Green stool is usually harmless and caused by something you ate. The most common explanation is a diet high in green vegetables or foods with green and blue dyes. In some cases, though, green stool signals that food is moving through your digestive system faster than normal, or that an infection is affecting your gut.
Why Stool Is Normally Brown
Your liver produces bile, a yellow-green fluid that helps break down fats during digestion. As bile travels through the intestines, bacteria in your gut convert it into a pigment called stercobilin, which is brown. This conversion takes time. If everything moves at a normal pace, your stool ends up the familiar brown color by the time it reaches the end of its journey.
When food passes through your intestines too quickly, those gut bacteria don’t have enough time to fully convert the green bile pigment into brown stercobilin. The result is stool that retains some or all of that original green color. This is why diarrhea from any cause, whether it’s a stomach bug, food intolerance, or stress, often comes out green.
Foods and Drinks That Turn Stool Green
The simplest explanation for green stool is chlorophyll, the pigment that makes plants green. Eating large amounts of spinach, kale, broccoli, or avocados can tint your stool noticeably. Pistachios are another common culprit, also because of their chlorophyll content. Matcha and other powdered green teas can do the same. Even blueberries, despite being dark purple, can produce shades of green as the pigments interact with digestive enzymes.
Food dyes are another frequent cause. Brightly frosted cupcakes, rainbow candy, and drinks with blue or green coloring can all change stool color within a day or two. If your stool turned green a day after eating something unusually colorful, the dye is almost certainly the reason. The color typically returns to normal within one to three bowel movements once the food clears your system.
Iron Supplements and Antibiotics
Iron supplements are one of the most reliable stool-color changers. They can turn stool dark green or even blackish-green, which sometimes alarms people who aren’t expecting it. This is a normal side effect of unabsorbed iron reacting with digestive fluids, not a sign of internal bleeding. The color change typically starts within a few days of beginning supplementation and continues as long as you take it.
Certain antibiotics can also tint stool yellow or green. Antibiotics disrupt the normal balance of gut bacteria, which are the very organisms responsible for converting green bile into brown pigment. With fewer bacteria doing that work, more green pigment passes through unchanged. If your stool turns green during a course of antibiotics, it should return to its usual color once you finish the medication and your gut bacteria repopulate.
Rapid Transit and Digestive Upset
Any condition that speeds up digestion can produce green stool, even without an infection. Stress, anxiety, irritable bowel syndrome, and food intolerances all accelerate the movement of food through your intestines. When transit time drops significantly, bile doesn’t have the hours it needs for bacterial breakdown, and stool comes out green.
People who have had their gallbladder removed sometimes notice more frequent green stools. Without the gallbladder to store and release bile gradually, bile flows directly into the small intestine in a more continuous stream. This can overwhelm the gut’s ability to process it, especially after fatty meals, leading to looser, greener stool.
Infections That Cause Green Stool
Bacterial infections like Salmonella and E. coli can produce green diarrhea. These infections inflame the intestinal lining, trigger rapid transit, and disrupt the normal gut bacteria that would otherwise convert bile to its brown form. Green stool from an infection is rarely the only symptom. It typically comes alongside cramping, nausea, fever, or watery diarrhea that lasts more than a day or two.
Parasitic infections such as Giardia can also cause prolonged greenish, foul-smelling diarrhea. Viral gastroenteritis (stomach flu) may produce green stool simply because everything is moving through you so fast. In all of these cases, the green color itself isn’t the concern. What matters is the combination of symptoms: how long the diarrhea lasts, whether you see blood or mucus, and whether you can stay hydrated.
Green Stool in Babies
Green stool in newborns and infants is extremely common and almost always normal. A baby’s very first bowel movements, called meconium, are sticky and greenish-black. During the first six weeks of life, both breastfed and formula-fed babies regularly produce stool that ranges from mustard yellow to green to light brown.
Some infant formulas can make a baby’s stool dark green. Breastfed babies sometimes produce bright, frothy green poo, which can happen when they take in more of the thinner, lower-fat milk at the beginning of a feeding (foremilk) and less of the fattier milk that comes later (hindmilk). This foremilk-hindmilk imbalance isn’t dangerous, but if it happens consistently and your baby seems gassy or fussy, adjusting feeding patterns, such as finishing one breast before switching, often helps.
When Green Stool Signals a Problem
An isolated episode of green stool with no other symptoms is almost never a reason for concern. It’s worth paying closer attention if the green color persists for more than a few days without an obvious dietary explanation, or if it comes with symptoms like persistent diarrhea, abdominal pain, fever, or unexplained weight loss.
The color that should prompt immediate attention isn’t green. Bright red stool may indicate bleeding in the lower digestive tract, and black, tarry stool can signal bleeding higher up, in the stomach or esophagus. If you see either of those colors and haven’t recently eaten beets, taken iron, or consumed something with red dye, that warrants prompt medical evaluation. Green, by comparison, sits comfortably in the range of normal variation for stool color.