Dark green poop is almost always harmless. It typically means you’ve eaten a lot of green vegetables, taken an iron supplement, or your food moved through your digestive system faster than usual. In most cases, your stool color will return to its normal brown within a day or two once the cause passes.
Why Stool Is Normally Brown
Your liver produces bile, a bright green fluid that helps you digest fats. As bile travels through your digestive tract, enzymes chemically break it down, gradually shifting its color from green to yellow to brown. That final brown shade is what most people see in a healthy bowel movement. When something interrupts or speeds up that process, the green color of bile shows through in the finished product.
Green Vegetables and Chlorophyll
The most common reason for dark green stool is simply eating a lot of green foods. Spinach, kale, and broccoli are high in chlorophyll, the pigment that makes plants green, and that same pigment can color your stool. Avocados, fresh herbs, matcha (powdered green tea), and even pistachios contain enough chlorophyll to have the same effect. The more you eat in a single sitting or over the course of a day, the more vivid the green.
This is completely normal and doesn’t mean the food wasn’t digested properly. You’re just seeing leftover plant pigment that your body didn’t absorb.
Food Dyes and Artificial Colors
Blue and purple food dyes can mix with the yellow-green bile in your gut and produce a surprisingly dark green result. Think frosted cupcakes, brightly colored candy, grape-flavored drinks, or blue sports drinks. If your stool turns green a day after eating something with heavy artificial coloring, that’s almost certainly the explanation.
Iron Supplements and Medications
Iron supplements are a well-known cause of darker stool. The color can range from dark green to nearly black, depending on the dose and form of iron you’re taking. This is common and usually nothing to worry about. However, if your stool turns truly black and looks tarry or sticky, or if you notice red blood, that’s a different situation. Black, tar-like stool can signal bleeding in the upper digestive tract and warrants a call to your doctor, especially if you also feel unwell.
Antibiotics can also cause green stool by disrupting the balance of gut bacteria that normally help break down bile. Once you finish the course, your stool color typically returns to normal.
Fast Transit Time
When food moves through your large intestine faster than usual, bile doesn’t have enough time to fully break down. The result is stool that still carries bile’s original green color. This happens most often with diarrhea, whether caused by a stomach bug, food intolerance, stress, or something you ate that didn’t agree with you. If the green color shows up alongside loose or watery stools, rapid transit is the likely culprit.
Infections and Gut Bugs
Certain infections cause a rapid “gush” of unabsorbed bile through the intestines, producing green diarrhea. Common culprits include Salmonella and E. coli (bacterial infections often linked to contaminated food), norovirus (the classic stomach flu), and Giardia, a waterborne parasite. With these infections, green stool is usually accompanied by other obvious symptoms: cramping, nausea, fever, or watery diarrhea that lasts more than a day or two.
If you have green diarrhea with a fever, bloody stool, or signs of dehydration (dizziness, dark urine, extreme thirst), that points toward an infection that may need medical attention rather than a simple dietary cause.
Green Stool in Babies
Green poop in infants is common and has its own set of causes. Breastfed babies sometimes produce green stool if they don’t finish feeding on one side before switching. The earlier milk in a feeding session is lower in fat, and missing the higher-fat “hind milk” can affect how the milk is digested, leading to green output. Babies on hypoallergenic formula (protein hydrolysate formulas used for milk or soy allergies) also tend to have greener stool. In newborns, the gut is still being colonized by bacteria, and the lack of a mature bacterial community can leave bile partially unprocessed.
Green baby poop on its own, without fever, unusual fussiness, or refusal to eat, is rarely a concern.
Dark Green vs. Black: An Important Distinction
The color that should get your attention isn’t green. It’s black. Very dark green stool from vegetables or supplements may look alarming in dim bathroom lighting, but there’s an important difference between dark green and true black, tarry stool. Tarry black stool has a distinct sticky consistency and an unusually strong smell. It can indicate bleeding in the stomach or upper intestine, where blood is digested along the way and turns dark. If you’re unsure whether what you’re seeing is very dark green or actually black, consider what you’ve eaten in the past 24 to 48 hours. A big spinach salad or iron pill is a reassuring explanation. No obvious dietary cause, combined with that tar-like texture, is worth a phone call to your doctor.