Green stool is usually caused by something you ate, particularly leafy green vegetables, foods with green dye, or iron supplements. Less commonly, it happens because food moved through your intestines too quickly for bile to fully break down. In most cases, it resolves on its own within a day or two and isn’t a sign of anything serious.
Why Stool Is Normally Brown
The brown color of normal stool comes from a pigment called stercobilin, and the process that creates it starts with your liver. When your body breaks down old red blood cells, it produces a green pigment called biliverdin. An enzyme called biliverdin reductase converts that green pigment into bilirubin, which is yellow-orange. Your liver then sends bilirubin into the intestine through bile.
Once bilirubin reaches your gut, bacteria finish the job. A single bacterial enzyme called bilirubin reductase converts bilirubin into urobilinogen, which then oxidizes into stercobilin, the dark orange-brown pigment that gives stool its familiar color. This entire chain depends on two things: enough time for gut bacteria to do their work, and a healthy population of those bacteria in the first place. When either of those conditions isn’t met, bilirubin (or even biliverdin) can pass through without being fully converted, leaving stool with a greenish tint.
Foods That Turn Stool Green
The most common cause of green stool is simply diet. Eating large amounts of chlorophyll-rich vegetables like spinach, kale, and broccoli can overwhelm the brown pigments your gut normally produces. The green chlorophyll essentially “wins” the color competition. You don’t need to eat an unusual amount for this to happen. A big salad, a green smoothie, or a couple of servings of cooked spinach can be enough.
Food dyes are another frequent culprit. Green or purple dyes found in drink mixes, ice pops, cake frosting, and candy can color your stool directly. This is especially common in children, who tend to consume more brightly colored foods and drinks. The effect is harmless and disappears once the dye clears your system, typically within one to two bowel movements.
Iron Supplements
Iron supplements are well known for changing stool color. Most people expect iron to turn stool black, and it often does at higher doses. But iron can also produce a dark green color, particularly at moderate doses or when combined with other foods. This is a normal chemical reaction between the iron and your digestive fluids, not a sign of a problem. If you’ve recently started taking iron and notice green or dark stool, the supplement is the likely explanation.
Rapid Transit Through the Gut
When food moves through your intestines faster than usual, bile doesn’t have enough time to be fully broken down by gut bacteria. Bile starts out green (from biliverdin) and only turns brown through a series of bacterial conversions that take time. If that transit is rushed, the stool comes out green.
Anything that speeds up digestion can cause this. Diarrhea is the most obvious example, regardless of its cause. A stomach bug, food that didn’t agree with you, stress, or even a strong cup of coffee on an empty stomach can push things through fast enough to produce green stool. If the diarrhea is brief and resolves on its own, the green color will too.
Infections That Cause Green Stool
Certain infections specifically trigger the kind of rapid intestinal flushing that leads to green diarrhea. Salmonella, a bacterial infection usually picked up from contaminated food, is one of the more common ones. The waterborne parasite giardia is another. Norovirus, the highly contagious stomach bug responsible for outbreaks on cruise ships and in schools, can also produce green-tinged stool. In all these cases, the green color happens because the infection causes your intestines to push contents through before bile pigments can be fully processed.
These infections typically come with other symptoms: nausea, cramping, fever, or watery diarrhea. The green stool itself isn’t the concern. It’s the infection and the risk of dehydration that matter. If diarrhea lasts more than a couple of days or you develop signs of dehydration like dizziness, dry mouth, or reduced urination, that warrants medical attention.
Bile Acid Malabsorption
In some people, green stool is a recurring issue rather than a one-time event. One possible explanation is bile acid malabsorption, a condition where bile acids aren’t properly reabsorbed in the lower part of the small intestine. Normally, about 95% of bile acids get recycled back to the liver. When that reabsorption fails, excess bile acids flood the colon, causing chronic watery diarrhea that can appear green.
This condition is associated with Crohn’s disease (particularly when it affects the lower small intestine), surgical removal of part of the ileum, radiation damage to the gut, and gallbladder removal. If you’ve had any of these and experience persistent loose, greenish stools, bile acid malabsorption could be the underlying cause. It’s treatable once diagnosed.
Green Stool in Babies
Green stool in infants is extremely common and rarely a cause for concern. Newborns pass meconium in their first few days of life, which is dark green to black. After that, breastfed babies can have stool that ranges from yellow to green depending on feeding patterns.
One specific cause in breastfed infants: if a baby doesn’t finish feeding on one breast before switching to the other, they may get more of the thinner, lower-fat foremilk and less of the higher-fat hindmilk. This imbalance can affect digestion and produce green stool. Babies who lack the typical balance of intestinal bacteria (which is normal in the early weeks of life) may also have green stool simply because those bacteria haven’t yet colonized in sufficient numbers to convert bile pigments to brown.
Formula-fed babies can develop green stool from protein hydrolysate formulas, which are specially designed for infants with milk or soy allergies. Iron-fortified formulas can have the same effect. In all of these cases, the green color is expected and not a sign of illness. If green stool in an infant is accompanied by diarrhea, fever, or poor feeding that lasts more than a few days, it’s worth a call to your pediatrician.
When Green Stool Signals a Problem
On its own, green stool is almost never an emergency. The color itself is not dangerous. What matters is the context: how long it lasts and what other symptoms come with it. Green stool that persists for more than a few days without an obvious dietary explanation is worth mentioning to a healthcare provider. Green stool accompanied by diarrhea raises the risk of dehydration, which is the more immediate concern, especially in young children and older adults.
The combination of green stool with high fever, severe abdominal pain, bloody stool, or signs of dehydration (extreme thirst, dark urine, lightheadedness) points toward an active infection or inflammatory condition that needs evaluation. A single episode of green stool after a spinach-heavy dinner is a completely different situation from weeks of green, loose stools with cramping.