Can Probiotics Change Stool Color?
Probiotics do not typically change stool color in any clinically significant way, though they may alter stool consistency and frequency. The available clinical evidence focuses on probiotics' effects on stool characteristics like consistency, frequency, and transit time—not color changes.
What Probiotics Actually Do to Your Stool
The evidence demonstrates that probiotics primarily affect:
- Stool consistency: Probiotics improve stool consistency (standardized mean difference: +0.55; 95% CI: 0.27,0.82), particularly with Bifidobacterium lactis strains 1
- Stool frequency: Probiotics increase bowel movements by approximately 1.3 per week (95% CI: 0.7,1.9 bowel movements/week) in adults with functional constipation 1
- Transit time: Probiotics reduce whole gut transit time by approximately 12.4 hours (95% CI: -22.3, -2.5 hours) 1
Why Color Changes Are Not Reported
Across multiple high-quality systematic reviews and meta-analyses examining probiotics for various gastrointestinal conditions, stool color changes are not mentioned as an adverse event or expected effect 1, 2. The most commonly reported gastrointestinal side effects are:
- Gas and flatulence as the gut microbiota adapts to new bacterial strains 3
- Temporary loose stools, especially in the first few days of use 3
- Abdominal bloating and mild discomfort 3, 4
These side effects are generally mild and transient, resolving within days 3.
When to Worry About Stool Color Changes
If you notice stool color changes while taking probiotics, the color change is almost certainly unrelated to the probiotic itself. Consider these more likely causes:
- Dietary factors: Foods, food dyes, or other supplements taken concurrently
- Medications: Iron supplements (black stools), bismuth subsalicylate (black stools), or antibiotics
- Underlying gastrointestinal conditions: Bleeding (black/tarry or red stools), bile duct obstruction (pale/clay-colored stools), or liver disease
Clinical Bottom Line
Probiotics are safe and well-tolerated with no evidence of causing stool color changes 2, 3, 5. In meta-analyses involving thousands of participants, adverse events from probiotics showed no significant difference compared to placebo groups (risk difference 0.00; 95% CI -0.01 to 0.01) 3. If stool color changes occur during probiotic use, investigate other causes rather than attributing it to the probiotic.