Green poo is almost always harmless. The most common reasons are eating lots of leafy greens, consuming foods with blue or green dye, or having food move through your gut faster than usual. In rare cases it signals an infection, but if you’re otherwise feeling fine, the color alone isn’t a concern.
Why Poo Is Normally Brown
Your liver produces bile, a yellow-green fluid that helps digest fats. When bile enters your small intestine, it starts out green. As it travels through the rest of your digestive tract, bacteria break it down into a pigment called stercobilin, which is orange-brown. That pigment is what gives stool its typical color.
The key factor is time. This chemical conversion from green to brown doesn’t happen instantly. It requires bacteria in your gut to add hydrogen atoms to the bile pigment across several steps. If food moves through your intestines faster than normal, bile doesn’t get fully processed, and your stool stays green. This is why diarrhea from any cause, whether it’s a stomach bug or something you ate, often comes out green.
Foods That Turn Stool Green
The most straightforward explanation is your diet. Chlorophyll, the pigment that makes plants green, can do the same to your stool when you eat enough of it. Spinach, kale, and broccoli are the usual suspects, but avocados, fresh herbs, matcha, and even pistachios contain enough chlorophyll to shift the color noticeably. You don’t have to eat an unusual amount. A big salad or a green smoothie can be enough.
Blueberries and blackberries are a less obvious trigger. Their deep blue pigments can mix with yellow-green bile and produce shades of green rather than the brown you’d expect. Artificial food coloring is another common cause. Green and blue dyes found in candy, cake frosting, fruit snacks, freeze pops, and brightly colored drinks will tint your stool on the way through. If you eat enough rainbow-colored candy, the mixed dyes can even turn stool black.
In all of these cases, the color change is temporary. Once the food clears your system (usually within a day or two), your stool returns to its normal brown.
Medications and Supplements
Iron supplements are one of the most common non-food causes of green (or very dark green, almost black) stool. Iron reacts with digestive enzymes and changes color as it passes through. If you’ve recently started an iron supplement and noticed a color change, that’s the likely explanation.
Antibiotics can also cause green stool, but for a different reason. They disrupt the gut bacteria responsible for converting bile from green to brown. With fewer bacteria doing that job, bile passes through less processed, keeping its original green tint. This usually resolves once you finish the course and your gut bacteria recover.
Infections and Faster Transit
When a stomach bug speeds everything through your digestive tract, bile doesn’t have time to change color. That’s why green diarrhea is common with food poisoning, viral gastroenteritis, and parasitic infections. The green color itself isn’t dangerous. It’s simply a sign that your gut is moving things along too quickly for the normal chemical process to finish.
If an infection is responsible, you’ll almost certainly have other symptoms: watery diarrhea, cramping, nausea, or fever. The green color on its own, without diarrhea or other signs, points toward diet or supplements rather than infection.
Green Stool in Babies
Green poo 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. The milk that comes first in a feeding session is lower in fat than the milk toward the end. When a baby switches breasts too early or doesn’t drain one side, the lower-fat milk can affect digestion and produce a greener result.
Babies on specialty formulas, particularly protein hydrolysate formulas used for milk or soy allergies, also tend to have green stool. This is a normal side effect of the formula and not a sign of a problem. Newborn gut bacteria are still developing too, and the lack of a fully established bacterial community means bile conversion is less efficient in the early weeks of life. As the baby’s microbiome matures, stool color typically shifts to the expected yellow-brown range.
When Green Stool Needs Attention
A single green bowel movement, or even a few days of them after a big spinach binge, doesn’t need investigation. The Mayo Clinic recommends contacting a healthcare professional if green stool persists for more than a few days without an obvious dietary explanation.
The more important signals are what accompanies the color change. Green stool paired with persistent diarrhea raises the risk of dehydration, especially in young children and older adults. Fever, blood in the stool, significant abdominal pain, or unexplained weight loss alongside a color change are reasons to seek prompt evaluation. The green color alone is almost never the problem. It’s the context around it that matters.