Dark green poop is almost always harmless. The most common causes are eating green vegetables, taking iron supplements, or having food move through your digestive tract faster than usual. In most cases, stool color returns to its typical brown within a day or two once the trigger passes.
Why Poop Is Normally Brown
Your liver produces bile, a bright yellow-green fluid that helps break down fats in the small intestine. As bile travels through the roughly 25 feet of intestine below, bacteria in your gut chemically transform it into a pigment called stercobilin, which is brown. This conversion takes time. The average transit through the colon alone is 30 to 40 hours, with anything up to about 72 hours considered normal. When that process completes without interruption, your stool comes out the familiar shade of brown.
Dark green stool simply means something interrupted or overwhelmed that color conversion, either by adding green pigment from food, speeding up transit so bile didn’t fully break down, or altering the gut bacteria that do the converting.
Foods That Turn Stool Green
Chlorophyll is the pigment that makes plants green, and eating enough of it will tint your stool. The most common culprits are spinach, kale, and broccoli, but avocados, fresh herbs, and matcha (powdered green tea) can do the same. Pistachios get their green color from chlorophyll too, so eating a large handful can have the same effect. Blueberries, surprisingly, can also produce green shades rather than blue.
Artificial food dyes are another frequent cause. Brightly frosted baked goods, green-colored candy, or drinks with food coloring can produce vivid stool colors. If you eat enough multicolored candy, the dyes can even mix together and produce near-black stool, which looks alarming but is purely cosmetic.
Iron Supplements and Medications
Iron supplements are one of the most reliable stool-color changers. The NHS notes that darker-than-usual stool is a common and expected side effect of taking iron. The color can range from dark green to nearly black, depending on the dose and the form of iron. This is harmless unabsorbed iron passing through and is not a reason to stop taking your supplement.
There is one important distinction: if your stool looks black and tarry (sticky, with a distinct foul smell) rather than simply dark, that can indicate bleeding in the upper digestive tract. If you’re taking iron and notice a tarry consistency along with feeling unwell, that warrants medical attention.
Certain antibiotics can also tint stool green or yellow. Antibiotics disrupt the normal gut bacteria responsible for converting bile pigments to brown stercobilin. With fewer bacteria doing that job, bile passes through in its original green state. Stool color typically normalizes after you finish the course of antibiotics and your gut bacteria repopulate.
Rapid Transit and Diarrhea
When food moves through your intestines faster than normal, bile doesn’t have enough contact time with gut bacteria to fully convert from green to brown. This is why diarrhea from any cause, whether it’s a stomach bug, food intolerance, or stress, often produces green stool. The faster things move, the greener the result. You might notice this during a bout of food poisoning or even after drinking too much coffee on an empty stomach.
Infections from bacteria like salmonella or parasites like giardia can cause prolonged green diarrhea because they both speed transit and inflame the intestinal lining, further reducing bile processing. If green diarrhea lasts more than a couple of days, comes with fever, or includes blood or mucus, that points toward an infection worth investigating.
Green Stool in Babies
Green poop in newborns and infants is especially common and rarely a concern. A newborn’s very first stools, called meconium, are thick, black, and tarry. Within the first few days, as breastfeeding or formula feeding begins, stool transitions to green or yellow with a more liquid consistency. This is a normal and expected progression.
Breastfed babies in particular can have stool that ranges from mustard yellow to bright green, sometimes within the same day. A green tint in a baby who is feeding well, gaining weight, and otherwise comfortable is not a problem. Foremilk (the thinner milk at the start of a feeding) contains less fat and can produce greener stool, while hindmilk (the fattier milk later in the feeding) tends to produce yellower stool.
When Green Stool Signals a Problem
Isolated dark green stool with no other symptoms is almost never dangerous. The situations worth paying attention to involve accompanying symptoms, not the color itself:
- Persistent diarrhea lasting more than 2 to 3 days, especially with fever or cramping, which may indicate a bacterial or parasitic infection.
- Black, tarry, or sticky stool that isn’t explained by iron supplements or dark foods like blueberries. This appearance can signal bleeding in the stomach or upper intestine.
- Bright red blood in or on the stool, which points to bleeding lower in the digestive tract.
- Unintended weight loss or ongoing pale/clay-colored stool, which can indicate problems with bile production or liver function.
If your stool is dark green and you recently ate a spinach salad, started an iron supplement, or had a bout of stomach upset, you can confidently chalk it up to one of those causes. The color should return to brown within one to three bowel movements once the trigger is gone.