Dark green poop is usually caused by something you ate, a supplement you’re taking, or food moving through your digestive system faster than normal. In most cases, it’s harmless and resolves on its own within a day or two. Understanding why it happens comes down to how your body processes bile, the green fluid your liver produces to help digest fat.
Why Poop Is Normally Brown
The brown color of a typical bowel movement starts with your red blood cells. After about 120 days, old red blood cells break down and produce a compound called heme. Your liver strips the iron out of heme and converts it into a green pigment called biliverdin. This gets secreted into bile and stored in your gallbladder, where it transforms into bilirubin, which is yellow.
When bile enters your large intestine, bacteria go to work breaking bilirubin down into the brown pigments that give stool its familiar color. That entire conversion, from green to yellow to brown, takes time. Anything that interrupts or speeds up this process can leave your stool looking green instead of brown.
Rapid Transit: The Most Common Non-Diet Cause
When food moves through your intestines faster than usual, bile doesn’t have enough time to complete its chemical transformation from green to brown. The result is stool that still carries that original green tint. This is why diarrhea often produces green or dark green stool, regardless of what you’ve eaten. Illnesses like stomach bugs, food poisoning, or any condition that speeds up digestion can trigger this.
If you’ve had a bout of diarrhea and noticed green stool, the color alone isn’t a sign of something serious. It simply means your gut moved things along before bacteria could finish their job. Once your digestion returns to normal speed, the brown color typically returns.
Foods That Turn Stool Dark Green
Chlorophyll, the pigment that makes plants green, is the biggest dietary culprit. Eating large amounts of green vegetables like spinach, kale, and broccoli floods your digestive system with more green pigment than it can fully break down. The stool that comes out still carries that deep green color, sometimes dark enough to look almost black in dim bathroom lighting.
The list of chlorophyll-rich foods goes beyond salads. Avocados, fresh herbs like parsley and cilantro, matcha (powdered green tea), and pistachios all contain enough green pigment to shift your stool color. Blueberries can also produce shades of green, which surprises most people expecting blue or purple.
Artificial food coloring is another common trigger. Brightly dyed frosting, drink mixes, ice pops, and candy continue tinting whatever they touch as they travel through your digestive tract. Green, blue, or purple dyes in particular can produce noticeably dark green stool a day or so after you eat them.
Iron Supplements and Medications
Iron supplements are one of the most frequent non-food causes of dark green (or even blackish-green) stool. Your body absorbs only a fraction of the iron in each pill, and the unabsorbed iron oxidizes as it passes through your gut, darkening the stool considerably. This is completely expected and not a reason to stop taking your supplement.
Antibiotics can also change stool color by disrupting the balance of gut bacteria. Since those bacteria are responsible for converting green bile pigments into brown ones, fewer bacteria means less conversion, and greener stool. This effect usually resolves once you finish the course of antibiotics and your gut flora recovers.
Dark Green vs. Black and Tarry
The important distinction to make is between dark green stool and true black, tarry stool, which can indicate bleeding somewhere in the upper digestive tract. Stool from internal bleeding (called melena) is jet black, has a sticky or tarry consistency, and produces a distinctly strong, offensive odor that’s noticeably different from a normal bowel movement. The smell comes from blood being digested as it travels through the GI tract.
Dark green stool from food or supplements doesn’t have that same sticky texture or unusual odor. It may look very dark, especially under poor lighting, but if you can see green tones when you look closely, it’s almost certainly not bleeding. A small amount of GI bleeding can sometimes appear more dark brown than black, so color alone isn’t always definitive. Texture and smell are more reliable clues.
Iron supplements and bismuth subsalicylate (the active ingredient in Pepto-Bismol) can also stain stool black without any bleeding involved, which adds to the confusion. If you’re taking either of these and notice very dark stool, the supplement is the likely explanation.
When the Color Matters
A single episode of dark green stool after a big spinach salad or a round of iron supplements is nothing to worry about. The same goes for green stool during a short bout of diarrhea. If the color persists for more than a few days after you’ve stopped eating green foods and aren’t taking iron, or if it’s accompanied by persistent diarrhea, abdominal pain, or fever, that’s worth getting checked. These symptoms together could point to an infection, inflammatory condition, or malabsorption issue that needs attention.
True black, tarry stool with an unusually foul smell warrants prompt medical evaluation regardless of what you’ve been eating, since upper GI bleeding requires treatment.