Seven mechanisms that turn stool green, ranked from most to least common.
Green stool almost always has an everyday explanation. Here are the causes, from the most common to the rarest, with the mechanisms that produce the colour.
When stool moves through the colon faster than usual, the bacteria that normally convert green bile into brown pigment do not have time to act — so the stool exits still green. This is why green stool and loose stool travel together. Acute diarrhoea affects most adults about once a year, and the green colour simply confirms the speed. It resolves as the diarrhoea settles, usually within 1–3 days.
Spinach, kale, broccoli, spirulina, wheatgrass, and green smoothies contain chlorophyll, a green pigment that can survive digestion and colour the stool. A large green salad or a daily green smoothie is a very common explanation — especially if the stool is otherwise normal in consistency. No action needed beyond knowing the cause.
Green, blue, and purple food colourings (in sweets, icing, drinks, and even some cereals) are not fully absorbed and can dye stool green. Blue or purple dye mixed with yellow bile can also read as green. If the colour appeared after a bag of green or blue sweets, that is your answer.
Iron supplements commonly darken stool and can turn it green or nearly black. Certain antibiotics (especially those used for gut infections), bismuth products (Pepto-Bismol — which usually turns stool black), and some laxatives can also change colour. If the colour change started when the medication did, the medication is the explanation.
Antibiotics temporarily change the gut's bacterial population — the very bacteria that convert bile to brown. Around 5–25% of people taking antibiotics develop diarrhoea, and green stool can persist for a week or two as the microbiome recovers. Probiotics and time usually restore normal colour.
Conditions that speed bile through the gut — bile acid malabsorption, gallbladder removal, or a bout of food poisoning — leave more green bile in the stool. Bile acid malabsorption is increasingly recognised as a cause of chronic diarrhoea, affecting a meaningful share of people with IBS-type symptoms. Green, watery stool with urgency after gallbladder surgery is a classic presentation.
Some infections (for example giardiasis and salmonella) cause rapid transit with green, watery stool, often with cramps, fever, or nausea. The green colour itself is not diagnostic — the accompanying symptoms are what matter. Most infectious diarrhoea clears within a week; persistent cases need a stool test.
Green stool is a colour, not a diagnosis — the underlying story (diarrhoea, diet, medication) is what matters. The one scenario that deserves real attention is persistent green, watery stool, which can indicate bile malabsorption or a lingering infection rather than a one-off. See when to see a doctor for the thresholds.
Most causes are harmless, but these versions need attention:
If any fit, use the Doctor page.
The pathway is mostly history. The clinician will ask what you ate in the previous 48 hours, what medications and supplements you take, whether the stool is loose or formed, how long the colour has persisted, and whether fever, pain, or blood accompany it. That conversation resolves most cases — green food, iron, antibiotics, or a stomach bug each have a clear signature. A stool sample is added when diarrhoea persists beyond a few days, especially with fever or recent travel, to check for infection. Blood tests appear only when there are signs of malabsorption — weight loss, fatigue, or deficiency. Bile acid malabsorption, the main cause of persistent green watery stool in otherwise healthy people, is diagnosed by response to treatment rather than by any single dramatic test.