Green Stool: Causes, Symptoms & When to See a Doctor

Green stool is usually benign and diet-related, but persistent green diarrhea can indicate medical issues.

Common Benign Causes

• Green foods: Leafy greens (spinach, kale), matcha, green food coloring (icing, drinks, processed foods).
• Iron supplements: Ferrous sulfate → dark green/black stool.
• Rapid transit: Bile (green) doesn't have time to be converted to brown stercobilin by gut bacteria. Causes: diarrhea, laxatives, gastroenteritis, IBS-D, food poisoning.
• Medications: Antibiotics (alter flora), indomethacin, some antacids.

Potential Medical Causes

• Bile acid malabsorption: Terminal ileum resection/Crohn's → bile acids reach colon → secretory diarrhea (green/yellow).
• Infections: Salmonella, Giardia, C. difficile, norovirus – green diarrhea.
• Liver/gallbladder: Obstruction (stones, stricture) → pale/clay stool (not green); but biliary fistula → green.
• Malabsorption syndromes: Celiac, pancreatic insufficiency.

When to See a Doctor

• Green stool >3-5 days without dietary explanation.
• Accompanied by: fever, severe abdominal pain, blood/mucus in stool, dehydration signs, weight loss.
• Recent antibiotics + diarrhea (rule out C. diff).
• Immunocompromised.

Diagnostic Clues

• Stool culture, O&P, C. diff toxin, calprotectin, fecal fat, bile acids.
• Blood: CBC, LFTs, electrolytes.

Key Point

Isolated green stool after spinach salad or iron pills = normal. Persistent + systemic symptoms = investigate.