Metronidazole and Cholestyramine: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Cholestyramine
Metronidazole
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Decreased metronidazole effectiveness
Interaction Deep Dive
Concomitant administration of oral metronidazole and cholestyramine reduces the absorption of metronidazole. The bioavailability of metronidazole was decreased by 21% in a single dose study with healthy volunteers1.
Why it happens (mechanism)
Decreased metronidazole absorption
Common questions
Can I take Metronidazole and Cholestyramine together?
Decreased metronidazole effectiveness Always confirm with your pharmacist or prescriber before making any change.
How serious is the Metronidazole and Cholestyramine interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "rapid". Effects can appear quickly, often within about 24 hours of combining the drugs.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Metronidazole or Cholestyramine need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (1)
- Molokhia AM: Effect of concomitant oral administration of some adsorbing drugs on the bioavailabiity of metronidazole. Drug Dev Ind Pharm 1987; 13:1229.
Keep reading about Cholestyramine
Keep reading about Metronidazole
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
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