Dark green poop is almost always caused by something you ate, drank, or swallowed as a supplement. It looks alarming, but in the vast majority of cases it’s harmless and resolves on its own within a day or two. The most common culprits are leafy greens, iron supplements, and foods with artificial coloring.
Why Stool Turns Green in the First Place
Your liver produces bile, a yellowish-green fluid that helps digest fats. As bile travels through your intestines, bacteria break it down and gradually shift its color from green to the familiar brown you’re used to seeing. This process takes time. The average transit through the colon alone is 30 to 40 hours, and anything up to about 72 hours is considered normal.
When food moves through your system faster than usual, bile doesn’t have enough time to fully break down. The result is stool that still carries that green pigment. This is why diarrhea, stomach bugs, and anything that speeds up digestion can leave you with green poop, even if you haven’t eaten anything green at all.
Foods That Turn Stool Dark Green
Chlorophyll, the pigment that makes plants green, passes through your digestive system and can color your stool on the way out. Spinach, kale, and broccoli are the usual suspects, but the list is longer than most people realize. Avocados, fresh herbs, matcha, and even pistachios contain enough chlorophyll to shift your stool color noticeably, especially if you eat a large serving.
Blueberries can also produce greenish shades. And artificial food coloring is a surprisingly powerful tint. Brightly frosted cupcakes, colored drink mixes, ice pops, and candy continue dyeing whatever they touch as they move through your gut. Blue and purple dyes, in particular, can mix with the yellow of bile and create a deep green that looks unnatural.
Supplements and Medications
Iron supplements are one of the most reliable causes of dark green (sometimes almost black) stool. If you recently started taking iron, that’s very likely your answer. The color change is a normal side effect, not a sign that anything is wrong.
Several common medications can also be responsible:
- Antibiotics disrupt the normal balance of gut bacteria, which are the very microbes responsible for turning bile from green to brown. Without enough of them doing their job, stool stays greener.
- Bismuth subsalicylate, the active ingredient in some antidiarrheal medications, reacts with sulfur in your digestive tract and can produce dark green or black stool.
- Antacids containing aluminum hydroxide can cause greenish stool as a side effect.
Infections and Digestive Illness
When a stomach bug or food poisoning hits, your intestines push everything through much faster than normal. That rapid transit means bile stays green, and you may notice bright or dark green diarrhea. Bacterial infections like Salmonella and parasites like Giardia are known to cause this. The green color itself isn’t the problem in these cases. It’s really just a visual sign that your gut is moving things along at high speed.
If you have green diarrhea along with fever, cramping, or vomiting, the infection itself is what matters, not the color. Dehydration is the most immediate risk with any prolonged bout of diarrhea, so staying on top of fluid intake is the priority.
Dark Green vs. Black Stool
This distinction matters. Dark green stool from food, iron, or fast digestion is harmless. Truly black, tarry stool with a sticky texture and strong odor can signal bleeding in the upper digestive tract (the stomach or upper intestine). Blood that’s been digested turns very dark, almost like coffee grounds or tar.
The tricky part is that iron supplements and bismuth can also make stool look black, which creates confusion. If you’re taking either of those, the dark color is expected. If you’re not, and your stool looks genuinely black and tarry rather than just very dark green, that warrants prompt medical attention.
Green Stool in Babies
Green poop in infants is common and usually not a concern. Newborns pass meconium in their first days of life, which is naturally dark green to black. After that transitional period, several things can keep a baby’s stool green: switching to a protein hydrolysate formula (used for milk or soy allergies), not finishing a full feeding on one breast (which means the baby gets less of the higher-fat hindmilk), or simply having an immature gut that hasn’t yet developed a full population of the bacteria that turn bile brown. Diarrhea in babies can also produce green stool for the same transit-time reasons it does in adults.
When the Color Should Concern You
A single episode of dark green stool, or even a few days of it after a big salad or starting a new supplement, is not a red flag. The Mayo Clinic recommends contacting a healthcare provider if green stool persists for more than a few days without an obvious dietary explanation. You should also seek immediate care if green diarrhea is severe enough to cause signs of dehydration: dry mouth, dizziness, dark urine, or (in children) no tears when crying.
The fastest way to figure out your own cause is to think back 24 to 48 hours. Did you eat a large amount of greens, drink a green smoothie, or snack on something with food coloring? Did you start iron, an antibiotic, or an antacid? If the answer to any of those is yes, you almost certainly have your explanation. Give it a day or two of normal eating and the color will typically return to brown on its own.