Dark green stool is almost always caused by something you ate, a supplement you’re taking, or food moving through your digestive system faster than usual. It’s rarely a sign of anything serious. The most common culprit is a large serving of leafy greens like spinach or kale, but iron supplements, food dyes, and even infections that speed up digestion can all turn your poop noticeably green.
How Bile Makes Stool Green
Your liver produces bile, a bright green fluid that helps break down fats during digestion. As bile travels through your intestines, gut bacteria gradually break it down and change its color from green to yellowish-brown. That’s why normal stool is brown: the bile has had enough time to be fully processed on its journey through your digestive tract.
When something interrupts that process, the green color sticks around. This can happen in two ways: either you’ve eaten something with so much green pigment that it overwhelms the color change, or food is moving through your intestines too quickly for bacteria to finish converting the bile. Both result in dark green stool.
Foods That Turn Stool Dark Green
Chlorophyll is the pigment that makes plants green, and eating a lot of it is the single most common reason for green stool. Spinach, kale, and broccoli are the biggest offenders, but avocados, pistachios, fresh herbs, and matcha (powdered green tea) all contain enough chlorophyll to shift your stool color noticeably. The darker and more concentrated the green food, the more dramatic the effect. A big spinach salad or a green smoothie packed with kale can produce stool that looks almost forest green within a day or two.
Blueberries and blackberries can also cause dark green stool, which surprises people who expect them to produce purple or blue. The deep blue pigments in these fruits mix with the yellow-green bile already in your gut, and the combination often reads as dark green rather than blue. Artificial food coloring, especially green and blue dyes found in candy, frosting, freeze pops, and brightly colored drinks, does the same thing.
Supplements and Medications
Iron supplements are a well-known cause of dark green (and sometimes black-green) stool. Your body only absorbs a portion of the iron you swallow, and the unabsorbed iron oxidizes as it passes through your intestines, tinting your stool dark green or black. This is harmless and expected. If you’ve just started an iron supplement and notice the color change, that’s why.
Antibiotics can also produce green stool through a different mechanism. A strong course of antibiotics kills off some of the gut bacteria responsible for converting bile from green to brown. With fewer bacteria doing that job, more green bile pigment survives the trip through your intestines. Laxatives and any medication that speeds up digestion can have a similar effect by reducing transit time.
Infections and Rapid Digestion
When your body is fighting a gastrointestinal infection, it often pushes food through your intestines much faster than normal. Bacterial infections like Salmonella and E. coli, viral infections like norovirus, and parasites like Giardia can all cause this rapid transit. The result is watery diarrhea that looks green because bile hasn’t had time to change color. If your dark green stool comes with diarrhea, cramping, fever, or nausea, an infection is a more likely explanation than anything you ate.
Conditions that cause chronic malabsorption or ongoing rapid transit, like celiac disease or irritable bowel syndrome during a flare, can also produce persistently green stool. In these cases, the green color tends to come and go rather than appearing once and resolving.
Dark Green Stool in Babies
If you’re a parent noticing dark green in your baby’s diaper, it’s almost certainly normal. A newborn’s very first stools (meconium) are thick, black, and tarry, then transition through dark green before settling into the yellow, seedy stools typical of breastfed babies or the tan-brown stools of formula-fed babies. Even after that transition, dark green poop in infants is usually just bile doing its job. Children’s Hospital Colorado notes that dark green baby poop is “completely normal” and “usually caused by bile.”
When the Color Matters
A single episode of dark green stool after a big salad or a new supplement is nothing to worry about. The color should return to brown within a day or two once the food or supplement clears your system. If green stool persists for two weeks or more without an obvious dietary explanation, that’s worth investigating, since it could point to an ongoing absorption issue or a low-grade infection.
The colors that do warrant immediate attention are bright red and black. Bright red stool can indicate bleeding in the lower digestive tract, while true black, tarry stool (distinct from the dark green-black of iron supplements) can signal bleeding higher up, like in the stomach. Dark green, on its own, is not a red flag. It’s one of the most common temporary color changes in stool and, in the vast majority of cases, resolves on its own once whatever caused it passes through your system.