Dark green stool is almost always harmless. The most common causes are foods rich in chlorophyll (like spinach or kale), iron supplements, and food moving through your intestines faster than usual. In most cases, the color returns to brown within a day or two once the trigger passes.
To understand why, it helps to know where brown stool color comes from in the first place, and what interrupts that process.
Why Stool Is Normally Brown
Your liver produces bile, a bright yellow-green fluid that helps digest fats. As bile travels through your digestive tract, bacteria in your intestines chemically break it down, gradually shifting its color from green to yellow to brown. That final brown pigment is what gives stool its typical color.
Anything that short-circuits this process, whether by adding green pigment directly or by rushing food through before bacteria can finish their work, can leave your stool looking green.
Foods That Turn Stool Dark Green
Chlorophyll is the most common culprit. It’s the pigment that makes plants green, and when you eat enough of it, it colors your stool the same way. Spinach, kale, and broccoli are the usual suspects, but avocados, pistachios, fresh herbs, and matcha (powdered green tea) are also packed with chlorophyll. A large salad or a green smoothie is more than enough to do it.
Blueberries and blackberries can also produce dark green stool. Their deep blue pigments mix with the yellow-green bile already in your intestines, and the combination reads as dark green rather than the purple you might expect.
Artificial food coloring is another frequent cause. Green and blue dyes found in candy, frosted cakes, freeze pops, and brightly colored drinks keep tinting whatever they touch all the way through your digestive tract. If you recently ate something with vivid coloring, that’s likely your answer.
Iron Supplements and Medications
Iron supplements are one of the most reliable ways to turn stool dark green or even blackish-green. The iron oxidizes as it moves through your gut, producing a deep, dark pigment. This is completely expected and not a sign that anything is wrong.
Some antibiotics can also shift stool color toward green or yellow-green. They do this by disrupting the normal gut bacteria responsible for converting bile pigments to brown. Once you finish the course of antibiotics and your gut bacteria recover, stool color typically returns to normal.
Fast Transit and Diarrhea
When food moves through your large intestine faster than usual, bile doesn’t have enough time to be fully broken down by bacteria. The result is stool that retains bile’s original green color. This is why diarrhea from any cause, whether it’s a stomach bug, food intolerance, or stress, often produces green stool.
Bacterial infections like Salmonella and E. coli, viral infections like norovirus, and parasites like Giardia can all cause this rapid “gush” of unprocessed bile through the intestines. In these cases, the green color is a side effect of the speed, not the infection itself. You’ll usually also have watery diarrhea, cramping, or nausea if an infection is involved.
After Gallbladder Removal
If you’ve had your gallbladder removed, green stool can become a recurring visitor. Without the gallbladder to store and concentrate bile, more bile acids flow directly into your large intestine. This excess bile acts as a mild laxative, speeding transit and sometimes leaving stool green. For most people this improves over weeks to months as the body adjusts, though some notice looser, greener stools occasionally for longer.
Green Stool in Babies
Green stool in infants is common and rarely a concern. Breastfed babies may produce green stool if they don’t fully empty one breast before switching to the other, since they miss some of the higher-fat milk that slows digestion. Babies on hydrolyzed formula (used for milk or soy allergies) also tend to have greener stool. Newborns who are exclusively breastfed sometimes produce green stool simply because their gut bacteria haven’t fully established yet.
If a baby has green diarrhea along with fever, reduced wet diapers, or irritability, that’s worth a call to the pediatrician. Otherwise, green on its own is just a color.
When Dark Green Becomes a Concern
Green stool by itself, with no other symptoms, is not a medical issue. It’s worth paying attention, though, if the color is so dark it looks almost black, especially if it’s also tarry or has a strong, foul smell. True black, tarry stool can indicate bleeding in the stomach or upper small intestine, and that requires immediate medical attention.
The distinction matters: iron supplements produce a dark greenish-black that’s firm or normal in texture, while bleeding produces stool that’s sticky, tar-like, and distinctly foul-smelling. If you’re taking iron, you can generally attribute the color to the supplement. If you’re not, and the stool looks tarry, that’s a different situation entirely.
Bright red blood in stool, lightheadedness, persistent abdominal pain, or fever alongside green diarrhea lasting more than a couple of days are all reasons to get evaluated. Temporary color changes that resolve on their own, which is the vast majority of cases, are not.