Dark green poop is almost always caused by something you ate, a supplement you’re taking, or food moving through your digestive system faster than usual. It’s rarely a sign of something serious. To understand why it happens, it helps to know what gives stool its normal brown color in the first place, and what can interrupt that process.
How Stool Gets Its Color
Your liver produces a yellow-green substance called bile, which gets released into your small intestine to help digest fats. Bile contains a pigment called bilirubin, and as food travels through your intestines, bacteria break bilirubin down into new compounds. The final product, stercobilin, is a dark orange pigment that gives stool its characteristic brown color.
This breakdown process takes time. Food spends about six hours passing through your stomach and small intestine, then another 36 to 48 hours moving through the large intestine. If anything speeds up that timeline, the bacteria in your gut don’t get enough time to fully convert bile’s green pigments into brown ones. The result: your stool comes out green.
Fast Transit: The Most Common Cause
When food moves through your colon too quickly, bile stays partially intact and stains your stool green. This is the single most common explanation for unexpectedly green poop, and it often happens alongside loose stools or mild diarrhea. Anything that accelerates digestion can trigger it: stress, a stomach bug, too much coffee, a meal that didn’t agree with you, or even vigorous exercise.
If your green stool coincided with a bout of diarrhea and then returned to normal within a day or two, fast transit was likely the reason. No action needed.
Foods That Turn Stool Green
Chlorophyll, the pigment that makes plants green, can do the same to your stool when you eat enough of it. The biggest culprits are dark leafy greens like spinach, kale, and broccoli. Avocados, fresh herbs, and matcha (powdered green tea) can also contribute. The more you eat in a single sitting, the more vivid the color change.
Artificial food dyes are another common cause. Brightly colored frosting, candy, sports drinks, or anything with synthetic green or blue dye can tint your stool a surprising shade of green. The dye continues coloring whatever it touches as it moves through your digestive tract. If you recently ate something with vivid coloring, that’s your likely answer.
Iron Supplements and Medications
Iron supplements are one of the most reliable ways to turn your stool dark green or even black. Whether you take iron as a standalone tablet, a liquid, or as part of a multivitamin, darkened stool is a well-known and harmless side effect. The color change comes from unabsorbed iron reacting with other compounds in your gut.
Certain over-the-counter stomach medications can produce a similar effect. Products containing bismuth subsalicylate (the active ingredient in Pepto-Bismol and Kaopectate) can turn stool very dark green or black. Some antibiotics also alter stool color, shifting it toward green or yellow by disrupting the gut bacteria responsible for bile breakdown.
Gut Infections
Several common infections produce green-colored diarrhea. Salmonella (from contaminated food), giardia (a waterborne parasite), and norovirus (the classic “stomach bug”) all cause stool to turn green. In these cases, the green color happens for two reasons at once: the infection speeds up transit time so bile can’t be fully processed, and the infection itself disrupts normal bacterial activity in the gut.
The giveaway that an infection is involved is usually obvious. You won’t just notice a color change. You’ll also have watery or frequent diarrhea, cramping, nausea, or fever. Most of these infections resolve on their own within a few days, though persistent symptoms or signs of dehydration warrant a call to your doctor.
Bile Acid Malabsorption
In some cases, green stool is caused by bile acids that never get reabsorbed by the small intestine. Normally, your body recycles most of the bile it produces, reabsorbing it near the end of the small intestine. When this process fails, excess bile passes into the colon and colors the stool green.
This can happen with liver or gallbladder disease, after surgical removal of the gallbladder, or in people with disorders affecting the small intestine (like Crohn’s disease). If your stool is persistently green and you haven’t changed your diet or started new supplements, bile acid malabsorption is worth discussing with a doctor, especially if you also have chronic diarrhea.
Green Stool in Babies
If you’re a parent searching this question about your infant, the explanation is usually straightforward. A newborn’s very first bowel movements, called meconium, are sticky and greenish-black. This is completely normal and clears within the first few days of life.
After that initial period, green stool in formula-fed babies is often caused by iron-fortified formula. Check the label on your baby’s formula. If it contains added iron, the green color is an expected side effect and not a concern.
When Green Stool Signals a Problem
Green stool on its own, even very dark green, is almost never dangerous. The colors that do warrant immediate medical attention are bright red and black (truly black, like tar, not just dark green). Both can indicate bleeding somewhere in the digestive tract.
Green stool paired with ongoing diarrhea lasting more than a few days, fever, significant abdominal pain, or signs of dehydration is worth getting checked out. The same applies if your stool has been persistently green for weeks without an obvious dietary explanation. In most cases, though, the answer is sitting on your plate or in your medicine cabinet.