blood sausage can all turn your poop noticeably dark brown or black. In most cases, the color shift is harmless and temporary. But truly black, tarry stool with a strong foul smell can signal bleeding in the upper digestive tract, which needs medical attention.
Foods and Supplements That Darken Stool
The most common reason for unexpectedly dark poop is dietary. Blueberries, black licorice, and blood sausage are well-known culprits. Foods with dark artificial dyes can do the same. If you recently ate any of these and feel fine otherwise, the color change is almost certainly harmless and will resolve on its own within a day or two.
Iron supplements are another frequent cause. Black stools are a normal side effect of taking iron tablets, and this happens to nearly everyone on oral iron. The unabsorbed iron oxidizes in your gut and darkens the stool. If you recently started an iron supplement and noticed the change, that’s your answer.
Medications That Turn Stool Black
Pepto-Bismol and similar bismuth-based stomach remedies reliably produce black stool, and the chemistry behind it is straightforward. Bismuth reacts with small amounts of sulfur naturally present in your saliva and digestive system, forming a compound called bismuth sulfide, which is jet black. The same reaction can temporarily darken your tongue. This is completely harmless and clears up once you stop taking the medication.
Activated charcoal, sometimes taken for digestive issues, also turns stool very dark or black.
How to Tell If It’s Blood
This is the critical distinction. Stool that’s dark because of food, iron, or bismuth looks dark but otherwise behaves normally. It doesn’t have an unusual smell beyond what you’d typically expect.
Stool that contains digested blood, called melena, is different in several specific ways. Classic melena is jet black with a tarry, sticky consistency, almost like roofing tar. It has a particularly strong, offensive odor that’s noticeably worse than usual. That smell is a byproduct of blood being broken down as it passes through your digestive tract. You won’t notice this distinctive smell with stool that’s been stained black by food or medication.
A small amount of bleeding may look more dark brown than fully black, so color alone isn’t always a reliable indicator. The sticky texture and unusually foul smell together are the more telling signs. In some cases, upper digestive bleeding can also cause looser or more watery stools than normal.
What Causes Bleeding in the Upper Gut
When blood originates high in the digestive tract (the stomach or the first part of the small intestine), stomach acid breaks down the hemoglobin in the blood before it exits the body. This chemical transformation is what produces the black, tarry appearance. By contrast, bleeding lower in the digestive tract tends to produce bright red or maroon-colored stool because the blood hasn’t been exposed to stomach acid.
Common causes of upper digestive bleeding include stomach ulcers, inflammation of the stomach lining, and tears in the esophagus. Heavy use of anti-inflammatory painkillers like ibuprofen or aspirin increases the risk of stomach ulcers, so if you take these regularly and notice tarry black stool, that connection is worth paying attention to. Alcohol use and certain infections can also contribute to stomach lining irritation that leads to bleeding.
Symptoms That Signal an Emergency
Dark stool on its own, without other symptoms, is rarely an emergency. But if black, tarry stool appears alongside any of the following, it points to active bleeding that may need urgent care:
- Dizziness, lightheadedness, or fainting: These suggest enough blood loss to affect your circulation.
- Vomiting blood or material that looks like coffee grounds: This confirms bleeding in the upper digestive tract.
- A fast heart rate or feeling of weakness: These are signs your body is compensating for blood loss.
- Pale skin or cold, clammy hands: These suggest your body is going into shock from significant blood loss.
If you’re experiencing any of these alongside very dark stool, that combination warrants emergency care rather than a wait-and-see approach.
A Simple Way to Narrow It Down
Before worrying, think back over the last 24 to 48 hours. Did you take Pepto-Bismol, start iron supplements, eat a pint of blueberries, or have black licorice? If so, give it a day or two. The color should return to normal once the substance clears your system.
If you can’t trace the dark color to anything you ate or took, pay close attention to the texture and smell. Normal-textured dark stool is far less concerning than sticky, tarry stool with an unusually strong odor. And if the dark color persists for more than two or three days without an obvious dietary explanation, that’s worth getting checked out. A simple stool test can detect hidden blood that isn’t visible to the naked eye, which helps clarify whether bleeding is involved.