Can Metronidazole Cause Black Stools? Distinguishing Benign vs. Serious Causes

October 15, 2025 •

3 min read

While less common, some patients taking metronidazole report noticing changes in their stool color. This can be an alarming sight, leading many to question, can metronidazole cause black stools? Understanding the potential causes, from benign side effects to more serious conditions, is essential for patients.

Quick Summary

Taking metronidazole may result in darker stools, especially in combination with other medications like bismuth subcitrate. This is often a harmless, temporary side effect. Serious black, tarry stools (melena) require immediate medical evaluation.

Key Points

Less Common Side Effect: Metronidazole alone is not typically a cause of black stools, but it is listed as a less common side effect in some sources.
Combination Therapy: When used with bismuth subsalicylate (e.g., for H. pylori treatment), harmless black tongue and stool are a common and expected side effect.
Benign vs. Serious: Harmless, darker stool from medication differs significantly from melena, which is black, tarry, and foul-smelling stool caused by GI bleeding.
When to Seek Medical Attention: Immediately contact a doctor if black stool is tarry, has a strong odor, or is accompanied by symptoms like dizziness, abdominal pain, or vomiting blood.
Management and Mitigation: Taking metronidazole with food can minimize stomach upset, while avoiding alcohol and staying hydrated are crucial for managing side effects.
Rule Out Other Causes: Consider other potential causes of black stool, including iron supplements and dark-colored foods, before concluding it's medication-related.

Is It Normal to Have Dark Stools While Taking Metronidazole?

Metronidazole, commonly known by the brand name Flagyl, is a powerful antibiotic used to treat various bacterial and parasitic infections. Like many medications, it can cause a range of side effects, particularly affecting the gastrointestinal (GI) tract. While it's not a frequently cited side effect for metronidazole alone, some patients do report darker stool.

It is important to note a key distinction regarding metronidazole-induced stool changes. When metronidazole is prescribed as part of a combination therapy for a specific infection, like H. pylori, other medications in the regimen can be the primary cause of stool discoloration. For instance, if you are also taking a medication containing bismuth subsalicylate (the active ingredient in Pepto-Bismol), black tongue and stool are expected and harmless side effects. However, even without bismuth, some drug references list black, tarry stools as a less common side effect of metronidazole.

When Are Black Stools a Cause for Concern?

While some causes of black stool are benign, it's crucial to distinguish harmless discoloration from melena, which is black, tarry, and foul-smelling stool caused by upper GI bleeding. This occurs when blood is digested, turning black as it travels through the digestive system.

Other Potential Causes of Dark Stool

Before assuming a serious problem, consider other causes of dark or black stool, which can include:

Dietary factors: Eating large quantities of dark-colored foods like black licorice, blueberries, or blood sausage.
Iron supplements: Taking iron supplements can cause stools to appear dark or black.
Other medications: As mentioned, medicines containing bismuth subsalicylate are a common cause.

Comparison: Harmless Discoloration vs. GI Bleed (Melena)

This table helps differentiate between benign and serious causes of black stool.


Feature	Harmless Discoloration (Medication/Food)	Upper GI Bleed (Melena)
Color	Dark, greyish, or greenish-black. Not typically shiny or sticky.	Deep black, shiny, and tarry (like wet asphalt).
Odor	Normal fecal odor or slight change.	Distinct, very foul, and lingering odor.
Consistency	Can vary, but not typically sticky or tarry.	Sticky, thick, and tarry.
Associated Symptoms	May have other medication side effects (nausea, metallic taste) but generally no severe symptoms.	Can be accompanied by dizziness, weakness, abdominal pain, shortness of breath, or vomiting blood.
Significance	Benign; should resolve after stopping the medication or consuming the food.	Serious medical condition requiring immediate attention.

Management and When to See a Doctor

If you notice dark or black stools while on metronidazole, the first step is to assess the stool's characteristics and any accompanying symptoms. If you are also taking a bismuth product, harmless discoloration is the most likely cause. However, if the stool is tarry, has a foul odor, or you experience any of the serious symptoms listed above, seek medical help immediately.

How to Minimize Gastrointestinal Side Effects

Take with food: Oral metronidazole can cause stomach upset, nausea, and diarrhea. Taking the medication with a meal or snack can help reduce these effects.
Stay hydrated: Drinking plenty of water can help manage side effects like diarrhea and prevent dehydration.
Avoid alcohol: Do not consume alcohol during your metronidazole treatment and for at least 72 hours afterward, as it can cause a severe reaction with symptoms like nausea, vomiting, and flushing.
Consider probiotics: Some healthcare providers suggest taking a probiotic supplement, separated by at least two hours from your antibiotic dose, to help restore healthy gut bacteria and minimize GI side effects.

If you are concerned about your side effects, contact your doctor. They may be able to adjust your dose or provide an alternative medication. Do not stop taking your antibiotic without consulting a healthcare professional, as incomplete treatment can cause the infection to return and may contribute to antibiotic resistance.

Conclusion

While the sight of black stool can be startling, it is not a common side effect of metronidazole alone. When it does occur, it may be a benign discoloration, especially in combination therapies involving bismuth. True melena, which is black, tarry stool indicative of GI bleeding, requires urgent medical evaluation, particularly if accompanied by other concerning symptoms like pain or dizziness. Patients should always consult their healthcare provider if they have persistent questions or experience severe side effects while on metronidazole.

Frequently Asked Questions

While it's a less common side effect, some sources list black, tarry stools as a potential side effect of metronidazole. However, other medications or dietary factors are more frequently the cause of dark stool discoloration.

Dark stool from medication is typically a benign discoloration, while black, tarry, and foul-smelling stool, known as melena, is a sign of bleeding in the upper gastrointestinal tract. The key difference lies in the sticky, tarry appearance and strong, unpleasant odor of melena.

Yes. Medications containing bismuth subsalicylate (like Pepto-Bismol) and iron supplements are known to cause harmless darkening or blackening of the stool.

First, assess if the stool is truly tarry, sticky, and foul-smelling. Consider recent food intake or other medications. If it is tarry or if you have other symptoms like abdominal pain or dizziness, contact your doctor immediately. If the stool is simply darker and you have no other symptoms, monitor it and inform your doctor at your next appointment.

Benign stool discoloration caused by medication is temporary and should resolve shortly after you stop taking the medication. If it persists for more than a couple of days after finishing treatment, consult your doctor.

Yes, metronidazole is known to cause a 'furry' or black tongue in some patients, though this is also often associated with bismuth in combination therapies. This is a temporary and harmless side effect that should go away after you complete your treatment.

Do not stop taking your prescribed medication without first consulting your healthcare provider. They can help determine the cause of the stool color change and provide appropriate guidance. Stopping an antibiotic prematurely can lead to a worsening infection.