Can B1, B6, and B12 Supplements Cause Dark Stool?
B1 (thiamine), B6 (pyridoxine), and B12 (cobalamin) supplements do not cause dark stools as a recognized side effect. These water-soluble vitamins are not associated with stool color changes in clinical guidelines or pharmacological literature.
Known Side Effects of B-Vitamin Supplementation
Thiamine (Vitamin B1)
- No gastrointestinal effects on stool color are documented 1
- High IV doses (>400 mg) may rarely induce nausea, anorexia, and mild ataxia, but not stool changes 2
- Excess thiamine is excreted in urine without causing stool discoloration 2
Pyridoxine (Vitamin B6)
- The primary concern with B6 is neuropathy and photosensitivity at excessive doses, not gastrointestinal effects 1
- Doses above 100 mg/day can cause sensory neuropathy 1
- No stool color changes are listed among adverse effects in clinical guidelines 1
- Vitamin B6 overload from supplementation can cause polyneuropathy but does not affect stool appearance 3, 4
Cobalamin (Vitamin B12)
- No known adverse effects on stool color or appearance 1
- The guideline literature lists "unknown" for effects of excess B12 1
- B12 deficiency can cause pernicious anemia and neurologic deterioration, but supplementation does not alter stool color 1
Important Clinical Distinction
Dark or black stools warrant immediate medical evaluation for gastrointestinal bleeding, which appears as melena (tarry, black stools) and represents a potentially serious condition unrelated to B-vitamin supplementation.
Alternative Causes of Dark Stools to Consider:
- Iron supplementation is a well-documented cause of dark or black stools 1
- Bismuth-containing medications (e.g., Pepto-Bismol)
- Foods with dark pigments (blueberries, black licorice, dark leafy greens)
- Gastrointestinal bleeding (requires urgent evaluation)
Clinical Recommendation
If a patient taking B1, B6, and B12 supplements develops dark stools, investigate other causes rather than attributing it to the B vitamins. Check for concurrent iron supplementation, dietary factors, or signs of gastrointestinal bleeding. The B-vitamin complex itself is not responsible for stool color changes based on established pharmacological profiles 1.