Ondansetron and Metronidazole: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Ondansetron
No brand names on recordMetronidazole
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
An increased risk of QT interval prolongation
Interaction Deep Dive
Concomitant use of metroNIDAZOLE other QT prolonging drugs can cause additive QT interval prolongation12. MetroNIDAZOLE can cause QT prolongation 4 and has caused torsades de pointes with concomitant amiodarone administration 5. Susceptible patients may require ECG monitoring and avoidance of medications known to cause QT prolongation 3.
Why it happens (mechanism)
Additive QT interval prolongation
Literature reports
3 reports — tap to read
a) A 71-year-old woman developed QT prolongation 2 days after initiation of IV metroNIDAZOLE for nosocomial pneumonia. She was admitted with atrial fibrillation, and QTc was 396 msec. Medical history included COPD, diabetes, mellitus, hypertension, coronary artery disease, and QT prolongation with moxifloxacin. She received diuretics, ACE inhibitors, omeprazole, and bronchodilators and developed nosocomial pneumonia. Two days after initiation of metroNIDAZOLE, QTc was 559 msec and T waves were inverted. MetroNIDAZOLE was discontinued and ECG returned to normal within 48 hours 3.
b) A 90-year-old woman with no known history of structural heart disease developed QT prolongation after metroNIDAZOLE treatment for aspiration pneumonia. QTc was 324 msec on admission and 703 msec the next day, after initiation of cefTRIAXone 1 g/day IV and metroNIDAZOLE 500 mg 3 times daily. MetroNIDAZOLE was discontinued and QTc gradually returned to initial value without significant arrhythmias. The patient was advised to avoid medications known to cause QT prolongation 4.
c) A 71-year-old woman with antibiotic-induced pseudomembranous colitis developed ECG QTc interval prolongation and torsades de pointes with concurrent amiodarone 450 mg bolus followed by 900 mg/day IV and metroNIDAZOLE 1500 mg/day oral administration. Baseline QTc interval was 440 msec. Amiodarone was added after atrial fibrillation developed with 3 days of amiodarone therapy. Conversion to sinus rhythm occurred 2 days later; however, the follow-up ECG revealed a QTc interval of 625 msec. Symptoms progressed to sustained torsades de pointes-variant ventricular tachycardia that required emergent cardioversion/defibrillation to restore normal sinus rhythm. Amiodarone and metroNIDAZOLE were immediately withdrawn, and the QTc interval slowly returned to baseline values without further clinically significant arrhythmia events 5.
Common questions
Can I take Ondansetron and Metronidazole together?
An increased risk of QT interval prolongation Always confirm with your pharmacist or prescriber before making any change.
How serious is the Ondansetron 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 Ondansetron 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 (5)
- Product Information: LIKMEZ(TM) oral suspension, metronidazole oral suspension. Saptalis Pharmaceuticals LLC (per FDA), Hauppauge, NY, 2023. DailyMed
- Product Information: PYLERA(R) oral capsules, bismuth subcitrate potassium, metronidazole, tetracycline HCl oral capsules. H2-Pharma, LLC (per FDA), Montgomery, AL, 2025. DailyMed
- Altin C, Kanyilmaz S, Baysal S, et al: QT interval prolongation due to metronidazole administration. Anadolu Kardiyol Derg 2011; 11(5):468-469. PubMed
- Cohen O, Saar N, Swartzon M, et al: First report of metronidazole-induced QT interval prolongation. Int J Antimicrob Agents 2008; 31(2):180-181. PubMed
- Kounas SP, Letsas KP, Sideris A, et al: QT interval prolongation and torsades de pointes due to a coadministration of metronidazole and amiodarone. Pacing Clin Electrophysiol 2005; 28:472-473. PubMed
Keep reading about Ondansetron
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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