What causes green stool?

What Causes Green Stool

Green stool is most commonly caused by rapid intestinal transit, dietary factors (particularly green vegetables and food dyes), bile pigment, or certain medications—it is typically benign and does not indicate serious pathology unless accompanied by other concerning symptoms.

Common Benign Causes

Dietary Factors

• Green vegetables high in chlorophyll (spinach, kale, broccoli) can directly color stool green
• Food dyes and additives in processed foods, beverages, and candies frequently cause green discoloration
• Iron supplements can produce greenish-black stools

Rapid Transit Time

• When stool moves through the intestines too quickly, bile doesn't have adequate time to break down completely from its natural green color (biliverdin) to brown (stercobilin)
• This can occur with:
  • Mild diarrhea from any cause 1
  • Dietary factors including excessive caffeine intake 1
  • Medications that increase gut motility 1

Medications

• Antibiotics can alter gut flora and transit time, leading to green stool 1
• Magnesium-containing products may accelerate transit 1
• Various other medications affecting intestinal function 1

When to Investigate Further

Red Flags Requiring Evaluation

While green stool alone is benign, investigate if accompanied by:

• Persistent diarrhea lasting >3 months 1
• Blood or mucus in stool (suggests inflammatory or infectious colitis) 1
• Unintentional weight loss 1
• Nocturnal diarrhea 1
• Fever or severe abdominal pain 2

Pathologic Causes to Consider

If green stool persists with concerning features, consider:

Infectious causes:

• Bacterial infections (Salmonella, Campylobacter, Giardia) can cause rapid transit and green diarrhea 3, 4
• C. difficile following antibiotic use 1

Malabsorption syndromes:

• Bile acid malabsorption causes rapid transit and can produce green stool 1, 5
• Celiac disease may present with altered stool color due to malabsorption 1

Inflammatory conditions:

• Inflammatory bowel disease (Crohn's, ulcerative colitis) 1
• Microscopic colitis 1

Clinical Approach

Initial Assessment

• Detailed dietary history focusing on green vegetables, food dyes, and supplements
• Medication review including antibiotics, magnesium, and other agents 1
• Duration and pattern of symptoms 1
• Associated symptoms (blood, weight loss, fever) 1

When Testing Is Warranted

Testing is not needed for isolated green stool without other symptoms that resolves quickly 3.

Pursue investigation if:

• Symptoms persist >6 weeks 1
• Alarm features present (blood, weight loss, nocturnal symptoms) 1
• Recent antibiotic exposure (test for C. difficile) 1
• Severe or bloody diarrhea (obtain stool studies) 3

Key Pitfall to Avoid

Do not over-investigate isolated green stool in otherwise healthy patients—this is almost always dietary or related to rapid transit and resolves spontaneously. Focus investigation on patients with persistent symptoms or alarm features that suggest organic disease 1.