Green stool is almost always harmless. It typically means food moved through your digestive system faster than usual, or you ate something with strong green pigment. In most cases, your stool will return to its normal brown color within a day or two without any treatment.
Why Stool Is Normally Brown
The brown color of stool comes from a surprisingly long chemical journey. Your liver breaks down old red blood cells and converts part of them into a green pigment called biliverdin. This gets secreted into bile and passes to your gallbladder, where it transforms into bilirubin, which is yellow. From there, bilirubin enters your intestines, and bacteria in your large intestine gradually break it down into brown-colored compounds. These final pigments are what give stool its characteristic color.
When anything disrupts this process, whether food passes through too quickly for bacteria to finish the job, or a strong pigment overwhelms the brown, your stool can come out green.
Rapid Transit: The Most Common Cause
If food moves through your intestines faster than normal, bile doesn’t have enough time to fully break down from green to brown. This is the single most frequent reason for green stool, and it happens during bouts of diarrhea, stomach bugs, food intolerances, or even just a stressful day that speeds up your gut. You might also notice the stool is looser than usual, which makes sense since the colon had less time to absorb water.
Foods That Turn Stool Green
Chlorophyll, the pigment that makes plants green, can do the same to your stool. Eating a lot of spinach, kale, broccoli, avocados, herbs, or matcha can produce noticeably bright green stool. Pistachios are another common culprit, since they’re packed with chlorophyll along with other plant pigments. The more you eat, the more vivid the color change tends to be.
Artificial food dyes are another frequent cause. Brightly colored frosting, candy, sports drinks, and flavored cereals can tint stool green, blue-green, or other unexpected shades. If you eat enough of multiple colors, they can even mix together and produce very dark or black-looking stool, which can be alarming but is harmless in that context.
Medications and Supplements
Iron supplements commonly turn stool dark green, sometimes so dark it looks nearly black. This is normal and some physicians actually consider it a sign the supplement is being absorbed properly. If the color bothers you, a lower dose may reduce the effect.
Certain antibiotics can also tint stool green or yellow-green. Antibiotics alter the balance of bacteria in your gut, and since those bacteria are responsible for converting bile pigments to brown, disrupting them can leave stool greener than usual. This typically resolves after you finish the course of medication.
Green Stool in Babies
Green stool in infants is common and has several specific causes. Newborns pass meconium in their first few days, which is a dark greenish-black substance that’s completely normal. As feeding gets established, stool color can vary widely.
Breastfed babies sometimes produce green stool if they don’t finish feeding on one side before switching. This means they get more of the thinner, lower-fat milk at the start of a feeding and miss the higher-fat milk that comes later, which can affect digestion and stool color. Breastfed infants may also have green stool simply because they haven’t yet developed a full population of intestinal bacteria to complete the bile conversion process.
Babies on specialized protein hydrolysate formulas, used for milk or soy allergies, frequently have green stool as well. And of course, diarrhea in infants produces green stool for the same rapid-transit reason it does in adults.
When Green Stool Signals a Problem
Green stool on its own, even for a couple of days, is rarely a concern. But certain accompanying symptoms change the picture. If green stool comes with persistent diarrhea, fever, abdominal pain, or visible blood or mucus, something more may be going on. Bacterial infections like Salmonella, parasites like Giardia, and inflammatory bowel conditions can all produce green stool alongside these other symptoms.
If your stool is bright red or black (and you haven’t been eating beets, taking iron, or consuming dark-colored foods), that could indicate bleeding in your digestive tract and warrants prompt medical attention. Green stool that continues for more than a few days without an obvious dietary explanation is also worth bringing up with a healthcare provider.
For the vast majority of people, though, the answer is simple: you ate something green, something moved through your system quickly, or a supplement changed the color. Once the cause passes, so does the color.