Dark green poop is almost always harmless and caused by something you ate, a supplement you’re taking, or food moving through your digestive system faster than usual. In most cases, the color returns to normal within a day or two without any intervention.
How Stool Gets Its Color
Your liver produces bile, a digestive fluid that starts out green. As bile travels through your intestines, bacteria break it down into a compound called stercobilin, which gives stool its typical brown color. This process takes time. The average transit through the colon alone is 30 to 40 hours, and up to 72 hours is still considered normal.
When food moves through your gut faster than usual, bile doesn’t get fully processed. It stays partially green, tinting your stool anywhere from bright green to a deep, dark green. This is the single most common explanation for green poop that isn’t linked to diet, and it can happen after a stomach bug, a period of stress, increased physical activity, or even a large cup of coffee on an empty stomach.
Foods That Turn Stool Green
Chlorophyll, the pigment that makes plants green, can do the same to your stool. Spinach, kale, broccoli, arugula, and green smoothies are frequent culprits. The more you eat in a single sitting, the more vivid the result. A big salad at dinner can produce noticeably green stool the next morning.
Artificial food dyes are another common cause, and they’re easy to overlook. Blue and purple dyes (found in grape soda, sports drinks, candy, frosted baked goods, and some cereals) mix with the yellow-green bile already in your gut and create a dark green color. If your stool turned green without any leafy greens in your recent meals, think back to anything brightly colored you ate or drank in the past 24 to 48 hours.
Iron Supplements and Medications
Iron supplements are one of the most reliable causes of dark green or even black stool. Two things happen at once: iron can speed up intestinal movement, which gives bile less time to convert to brown, and the iron itself reacts with digestive juices to produce a dark pigment. The combination often results in very dark green stool, especially in the first week or two of supplementation.
If you’ve recently started iron tablets and the color change is bothering you, splitting your daily dose into two smaller amounts can reduce both the stool changes and the stomach upset that often comes with it. Taking iron alongside vitamin C (a glass of orange juice works) improves absorption, which means a lower dose can be equally effective.
Certain antibiotics can also cause green stool by disrupting the gut bacteria responsible for converting bile to its brown form. This typically resolves once the course of antibiotics is finished and your gut bacteria repopulate.
Rapid Transit and Digestive Upset
Diarrhea from any cause, whether it’s food poisoning, a viral illness, or anxiety, pushes food through the intestines too quickly for bile to fully break down. The faster the transit, the greener the result. This is why a bout of stomach flu often produces watery, green stool even if you haven’t eaten anything green.
Bile acid malabsorption is a less common but underrecognized condition where your intestines fail to reabsorb bile acids properly. Excess bile acids flood the colon, causing watery stool, urgency, and sometimes a greenish color. This can occur on its own or alongside conditions like Crohn’s disease or after gallbladder removal. If you’re experiencing persistent watery diarrhea with urgency and it doesn’t match anything in your diet, bile acid malabsorption is worth discussing with a gastroenterologist.
Dark Green Stool in Newborns
If you’re a new parent searching this, the answer is different. A newborn’s first bowel movements are a substance called meconium: a dark brown or green, tar-like stool that’s been building up in the baby’s intestines before birth. Meconium typically passes within the first 24 to 48 hours of life. Over the next few days, stool transitions to a yellowish-green color as the baby starts digesting milk, eventually settling into the mustard-yellow shade that’s normal for breastfed infants. This progression is completely expected.
When the Color Matters
An isolated episode of dark green stool with no other symptoms is rarely a concern. The color alone isn’t a problem. What matters is the context around it.
Pay attention if green stool lasts more than a few days without an obvious dietary explanation, or if it comes with fever, severe abdominal pain, blood or mucus in the stool, or unintentional weight loss. These combinations can signal an infection, inflammatory bowel disease, or a malabsorption issue that needs evaluation.
One important distinction: very dark green stool can sometimes look black under bathroom lighting. True black, tarry stool (with a distinctive foul smell) can indicate bleeding in the upper digestive tract and warrants prompt medical attention. If you’re unsure whether your stool is very dark green or actually black, checking under brighter light or placing a small amount on a white paper towel can help you tell the difference.