Why Is My Stool Green?
Green stool is one of the most common colour variations people notice, and it is almost always harmless. However, the frequency and context of green stool matter. An occasional green bowel movement after a large salad is meaningless, while persistently green stool without an obvious dietary explanation may point to a digestive issue worth investigating.
To understand green stool, you need to understand bile. Bile is produced by the liver and stored in the gallbladder. When you eat, bile is released into the duodenum to help digest fats. Fresh bile is bright yellow-green. As it travels through the intestines, bacteria chemically transform the bile pigments from green to yellow to brown. If this transformation is interrupted or overwhelmed, stool retains its green colour.
Dietary Causes: The Most Common Explanation
The simplest and most frequent cause of green stool is eating large quantities of green or green-pigmented foods:
- Leafy greens — spinach, kale, rocket, Swiss chard, and broccoli contain chlorophyll that can colour stool green when consumed in large amounts
- Green smoothies and juices — concentrated green vegetable drinks are a very common cause
- Matcha and green tea — in large quantities
- Green food colouring — found in sweets, icing, and certain processed foods
- Iron supplements — iron reacts with digestive fluids to produce dark green or black stool
If you can identify a clear dietary source and the green colour resolves within one to two days after removing that food, no further investigation is needed.
Rapid Transit Time
The second most common cause of green stool is food moving through the colon faster than normal. As explained above, bile starts green and is gradually transformed to brown by bacterial action as it moves through the large intestine. When transit is rapid, bile arrives at the rectum before this transformation is complete, producing green stool.
Rapid transit green stool is typically also loose or watery. Common triggers include:
- Gastroenteritis or food poisoning
- Stress and anxiety, which accelerate colonic motility
- Excessive caffeine intake
- IBS-D flares
- Use of laxatives or magnesium supplements in excess
Bile Acid Malabsorption
A less recognised but important cause of persistently green, loose stool is bile acid malabsorption (BAM). Normally, approximately 95 percent of bile acids are reabsorbed in the terminal ileum and recycled back to the liver. When this reabsorption fails, excess bile acids reach the colon, where they stimulate water and electrolyte secretion, causing watery, green-tinged diarrhoea.
BAM affects an estimated one in three people diagnosed with IBS-D but is rarely tested for. It can result from ileal inflammation (as in Crohn's disease), ileal resection, cholecystectomy (gallbladder removal), or idiopathic causes where the feedback mechanism controlling bile production malfunctions.
Infections and Parasites
Certain infections can produce distinctly green stool:
- Salmonella — can produce green, foul-smelling diarrhoea alongside fever and cramping
- Giardia — this waterborne parasite classically produces greasy, green, extremely foul-smelling stool with bloating and sulphurous belching
- Clostridioides difficile — C. diff infection can produce green, watery diarrhoea with a characteristic odour, particularly after recent antibiotic use
Infection-related green stool is usually accompanied by other symptoms such as fever, cramping, nausea, and urgency. If these are present, a stool culture or PCR test can identify the pathogen.
Antibiotic-Related Green Stool
Antibiotics disrupt the gut bacteria responsible for transforming bile pigments from green to brown. During and shortly after a course of antibiotics, green stool is common and usually resolves as the microbiome recovers over the following weeks. Supporting microbiome recovery with fermented foods and probiotic supplementation can accelerate the return to normal stool colour.
When Green Stool Needs Attention
Green stool warrants investigation if it persists for more than two weeks without a clear dietary explanation, is accompanied by diarrhoea that does not resolve, occurs with weight loss or dehydration, or follows a recent course of antibiotics and is accompanied by severe cramping. Tracking your stool colour alongside your diet using GutIQ makes it straightforward to distinguish dietary causes from something that may need medical evaluation.