What are the causes of green-colored stools (diarrhea) in patients?

Causes of Green-Colored Stools

Green-colored stools are most commonly caused by rapid intestinal transit, dietary factors (particularly green vegetables and food dyes), iron supplementation, or bile pigment malabsorption—none of which are typically addressed in standard chronic diarrhea guidelines because they rarely indicate serious pathology.

Primary Mechanisms of Green Stool Coloration

The green color in stools results from specific physiological and dietary factors that alter normal stool pigmentation:

Rapid Transit Time

• When intestinal contents move too quickly through the colon, bile pigments (biliverdin) don't have adequate time to be converted to stercobilin (the normal brown pigment), resulting in green stools 1
• This occurs in secretory diarrhea where increased intestinal motility is the result rather than the cause of fluid imbalance 1
• Any condition causing diarrhea with increased stool frequency can produce green coloration due to accelerated transit 2

Dietary Causes

• Green leafy vegetables (spinach, kale, broccoli) contain chlorophyll that can directly color stools green
• Food dyes and artificial coloring agents, particularly in processed foods and beverages, are common culprits
• These dietary causes are benign and self-limited

Bile Pigment Disorders

• Bile salt malabsorption can result in excess bile acids reaching the colon, which may contribute to both diarrhea and green stool coloration 1
• This pathophysiology involves disturbances in enteral water and electrolyte balance 1

Medications and Supplements

• Iron supplementation frequently causes dark green to black stools
• Certain antibiotics can alter gut flora and bile metabolism, potentially producing green stools
• This is distinct from the diarrhea caused by antibiotics themselves (such as C. difficile infection) 3

Clinical Context and Red Flags

While green stools alone are rarely concerning, certain accompanying features warrant investigation:

When to Investigate Further

• If green diarrhea persists beyond 4 weeks, it meets criteria for chronic diarrhea requiring systematic evaluation 4, 5
• Presence of blood in stool, weight loss, anemia, or palpable abdominal mass (red flag symptoms) necessitates urgent gastroenterology referral 2
• Signs of severe dehydration, persistent fever, or immunocompromised status require diagnostic workup 2

Factitious Diarrhea Consideration

• In patients with chronic green diarrhea and extensive negative workup, consider factitious diarrhea, which accounts for 20% of cases at tertiary referral centers 3
• Certain laxatives can produce colored stools: bisacodyl turns stool purple-blue, while phenolphthalein and anthraquinones turn it red 3
• Faecal osmolality <290 mosmol/kg indicates dilutional diarrhea from added water 6, 7

Practical Approach

For isolated green stools without other symptoms:

• Review dietary intake over the preceding 48-72 hours for green vegetables, food dyes, or iron supplements
• If dietary cause identified, reassure the patient—no further workup needed
• If no obvious dietary cause and stools normalize within days, observation is appropriate

For green diarrhea with concerning features:

• Evaluate as chronic diarrhea if duration exceeds 4 weeks 4, 5
• Assess for red flag symptoms requiring urgent referral 2
• Consider stool testing for infectious causes if acute presentation with fever or bloody stools 3
• In refractory cases with negative workup, measure faecal osmolality to exclude factitious diarrhea 6, 7

Common Pitfalls

• Do not assume green color alone indicates infection or serious pathology—most cases are benign and self-limited
• Do not order extensive testing for isolated green stools without diarrhea or other symptoms
• In chronic cases, do not overlook factitious diarrhea, which becomes increasingly likely as investigations return negative 3
• Remember that the Bristol Stool Form Scale helps distinguish true diarrhea from patient perception of loose stools 4