Green poop is almost always harmless. The most common cause is simply that food moved through your intestines faster than usual, which prevented bile from completing its normal color change from green to brown. Diet, supplements, and mild infections can also turn your stool green.
Why Stool Is Normally Brown
Your liver produces bile, a yellowish-green fluid that gets stored in your gallbladder and released into your small intestine to help break down fats. As bile travels through your digestive tract, bacteria in your colon break it down into a compound called stercobilin, which gives stool its characteristic brown color. About 80% of bile pigment gets converted this way before leaving your body.
When anything disrupts that process, whether by speeding up digestion or overwhelming the gut with green pigments, stool can come out green instead.
Rapid Transit: The Most Common Cause
If food passes through your large intestine too quickly, bile doesn’t have enough time to be fully broken down by gut bacteria. The result is stool that still carries bile’s original green color. This is why diarrhea from any cause, whether it’s a stomach bug, food intolerance, or stress, often produces green stool. The green color itself isn’t the problem; it’s just a visible sign that digestion was rushed.
Foods That Turn Stool Green
Eating large amounts of chlorophyll-rich foods is the other everyday explanation. Spinach, kale, broccoli, and other dark leafy greens contain enough plant pigment to tint your stool noticeably, especially if you’ve recently started a new smoothie habit or increased your vegetable intake. Green food coloring in processed foods, candy, and frosted baked goods can do the same thing. The color change typically lasts a day or two after you stop eating the food in question.
Supplements and Medications
Iron supplements are one of the most reliable stool-color changers, turning poop dark green or even black. This is a normal chemical reaction between iron and digestive enzymes, not a sign of a problem.
Several other supplements can also cause green stool:
- Chlorophyll-based supplements like wheatgrass, spirulina, barley grass, chlorella, and blue-green algae
- Herbal laxatives such as senna, cascara sagrada, and rhubarb
- Fiber supplements
- Yerba mate tea
Certain medications cause green stool indirectly by triggering diarrhea as a side effect. Antibiotics are a common culprit because they disrupt the gut bacteria responsible for converting bile pigments to brown. When those bacterial populations are temporarily reduced, stool stays greener. Metformin and some antidepressants can have a similar effect through increased intestinal motility.
Infections and Digestive Illness
Bacterial and parasitic infections sometimes produce green diarrhea. Salmonella, for example, causes intense inflammation in the intestinal lining that speeds up transit dramatically. Parasitic infections like giardia can do the same. In these cases, green stool is just one symptom alongside cramping, nausea, fever, or watery diarrhea. The green color comes from the same mechanism: bile rushing through too fast to change color. If your green stool comes with a fever, severe abdominal pain, or lasts more than a few days, that pattern points toward an infection rather than a dietary cause.
Green Stool in Babies
Green poop in infants is common and usually not concerning, but the causes differ from adults. Newborns pass meconium in their first few days, a dark greenish-black stool that’s entirely normal. After that transition, green stool in breastfed babies can happen when the baby doesn’t finish feeding on one side, missing the higher-fat milk that comes later in a feeding. That fat content affects how the milk is digested and what color comes out the other end.
Babies on protein hydrolysate formula, which is used for milk or soy allergies, also tend to produce green stool. So do breastfed infants who haven’t yet developed a full population of intestinal bacteria, since those bacteria are needed to convert bile to its brown form. Diarrhea in babies causes green stool for the same transit-time reason it does in adults.
When Green Poop Signals a Problem
A single episode of green stool, or even a few days of it after a dietary change, is not a medical concern. The color alone is almost never dangerous. What matters more is the context around it. Green stool paired with persistent diarrhea lasting more than two or three days, blood or mucus in the stool, significant abdominal pain, or fever suggests something beyond diet. Ongoing green stool with no clear dietary explanation and no resolution over a week or two is also worth investigating, as it could point to a malabsorption issue or chronic infection affecting how your gut processes bile.
If you recently started a new supplement, ate a pile of spinach, or had a brief bout of stomach illness, the green will resolve on its own once the cause passes. Most people see their stool return to brown within one to three days.