Green and runny stool almost always means food is moving through your digestive system faster than normal. Your liver produces bile, a green fluid that helps break down fats. Normally, bacteria in your intestines transform that bile from green to brown as it travels through your colon. When something speeds up the process, like an infection or a dietary trigger, bile doesn’t have time to complete that color change, and you end up with green, watery stool.
How Bile Makes Your Poop Green
Your body constantly recycles old red blood cells. During that process, a green pigment called biliverdin is produced, which gets converted into bilirubin and secreted into bile. As bile moves through your intestines, bacteria break bilirubin down further into compounds that are orange and brown. This is what gives healthy stool its typical color.
When stool moves through quickly, as it does during a bout of diarrhea, those bacteria simply don’t have enough contact time to finish the job. The bile stays green, and so does everything that comes out. This is the single most common reason green and runny stool appear together: rapid transit time through the gut.
Foods and Supplements That Turn Stool Green
Sometimes the color has nothing to do with transit time and everything to do with what you ate. Chlorophyll, the pigment that makes plants green, can do the same to your stool. Common culprits include spinach, kale, broccoli, avocados, matcha, fresh herbs, and even pistachios (which get their color from chlorophyll and other plant pigments). If you ate a large salad or a green smoothie recently, that alone could explain the color.
Artificial food dyes are another frequent cause. Bright frosting, candy, sports drinks, or anything with heavy coloring continues tinting your stool long after you eat it. Blue and green dyes are especially noticeable. Iron supplements can also turn stool dark green or nearly black, which catches many people off guard when they start a new supplement.
If the green color came from food or supplements, it’s harmless and will resolve once you stop eating that particular item or finish your course of supplements.
Infections That Cause Green Diarrhea
When green, runny stool arrives alongside cramping, nausea, or fever, an infection is the more likely explanation. Bacterial infections like Salmonella and parasitic infections like Giardia are well-known for producing watery, greenish diarrhea. Viral stomach bugs can do the same by inflaming the intestinal lining and speeding up digestion dramatically.
Infections cause green stool through the same basic mechanism: inflammation and irritation push contents through so fast that bile stays green. But infections also increase the volume of fluid your intestines secrete, which is why the stool is both green and watery rather than just discolored. Most infectious diarrhea resolves within a few days on its own, though staying hydrated during that time is critical.
Antibiotics and Gut Bacteria Disruption
If you recently started an antibiotic, that’s a likely explanation. Antibiotics can kill off large numbers of the normal, harmless bacteria in your colon. Those are the same bacteria responsible for converting green bile into brown pigments. With fewer of them working, bile passes through with its green color intact. Some antibiotics also irritate the gut lining directly, causing loose stools on their own.
In most cases, antibiotic-related diarrhea is mild and clears up after you finish the course. Occasionally, though, wiping out beneficial bacteria allows a harmful species called C. difficile to multiply unchecked. C. difficile produces toxins that damage the intestinal wall, causing persistent, watery diarrhea that can become severe. If diarrhea worsens while you’re on antibiotics or starts within a few weeks of finishing them, that warrants a conversation with your doctor.
Green Stool in Babies
Green, runny poop in infants is common and usually not a concern. Breastfed babies may produce green stool if they don’t finish feeding on one side before switching. The earlier milk (foremilk) is lower in fat, and when a baby gets mostly foremilk without the higher-fat hindmilk, it can speed digestion and produce green, loose stools. Letting your baby finish one breast before offering the other often resolves this.
Babies on specialized formula, particularly protein hydrolysate formulas used for milk or soy allergies, also tend to have green stools. This is a normal byproduct of how the formula is processed and digested. Breastfed newborns may also have green stool simply because their gut bacteria haven’t fully established yet. As the microbiome matures over the first weeks and months, stool color typically shifts toward the more familiar mustard yellow.
Staying Hydrated During Diarrhea
The bigger concern with green, runny stool isn’t usually the color. It’s the fluid loss. Diarrhea pulls water and electrolytes out of your body quickly, and dehydration can set in faster than most people expect, especially in children and older adults.
The World Health Organization recommends oral rehydration solutions (a mixture of clean water, salt, and sugar) as the primary treatment for diarrhea-related dehydration. These solutions are absorbed in the small intestine and replace both water and electrolytes lost in stool. You can buy pre-made oral rehydration packets at most pharmacies. Sports drinks are a distant second choice because they contain more sugar than is ideal, but they’re better than plain water alone when electrolytes are being lost rapidly.
Watch for signs of serious dehydration: sunken eyes, extreme drowsiness, very dark urine, or skin that stays tented when you pinch it rather than snapping back immediately. In infants, fewer than six wet diapers a day or no tears when crying are red flags.
When Green Diarrhea Needs Attention
A day or two of green, runny stool after a dietary change or a mild stomach bug is normal and resolves on its own. The Mayo Clinic recommends contacting a healthcare provider if green stool persists for more than a few days, particularly if it’s accompanied by diarrhea.
Seek more urgent attention if you notice blood in your stool, jet-black stool (which can indicate bleeding higher in the digestive tract), a high fever, severe abdominal pain, or signs of dehydration that aren’t improving with oral fluids. Unintentional weight loss alongside persistent changes in stool color or consistency also warrants investigation, as it can point to conditions affecting nutrient absorption.