Green stool is almost always caused by something harmless: the food you ate, a supplement you’re taking, or digestion that moved a little faster than usual. Your liver continuously produces bile, a yellow-green fluid that helps digest fat. As bile travels through your intestines, bacteria chemically transform it, shifting its color from green to brown. Anything that disrupts that process, whether by speeding up transit or overwhelming it with green pigment, can leave your stool looking green.
How Bile Gives Stool Its Normal Color
Your liver releases bile into the upper part of your small intestine every time you eat. Bile starts out yellow-green because of a pigment called bilirubin, which comes from the natural breakdown of old red blood cells. As bile moves through about 20 feet of intestine, gut bacteria go to work on that bilirubin, adding hydrogen atoms to it in a series of chemical reductions. The end product is a brown pigment called stercobilin, which is what gives healthy stool its characteristic color.
This conversion takes time. If anything shortens how long food spends in your intestines, the bacteria don’t finish the job, and the stool retains some of bile’s original green color. That single mechanism explains most cases of unexpectedly green stool.
Foods That Turn Stool Green
The most common cause is simply eating a lot of green or deeply pigmented food. Spinach, kale, broccoli, arugula, and other leafy greens contain high concentrations of chlorophyll, the pigment that makes plants green. Eat enough of it in a salad, smoothie, or juice and the chlorophyll passes through your system largely intact, tinting your stool. Matcha, wheatgrass shots, and green-colored health supplements have the same effect.
Artificial food dyes can do it too. Blue and green dyes used in candy, frosting, sports drinks, popsicles, and grape soda mix with the yellow pigments already in your digestive tract and produce a green result. A 2021 study had participants eat muffins dyed with royal blue food coloring and found the dye clearly changed stool color within a day or two. If your green stool appeared shortly after eating something brightly colored, that’s very likely the explanation, and it resolves on its own once the dye clears your system.
Rapid Transit and Diarrhea
When food moves through the large intestine too quickly, bile doesn’t have time to fully break down. The result is stool that still carries bile’s green tint. This is why green stool frequently accompanies diarrhea, regardless of the underlying cause. A stomach bug, food poisoning, stress, caffeine, or even a particularly large meal can all speed things up enough to produce green output. The color itself isn’t the problem here; it’s just a visible sign that transit was faster than normal.
Iron Supplements and Medications
Iron supplements are a well-known cause of unusual stool color. Iron can speed up intestinal movement, giving bile less time to convert from green to brown. On top of that, iron reacts with digestive juices in your gut to form iron oxide, which can tint stool dark green or black. Liquid and chewable iron formulations sometimes contain colorants that add to the effect. If you recently started an iron supplement and noticed a color change, it’s a normal side effect and not a sign of a problem.
Some antibiotics can also cause green stool by disrupting the balance of gut bacteria responsible for converting bile pigments. With fewer of those bacteria available, bile passes through less fully processed.
Infections That Cause Green Stool
Certain infections cause a rapid “gush” of unabsorbed bile through the intestines, producing green diarrhea. Bacterial infections like Salmonella and E. coli, viral infections like norovirus, and parasites like Giardia all fall into this category. The green color comes from the same mechanism as any other fast-transit situation, but infections tend to bring additional symptoms: cramping, fever, nausea, or watery diarrhea that lasts more than a day or two. If your green stool comes with those symptoms, especially after traveling, eating undercooked food, or drinking untreated water, an infection is worth considering.
Green Stool in Babies
Green stool in infants is extremely common and rarely concerning. Breastfed babies may produce green stool if they don’t finish feeding on one side before switching. The milk that comes first (foremilk) is lower in fat than the richer milk that follows (hindmilk), and that difference in fat content can change how bile is processed during digestion. Babies on protein hydrolysate formula, which is designed for infants with milk or soy allergies, also tend to have green stool. So do breastfed newborns whose guts haven’t yet developed a full population of the bacteria that convert bile pigments to brown.
Diarrhea in babies produces green stool for the same reason it does in adults: faster transit means less bile conversion. The meconium that newborns pass in their first few days is also dark green to black, which is completely normal.
When Green Stool Signals Something Deeper
A single episode of green stool, or even a few days of it, is almost never a medical concern. The color change usually traces back to something you ate or a temporary digestive speedup. It becomes worth investigating if green stool persists for more than a few weeks without an obvious dietary explanation, or if it’s accompanied by persistent diarrhea, fever, significant abdominal pain, or visible blood or mucus.
Chronic green stool can occasionally point to fat malabsorption, where your body isn’t properly digesting and absorbing dietary fat. This can reflect issues with the liver, gallbladder, pancreas, or intestinal lining. To evaluate this, a doctor may order a fecal fat test, which measures the amount of unabsorbed fat in a stool sample. A normal result is less than 7 grams of fat per 24 hours. Stool cultures can identify bacterial or parasitic infections if an infectious cause is suspected.
For the vast majority of people, though, the answer is straightforward: you ate a big spinach salad, took your iron pill, drank a blue sports drink, or had a bout of diarrhea. Once the cause passes, the color returns to normal within a day or two.