Green poo is almost always harmless. It usually means food moved through your gut faster than normal, you ate something packed with green pigment, or you’re taking a supplement like iron. All shades of brown and even green are considered typical stool colors.
To understand why green happens, it helps to know why poo is usually brown in the first place.
How Poo Gets Its Normal Brown Color
Your liver produces bile, a digestive fluid that starts out yellow-green. Bile contains a pigment called bilirubin, and when it’s released into your small intestine to help digest fats, bacteria in your gut get to work on it. These bacteria add hydrogen atoms to bilirubin through a series of chemical reactions, eventually converting it into a brown pigment called stercobilin. That final product is what gives healthy stool its familiar color.
The key detail: this conversion takes time. Bacteria need hours in the large intestine to fully break down bile’s green pigments into brown ones. Anything that shortens that window can leave you with green stool instead.
Fast Transit Is the Most Common Cause
When food and waste move through your large intestine faster than usual, bile doesn’t have time to complete its color change. The green pigments from early in the process pass through largely intact. 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.
Infections speed things up dramatically. Bacterial infections like Salmonella or E. coli, viral infections like norovirus, and parasites like Giardia can all cause a rapid “gush” of unabsorbed bile through the intestines. In these cases, green stool is just a visible side effect of your body flushing out the infection quickly. It’s the diarrhea itself, not the color, that matters most.
Green Foods and Drinks
Eating a lot of green vegetables is one of the simplest explanations. Spinach, kale, and broccoli are rich in chlorophyll, the pigment that makes plants green, and your body doesn’t fully break it down during digestion. Eat enough of these foods and chlorophyll will tint your stool bright green. The same goes for avocados, fresh herbs, pistachios, and matcha (powdered green tea), all of which carry enough chlorophyll to leave a visible mark.
Artificial food dyes can do this too. Blue or green dyes used in candy, frosting, sports drinks, and brightly colored cereals pass through your digestive system and mix with the yellow-green bile already present, sometimes producing a surprisingly vivid green. If your stool turns green a day after eating something with heavy food coloring, that’s your answer.
Iron Supplements and Medications
Iron supplements commonly turn stool a very dark green that can look almost black. This is normal and some physicians actually consider it a sign the supplement is being absorbed properly. If the color bothers you, reducing the dose with your doctor’s guidance will typically lighten things up. Certain antibiotics can also cause green stool by disrupting the gut bacteria responsible for converting bile pigments to brown. Once you finish the course, your normal bacterial balance restores itself and the color returns.
Green Stool in Babies
Parents often notice green poo in infants, and it’s rarely a concern. Newborns pass meconium, a dark greenish-black substance, in their first few days of life. After that transition, several things can keep stool green. Breastfed babies who don’t finish feeding entirely on one side may miss the higher-fat hindmilk, which changes how the milk is digested and can produce green stool. Babies on protein hydrolysate formula, used for milk or soy allergies, also tend to have greener poo.
Breastfed infants sometimes have green stool simply because they haven’t yet developed the full community of gut bacteria needed to convert bile pigments to brown. This resolves on its own as their microbiome matures. Diarrhea in babies, just like in adults, can also cause green stool through the same rapid-transit mechanism.
When Green Stool Deserves Attention
A single green bowel movement, or even a few days of green stool after a big salad or a bout of mild diarrhea, is not a red flag. Green stool that persists for more than a few days without an obvious dietary explanation, especially if it comes with fever, cramping, or ongoing diarrhea, may point to an infection worth investigating.
The colors that genuinely warrant prompt medical attention are bright red and black. Bright red can indicate bleeding in the lower digestive tract, while black, tarry stool can signal bleeding higher up, such as in the stomach. Green, by contrast, sits well within the range of normal variation.