Drug Interaction Report

Cimetidine and Metronidazole: Interaction Details

A pharmacist-reviewed analysis of the Cimetidine and Metronidazole interaction: the documented clinical effects, severity rating, onset, mechanism, and management considerations, with cited references throughout.

AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

Cimetidine

Tagamet
+

Metronidazole

Flagyl Flagyl® Flagyl® 375 Flagyl® ER Likmez MetroCream MetroGel MetroLotion
Dr. Brian Staiger, PharmD, BCPS
Medically reviewed by
Dr. Brian Staiger, PharmD, BCPS
Updated Aug 8, 2026
Editorial policy · Data sources
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Interaction severity
Major
Potentially serious — often needs a change or close monitoring.
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.

Of 83 documented Cimetidine interactions, 48 are rated major — including this one.
Worried about symptoms right now? Contact your pharmacist or prescriber, or call Poison Control at 1-800-222-1222 (US). Call 911 for an emergency.
Onset
unspecified
Evidence
theoretical
Severity
Major

What happens

Increased metroNIDAZOLE exposure

Interaction Deep Dive

Concomitant use of cimetidine, a CYP450 inhibitor, and metroNIDAZOLE, a CYP450 substrate, may decrease the metabolism and prolong the half-life of metroNIDAZOLE. Caution is advised with concurrent use of cimetidine and metroNIDAZOLE321. Adjustment of the metroNIDAZOLE dosage may be indicated when starting or stopping concomitant cimetidine, to maintain optimal metroNIDAZOLE blood levels 4.

Why it happens (mechanism)

Inhibition of CYP450-mediated metabolism of metroNIDAZOLE by cimetidine

Common questions

Can I take Cimetidine and Metronidazole together?

Increased metroNIDAZOLE exposure Always confirm with your pharmacist or prescriber before making any change.

How serious is the Cimetidine and Metronidazole interaction?

It is rated major. Potentially serious — often needs a change or close monitoring.

How quickly could this interaction happen?

The documented onset is "unspecified". The timing of this interaction is not well characterized.

How strong is the evidence for this interaction?

The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.

From our Q&A

Real reader questions about these medications, each personally answered by our pharmacist:

Questions for your pharmacist

  • Does my dose of Cimetidine or Metronidazole 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 a safer alternative to one of these medications for me?

References (4)

  1. Product Information: LIKMEZ(TM) oral suspension, metronidazole oral suspension. Saptalis Pharmaceuticals LLC (per FDA), Hauppauge, NY, 2023. DailyMed
  2. Product Information: Flagyl(R) oral tablets, metronidazole oral tablets. G.D. Searle LLC, New York, NY, 2010. DailyMed
  3. Product Information: HELIDAC(R) Therapy oral chewable tablets, tablets, capsule, bismuth subsalicylate metronidazole tetracycline HCl oral chewable tablets, tablets, capsule. Prometheus Laboratories, Inc. (per FDA), San Diego, CA, 2012.
  4. Product Information: cimetidine oral tablets, cimetidine oral tablets. Teva Pharmaceuticals USA (per DailyMed), Sellersville, PA, 2010. DailyMed
Was this write-up helpful?
Cimetidine

Keep reading about Cimetidine

All Cimetidine interactions (83)
Also serious with: Dofetilide Colchicine Octreotide Injection Meloxicam Injection
Metronidazole

Keep reading about Metronidazole

All Metronidazole interactions (278) Metronidazole drug information
Also serious with: Saquinavir Sparfloxacin Pimozide Piperaquine
Beyond drug–drug

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:

Cimetidine + Activated Charcoal Pu-erh Tea St. John's Wort Adrafinil African Wild Potato Banderol Beer Bitter Melon + 87 more
Metronidazole + Activated Charcoal American Hellebore Arsenic Cesium Ephedra Grapefruit Sida Cordifolia Adrafinil + 75 more

major · moderate · minor — check everything you take with our drug–supplement interaction checker.

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This information is for education, not a substitute for professional medical advice. Do not start, stop, or change any medication without talking to your pharmacist or prescriber.