Does Vitamin B12 Cause Dark Stools? Exploring the Link Between Supplements and Stool Color

October 12, 2025 •

4 min read

While normal stool color varies, persistent and sudden changes can be alarming. When beginning a new regimen of supplements, some people notice their stool has become darker, leading them to question, "Does vitamin B12 cause dark stools?". The answer is more complex, often involving co-ingredients like iron rather than the B12 itself.

Quick Summary

Taking high-dose B12 supplements or formulas containing iron can cause dark stools, but the vitamin itself is not the primary culprit. While typically harmless, it is important to distinguish this from serious medical issues like gastrointestinal bleeding. Monitoring changes and seeking medical advice for persistent symptoms is crucial.

Key Points

Iron is the likely culprit, not B12: Many supplements, including B-complexes, contain iron, which is the most common supplement-related cause of dark or black stool.
Excess B12 can be excreted: High doses of B12 are water-soluble and any excess is passed through the body, which can sometimes cause a mild darkening of stool.
Distinguish from Melena: Harmless dark stool from supplements should not be confused with melena, which is black, tarry, and foul-smelling stool indicating gastrointestinal bleeding.
Check your supplement ingredients: Always review the label of your supplement to identify if iron is an ingredient, as this is a very common cause for the color change.
Monitor for other symptoms: If dark stool persists after discontinuing the supplement or is accompanied by other symptoms like pain or dizziness, seek medical attention immediately.
Other medications can also darken stools: Medications containing bismuth (like Pepto-Bismol) and certain foods (like blueberries or black licorice) can also cause stools to darken.

Understanding the Connection Between Supplements and Stool Color

Stool color is typically a shade of brown, determined by the breakdown of bile in the digestive system. However, what we consume can directly impact this color. When a person starts a new supplement, like vitamin B12, and notices a change in their stool, it's natural to assume the two are related. The search results reveal that, while excess B12 can be excreted and potentially darken the stool, it is a less common and less pronounced effect than other ingredients frequently found alongside it.

The Role of Iron in Dark Stools

One of the most frequent and well-documented causes of dark or black stool among supplement users is iron. Many B-complex and multivitamin supplements contain iron, an essential mineral that can have a notable effect on stool color. The iron in these supplements reacts in the gastrointestinal tract, causing the stool to turn a greenish-black or blackish shade. This is a common and usually harmless side effect. If your B12 supplement is part of a multivitamin or B-complex, check the label for iron content.

Can Excessive B12 Alone Affect Stool Color?

For those taking pure B12 supplements, especially in very high dosages, it's possible for excess vitamins to alter stool color. Since B12 is a water-soluble vitamin, any amount not absorbed by the body is excreted. In some cases, this can lead to a darker stool color. However, this effect is often mild and not as dramatic or consistently reported as the change caused by iron. High-dose B12 intake is more commonly associated with bright yellow urine, another sign of the body flushing out excess vitamins.

Other Potential Digestive Side Effects of B12

High doses of vitamin B12, or the sudden introduction of a new supplement, can also sometimes cause minor gastrointestinal upset. Some people report experiencing diarrhea or other digestive changes when they start taking B12. Diarrhea, regardless of the cause, can alter stool color by speeding up the digestive process, which doesn't give bile enough time to fully break down into its normal brown color.

Benign vs. Concerning Causes of Dark Stool

It's critical to understand the distinction between a harmless change in stool color due to supplements and a potentially serious medical condition. Black, tarry, and foul-smelling stool, known as melena, can indicate bleeding in the upper gastrointestinal tract. This is a medical emergency and should not be confused with the darker, less alarming stool caused by dietary factors. The texture and smell are key differentiating features.


Comparison of Dark Stool Causes	Cause	Appearance of Stool	Other Features	When to Worry
Iron Supplements	Dark green or blackish	Can have a chalky texture; often resolves when supplement is stopped.	If accompanied by stomach pain, dizziness, or tarry texture.
Bismuth Medications	Blackish, sometimes with dark tongue	Typically resolves once medication is stopped.	Persists after stopping medication or accompanied by severe symptoms.
Excess B12/B-Complex	Mild darkening, can be excreted	Excreted through urine (bright yellow).	Rare for B12 alone to cause significantly dark stool.
Dark-Colored Foods	Dark green, black, or reddish	Color is temporary and depends on food (e.g., blueberries, licorice).	If the color doesn't resolve after a few days.
GI Bleeding (Melena)	Black, sticky, tarry, and foul-smelling	Often accompanied by other symptoms like abdominal pain, fatigue, dizziness, or vomiting blood.	Always. Seek immediate medical attention.

When to Contact a Healthcare Provider

While a change in stool color after starting a B12 or multivitamin supplement is often benign, it's important to be vigilant. You should always consult your doctor for persistent changes or if you experience additional symptoms. It is especially critical to seek immediate medical help if your stool is black and tarry, has a foul odor, or is accompanied by signs of bleeding such as abdominal pain, lightheadedness, or vomiting blood. A doctor can perform a fecal occult blood test to check for hidden blood and rule out more serious issues.

Monitor the Change: Note when the dark stool started and if it aligns with taking a new supplement or food. If you stop the supplement and the stool returns to a normal color, it was likely the cause.
Check the Label: Review your supplement label. If it contains iron, there is a high likelihood that the iron, not the B12, is responsible for the color change.
Observe Other Symptoms: Pay close attention to any other symptoms, especially those that might indicate gastrointestinal bleeding.

In conclusion, while does vitamin B12 cause dark stools is a common question, the answer points more directly to iron, a frequent co-ingredient in supplements. A noticeable change in stool color should always be monitored, and any concerning symptoms warrant a visit to a healthcare professional to ensure the cause is benign and not an indication of a more serious underlying issue. For further reading on bowel health, consult reliable sources like the MedlinePlus Medical Encyclopedia for information on black or tarry stools.

Frequently Asked Questions

While less common than with oral supplements containing iron, it is possible for B12 injections to cause a slight change in stool color in some individuals. However, the change is generally mild and not as pronounced as with iron-containing supplements. Any significant or persistent change should be discussed with a doctor.

No, if the dark stool is a result of iron supplements or excess B12, it is typically harmless. The color change is a normal side effect. The concern lies in distinguishing this benign side effect from more serious conditions, such as internal bleeding.

A black stool from gastrointestinal (GI) bleeding, known as melena, is typically sticky or tarry in texture and has a distinctively foul odor. Dark stool from supplements is usually not tarry and lacks the strong odor. Accompanying symptoms like abdominal pain, dizziness, and vomiting blood also point to a GI bleed.

Certain foods with dark pigments can cause stools to appear black or dark. Common culprits include black licorice, blueberries, and beets. The effect is temporary and should resolve once the food is out of your system.

If you suspect your supplement is the cause, you can try temporarily discontinuing it to see if your stool color returns to normal. Always consult a healthcare provider before making changes to your supplement regimen. If the color change persists after stopping, or if you have other symptoms, see a doctor.

No, many vitamin B12 supplements are formulated without iron, especially those that are pure B12 (cyanocobalamin or methylcobalamin). However, B-complex vitamins and general multivitamins often include iron. Always check the supplement facts label.

A vitamin B12 deficiency can cause digestive symptoms like diarrhea and weight loss, but it does not directly cause dark stools. If you have a deficiency, it's the treatment (which may involve supplements containing iron) that is more likely to alter stool color.