Dark green stool is almost always harmless. The color typically comes from something you ate, a supplement you’re taking, or food moving through your intestines faster than usual. In most cases, it resolves on its own within a day or two once the cause passes through your system.
To understand why stool turns green, it helps to know why it’s normally brown in the first place, and what interrupts that process.
Why Stool Is Normally Brown
Your liver produces bile, a digestive fluid that starts out yellow-green. As bile travels through your intestines, specific bacteria break down its pigments through a chain of chemical reactions. Only four known bacterial species in the gut can perform this conversion, including members of the Clostridium and Bacteroides families. The end product of their work is a compound called stercobilin, which gives stool its characteristic brown color.
When anything disrupts this process, whether it’s food moving too quickly, antibiotics killing off those bacteria, or an overwhelming amount of green pigment from food, your stool can stay green instead of completing that transition to brown.
Green Vegetables and Chlorophyll
The most common reason for dark green stool is simply eating a lot of green foods. Spinach, kale, broccoli, avocados, herbs, and matcha all contain high levels of chlorophyll, the pigment that makes plants green. Eat enough of it, and that same pigment colors your stool. Even pistachios can contribute, since their green color comes from chlorophyll and related plant compounds.
You don’t necessarily need to eat an unusual amount. A large spinach salad, a green smoothie packed with kale, or a day of particularly vegetable-heavy meals can be enough. The darker and more concentrated the green in the food, the more noticeable the effect.
Food Dyes and Processed Foods
Blue and purple food dyes are a surprisingly common culprit. When blue dye mixes with the yellow-green bile in your digestive tract, the result is green stool. This means foods you wouldn’t think of as “green” can cause the change: blue sports drinks, grape-flavored candy, purple frosting on a birthday cake, or brightly colored cereals. The effect is temporary and clears once the dye works its way through.
Iron Supplements
Iron supplements are well known for changing stool color. When iron reacts with digestive juices in your gut, it creates dark pigments that can turn stool anywhere from dark green to black. This happens with most oral forms of iron, including both pills and liquid formulations. Liquid and chewable iron supplements may also contain colorants that contribute to the green tint.
If the color change bothers you, splitting your daily dose into two smaller doses can reduce the effect. Different iron formulations (such as ferrous fumarate versus ferrous sulfate) may also produce less dramatic color changes. The dark color itself is not a sign that anything is wrong, it just means the iron is passing through your system.
Rapid Transit Through the Intestines
When food moves through your large intestine faster than normal, bile doesn’t have enough time to be fully broken down by gut bacteria. The result is stool that retains bile’s original green color. This is one of the reasons diarrhea often appears green regardless of what you ate.
Anything that speeds up digestion can cause this effect: a stomach virus, food poisoning, stress, caffeine, or simply a meal that didn’t agree with you. The green color in these cases is really just a sign of speed, not a separate problem.
Antibiotics and Medications
Antibiotics can turn stool green by wiping out the gut bacteria responsible for converting bile pigments to brown. Research has shown that certain antibiotic combinations can completely eliminate the bile-processing compounds from stool, leaving it green or yellow-green for the duration of treatment. Some antibiotics also speed up intestinal motility, compounding the effect.
Other medications, including certain laxatives and anti-inflammatory drugs, can also alter stool color. The change typically resolves within a few days to a couple of weeks after finishing the medication, as gut bacteria repopulate.
Bile Acid Malabsorption
In some people, bile acids aren’t properly reabsorbed in the small intestine. This condition, called bile acid malabsorption, causes excess bile to reach the colon, where it irritates the lining and triggers the colon to secrete extra water. The result is frequent, watery, urgent diarrhea that can appear green due to the high concentration of unprocessed bile.
This is more common in people who have had their gallbladder removed, though it can also occur with inflammatory bowel conditions or after certain intestinal surgeries. If you’re experiencing chronic watery diarrhea with cramping and urgency, especially after gallbladder removal, bile acid malabsorption is worth discussing with your doctor. It’s treatable but often underdiagnosed.
Green Stool in Babies
Green stool in breastfed infants often relates to the balance of milk they receive during feeding. Breast milk changes composition throughout a feeding session: the earlier milk (foremilk) is higher in lactose, while the later milk (hindmilk) is richer in fat. If a baby gets a large volume of foremilk without enough hindmilk, the excess lactose can cause green, watery, or foamy stools. This is sometimes called a foremilk-hindmilk imbalance and is more common when mothers have an oversupply of milk.
In formula-fed babies, iron-fortified formula is a frequent cause of green stool, following the same mechanism as iron supplements in adults. Occasional green stools in an otherwise healthy, growing baby are rarely a concern.
When the Color Matters
Green stool, even dark green, is considered a normal variation. The Mayo Clinic notes that all shades of brown and green fall within the typical range, and stool color alone rarely indicates a serious intestinal condition.
The colors that do warrant prompt attention are bright red and black. Bright red stool can indicate bleeding in the lower digestive tract, while true black, tarry stool (as opposed to the dark green-black from iron supplements) can signal bleeding higher up, such as in the stomach. If your stool is accompanied by severe abdominal pain, fever, blood, or the color change persists for more than a few days without an obvious dietary explanation, it’s worth getting checked out. But if you had a kale smoothie yesterday or started iron supplements last week, you likely have your answer.