Dark green stool is usually caused by something you ate, a supplement you’re taking, or food moving through your digestive system faster than normal. In most cases, it’s harmless and resolves on its own within a day or two. Understanding the specific cause helps you decide whether it’s worth paying attention to or simply the result of last night’s salad.
How Stool Gets Its Color
Your liver produces bile, a yellow-green fluid that helps digest fats. When bile is first secreted, it contains a pigment called bilirubin that gives it a yellowish color. As bile sits in the gallbladder or gets exposed to air, that pigment oxidizes into biliverdin, which is deep green. The bile stored in your gallbladder is typically a greenish-yellow to olive green.
Once bile enters your intestines, enzymes and bacteria chemically alter it as it travels through the digestive tract. This gradual breakdown is what shifts the color from green to the familiar brown you’re used to seeing. The entire process depends on time. If food moves through your system at a normal pace, bile completes its transformation. If something speeds that process up, bile doesn’t fully break down, and your stool stays green.
Foods That Turn Stool Dark Green
The most common explanation for dark green stool is diet. Chlorophyll, the pigment that makes plants green, can color your stool the same way. Eating large amounts of spinach, kale, broccoli, avocados, herbs, or matcha (powdered green tea) delivers enough chlorophyll to produce noticeably green stool. Pistachios can do the same, since their green color comes from chlorophyll along with other plant pigments.
Blueberries and blackberries are a less obvious culprit. Their deep blue pigments can temporarily turn brown stool green or even dark enough to look almost black in large quantities. Artificial food coloring, particularly green and blue dyes found in candy, frosting, freeze pops, and colored drinks, also continues tinting whatever it touches as it passes through your system. If you recently ate a cupcake with bright frosting or drank a vibrantly colored sports drink, that alone could explain what you’re seeing.
Diet-related green stool typically appears within 12 to 24 hours of eating the responsible food and clears up once that food has passed through your system.
Iron Supplements and Medications
Iron supplements are one of the most reliable causes of dark green to black stool. Unabsorbed iron reacts with compounds in your digestive tract, producing a dark color that can range from forest green to nearly black. This is a normal, expected side effect of oral iron tablets and not a reason for concern on its own. However, if you notice red streaks in the stool, sharp stomach pain, or cramping while taking iron, those are separate issues worth reporting to your doctor.
Antibiotics can also contribute to green stool by disrupting the balance of gut bacteria responsible for breaking down bile pigments. When those bacterial populations are temporarily reduced, bile passes through without being fully processed, leaving stool green. This typically resolves after you finish the antibiotic course and your gut bacteria recover.
Fast Digestion and Diarrhea
When food moves through your large intestine too quickly, bile doesn’t have time to complete its chemical transition from green to brown. Diarrhea is the most common reason for this. Whether triggered by a stomach bug, food intolerance, stress, or something else, any condition that accelerates digestion can leave you with green stool simply because transit time was too short for normal processing.
This is why green stool and diarrhea frequently show up together. The color itself isn’t the problem. It’s just a visible sign that your digestive system is running faster than usual. Staying hydrated matters here, since diarrhea can lead to dehydration quickly, especially in children and older adults.
Infections That Cause Green Stool
Certain gut infections produce green stool as part of a broader set of symptoms. Giardia, a waterborne parasite, causes smelly, greasy stools that can float and often appear green. Salmonella and other bacterial infections can also trigger rapid gut motility and watery diarrhea that comes out green because bile is being flushed through before it’s processed.
The distinguishing factor with infections is that green stool won’t be your only symptom. You’ll typically also have some combination of fever, cramping, nausea, and persistent watery diarrhea lasting several days. A single episode of green stool without other symptoms almost never points to an infection.
Green Stool in Babies
Parents often notice green stool in their infants, and it’s usually not alarming. Newborns pass meconium in their first days of life, which is dark green to black. After that transition, several things can keep stool green. Babies who don’t finish breastfeeding entirely on one side may miss some of the higher-fat hindmilk, which affects how the milk is digested and can produce green stool. Iron-fortified formula can darken stool color the same way iron supplements do in adults.
Babies on protein hydrolysate formula, used for milk or soy allergies, also commonly produce green stool. Breastfed infants who haven’t yet developed a full population of intestinal bacteria may have green stool simply because they lack the microbes needed to complete bile breakdown. Diarrhea in infants can cause green stool for the same transit-time reasons it does in adults, but dehydration is a more urgent concern in babies and warrants prompt attention.
When Green Stool Signals Something More
A single instance of dark green stool, especially if you can trace it to food, supplements, or a bout of diarrhea, is rarely concerning. The color alone doesn’t indicate disease. What matters is context: how long it lasts and what other symptoms accompany it.
Green stool that persists for more than a few days without an obvious dietary explanation is worth discussing with a healthcare provider. The same applies if green stool appears alongside high fever, bloody or tarry stools, significant abdominal pain, or signs of dehydration like dizziness, dark urine, or dry mouth. These combinations suggest something beyond normal digestion that may need evaluation.
For most people, the answer is straightforward: something you ate, something you’re taking, or a temporarily sped-up digestive system. Once the cause passes, your stool color returns to its usual brown within a day or two.