Black and green stool each have a range of causes, from completely harmless dietary triggers to signs of something that needs medical attention. The key distinction is whether the color change came with other symptoms or showed up on its own. In most cases, something you ate, drank, or took as a supplement is the explanation.
Why Stool Is Normally Brown
Your liver produces bile, a yellow-green fluid that helps digest fats. As bile travels through your digestive tract, enzymes chemically break it down, gradually shifting its color from green to brown. That breakdown process is what gives stool its typical medium-to-dark brown color. When something interrupts or speeds up that process, or when a pigment from food or medication overrides it, you see a color change.
Common Causes of Green Stool
The most frequent reason for green poop is simply eating a lot of green foods. Spinach, kale, broccoli, avocados, herbs, and matcha all contain chlorophyll, the pigment that makes plants green, and it can do the same to your stool. Pistachios have the same effect for the same reason. Even blueberries and blackberries, which aren’t green at all, can temporarily turn stool green. Artificial food coloring, especially green and blue dyes found in candy, fruit snacks, and freeze pops, is another common culprit.
The second major cause is rapid transit through the digestive tract. When food moves through your large intestine too quickly, often because of diarrhea, bile doesn’t have enough time to fully break down from green to brown. The result is green or greenish stool. This is why a stomach bug or food poisoning often produces green diarrhea specifically. Bacterial infections like Salmonella and E. coli, viral infections like norovirus, and parasites like Giardia can all cause this rapid “gush” of unabsorbed bile.
Antibiotics can also turn stool green by disrupting the balance of gut bacteria that normally help process bile. Iron supplements are another overlooked cause. Whether taken as a tablet or liquid, iron can produce dark green or even black stool. This change is harmless and resolves once you stop taking the supplement.
Common Causes of Black Stool
Black stool falls into two very different categories: harmless color changes from something you consumed, and internal bleeding. Telling them apart matters.
On the harmless side, iron supplements are the most common medication-related cause. Bismuth subsalicylate, the active ingredient in Pepto-Bismol and Kaopectate, also turns stool dark or black. Eating large amounts of blueberries can make stool so dark it looks nearly black. Black licorice does the same. And if you eat a handful of brightly colored candy, the mixed dyes can combine into a black shade. In all these cases, stool color returns to normal once the food or supplement leaves your system.
The concerning cause of black stool is upper gastrointestinal bleeding, a condition called melena. When blood is exposed to stomach acid and digestive enzymes during its journey through the gut, it turns dark and tarry. Melena typically originates in the stomach, the upper small intestine, or the lower esophagus. The stool is not just dark in color. It tends to be sticky, tar-like in consistency, and has a distinctly foul smell that’s different from normal stool.
What Causes Upper GI Bleeding
The most common source is a peptic ulcer, which is a sore in the lining of the stomach or upper small intestine. Severe inflammation of the stomach lining (gastritis) or esophagus can also bleed enough to darken stool. Other causes include ruptured veins in the esophagus or stomach, a tear in the esophagus from violent vomiting, erosion of the stomach lining, and in rarer cases, cancers of the stomach, esophagus, or pancreas.
How to Tell if Black Stool Is Serious
Start by thinking about what you’ve eaten or taken in the past day or two. If you recently took Pepto-Bismol, started an iron supplement, or ate a bowl of blueberries, that’s very likely your answer. The color should return to normal within a couple of days after you stop.
If you can’t trace the color to a food, supplement, or medication, pay attention to the stool’s texture and smell. Melena from internal bleeding is characteristically tarry and sticky, not just dark. It also comes with a particularly strong odor. And it doesn’t resolve after a day or two on its own.
The warning signs that black stool is a medical emergency include vomiting blood (which may look like dark coffee grounds), dizziness or fainting, feeling weak or unusually fatigued, cool or clammy skin, rapid pulse, and nausea. These are signs of significant blood loss and potentially shock. If black, tarry stool shows up alongside any of these symptoms, that warrants immediate medical care.
Green and Black Stool in Newborns
If you’re a new parent, dark stool in the first few days is completely expected. Within the first 24 to 48 hours of life, babies pass meconium, a thick, tar-like, odorless substance that’s black or dark greenish-brown. This is not a sign of bleeding. It’s made up of materials the baby ingested in the womb.
Over the next several days, stool transitions to a lighter greenish-brown or yellowish-green as the baby begins digesting milk. This progression from black to green to yellow-brown is a normal part of a newborn’s digestive system coming online.
When Color Changes Are Just Color Changes
Stool color varies more than most people realize. A single green or unusually dark bowel movement with no other symptoms is rarely a problem. The pattern to watch for is persistence: color changes lasting more than a few days without a clear dietary explanation, or color changes paired with symptoms like abdominal pain, diarrhea that won’t quit, unexplained weight loss, or any signs of bleeding. A one-off green stool after a kale salad or a dark stool after starting iron tablets is your digestive system doing exactly what you’d expect.