Green stool is almost always caused by something harmless: a food you ate, a supplement you’re taking, or a temporary bout of diarrhea that rushed everything through your gut too fast. The explanation comes down to bile, a yellow-green digestive fluid your liver produces to break down fats. Normally, bacteria in your intestines chemically transform bile from green to brown as it travels through. When that process gets interrupted, your stool stays green.
Why Stool Is Normally Brown
Your liver continuously produces bile, which starts out yellow-green. As bile enters your intestines, gut bacteria go to work on it. First, they strip it down using specialized enzymes. Then a bacterial enzyme called bilirubin reductase converts it into a compound called urobilinogen, which is further broken down into the brown pigment that gives stool its typical color.
This entire conversion depends on having the right bacteria present and enough transit time for them to do their job. Anything that disrupts either of those factors can leave your stool looking green. In healthy adults, the enzyme responsible for this conversion is present in nearly everyone. But in infants under one year old, it’s frequently absent, which is one reason green baby poop is so common.
Foods That Turn Stool Green
The most straightforward cause is eating a lot of green or dye-heavy foods. Chlorophyll, the pigment that makes plants green, can pass through your digestive tract and tint your stool directly. Common culprits include spinach, kale, broccoli, avocados, pistachios, matcha, and fresh herbs. You don’t need to eat an unusual amount. A large salad or a green smoothie can be enough.
Artificial food coloring is another frequent trigger. Brightly frosted cupcakes, candy, sports drinks, or anything with blue or green dye can produce a surprisingly vivid green the next day. Food dye tends to pass through within one to three bowel movements, so your stool color should return to normal once the dye clears your system.
Rapid Transit and Diarrhea
When food moves through your large intestine faster than usual, bile doesn’t have enough time to complete its chemical transformation from green to brown. This is probably the most common non-food explanation. Any cause of diarrhea, whether it’s a stomach bug, food intolerance, stress, or too much coffee, can speed things along enough to produce green stool.
You’ll often notice this during the first day or two of a gastrointestinal illness. As your digestion normalizes and transit time slows back down, the color returns to brown on its own. If you’re experiencing diarrhea alongside green stool, staying hydrated matters more than worrying about the color.
Infections That Cause Green Stool
Certain infections cause green stool specifically because they trigger rapid, watery diarrhea. Salmonella, norovirus, and the waterborne parasite giardia are well-known examples. These pathogens irritate the gut lining and cause it to flush contents through quickly, so bile passes out before bacteria can process it. The green color itself isn’t the infection. It’s a side effect of how fast everything is moving.
With most of these infections, you’ll also have cramping, nausea, or fever, so the green stool is rarely the only symptom. If it is, an infection is unlikely to be the cause.
Iron Supplements and Antibiotics
Iron supplements are a particularly common cause of unusual stool color, though they’re better known for turning stool black. The green version happens because iron can sometimes speed up intestinal movement, giving bile less time to break down fully. Some liquid or chewable iron formulations also contain colorants that contribute to the green tint. If your stool is a matte dark green or black while taking iron, that’s a normal side effect. A glossy, tarry black stool is different and worth flagging to your doctor, as it can indicate bleeding higher in the digestive tract.
Antibiotics can also cause green stool by disrupting the gut bacteria responsible for converting bile. Since the key enzyme that turns bile brown is produced by specific intestinal bacteria, a course of antibiotics can temporarily knock those populations down. People with inflammatory bowel disease are especially susceptible: over 30% of IBD patients lack sufficient levels of the bile-converting enzyme, compared to just 0.1% of the general population. This means green stool during a flare or after antibiotic use is more common in that group.
Green Stool in Babies
Green poop in infants is extremely common and rarely a concern. Newborns pass meconium in the first few days, which is dark green to black. As they transition to milk feeding, stool lightens but can remain green for several reasons. Breastfed babies who don’t fully finish one breast before switching may get more of the lower-fat foremilk, which can affect how the milk is digested and produce green stool. Babies on protein hydrolysate formula (used for milk or soy allergies) also tend to have green stool as a normal response to that formulation.
The underlying biology plays a role too. Infants haven’t yet built up the full population of gut bacteria needed to convert bile pigments to brown. Most babies develop adequate levels of the necessary enzyme by around one year of age, and stool color stabilizes after that.
When Green Stool Signals a Problem
Isolated green stool with no other symptoms is almost never a medical concern. It becomes worth paying attention to when it persists for more than a few days without an obvious dietary explanation, or when it’s accompanied by other symptoms like fever, severe cramping, blood in the stool, or signs of dehydration. In children, dehydration is the more immediate risk during any illness that produces both diarrhea and green stool, so watching for reduced urination, dry lips, or lethargy matters more than the stool color itself.