Metronidazole Drug Interactions: What to Avoid and What to Tell Your Doctor

Metronidazole (Flagyl) has a number of significant drug interactions that you and your prescriber need to know about before starting treatment. Some of these interactions are serious — even dangerous — while others require dose adjustments or closer monitoring. This guide covers the most important interactions, organized by severity, so you know exactly what to tell your doctor.

The Most Important Warning: Alcohol + Metronidazole

This is the interaction everyone on metronidazole needs to know. Do not drink alcohol — or consume products containing alcohol or propylene glycol — while taking metronidazole, and for at least 3 days after your last dose.

Metronidazole inhibits aldehyde dehydrogenase, an enzyme that normally breaks down acetaldehyde (a byproduct of alcohol metabolism). Without this enzyme working properly, acetaldehyde builds up in your blood, causing a disulfiram-like reaction:

Severe nausea and vomiting

Throbbing headache

Flushing (redness, warmth)

Rapid heartbeat (tachycardia)

Sweating and abdominal cramps

This reaction can last 30 minutes to several hours. Also avoid mouthwashes, cough syrups, cooking wines, and other products containing propylene glycol, as these can trigger a similar reaction.

Contraindicated Combinations: Do Not Take These Together

Disulfiram (Antabuse): Absolutely contraindicated. Using disulfiram within 2 weeks before starting metronidazole can cause psychotic reactions — a severe interaction affecting the central nervous system. If you take disulfiram, tell your doctor before any metronidazole prescription is written.

Dronabinol oral solution (Syndros): Syndros contains 50% dehydrated alcohol and 5.5% propylene glycol, which will trigger a disulfiram-like reaction with metronidazole. Do not use these together.

Serious Interactions: Require Dose Adjustment or Monitoring

Warfarin (Coumadin, Jantoven): Metronidazole significantly enhances the blood-thinning effect of warfarin by inhibiting the CYP2C9 enzyme that metabolizes it. INR can rise to dangerous levels, increasing your risk of bleeding. If you take warfarin, your doctor will likely reduce your warfarin dose during metronidazole treatment and monitor your INR closely.

Lithium: Metronidazole can increase lithium levels in the blood, potentially leading to lithium toxicity. Symptoms include tremor, confusion, nausea, and in severe cases, seizures. If you take lithium for bipolar disorder or another condition, tell your prescriber — lithium levels may need monitoring.

Busulfan: Metronidazole can increase plasma levels of busulfan, a chemotherapy drug, potentially increasing toxicity. This combination is particularly dangerous in cancer patients.

Phenytoin (Dilantin) and Phenobarbital: These seizure medications can increase the metabolism (clearance) of metronidazole, reducing its blood levels and potentially making it less effective. On the flip side, metronidazole may also increase phenytoin levels. Monitor for both reduced metronidazole efficacy and phenytoin toxicity.

Cimetidine (Tagamet): This heartburn medication slows the metabolism of metronidazole, potentially increasing its blood levels and the risk of side effects.

Moderate Interactions: Monitor Closely

CYP3A4 substrates (many medications): Metronidazole is a moderate inhibitor of CYP3A4, a liver enzyme responsible for metabolizing many medications. This means metronidazole can increase blood levels of drugs processed by CYP3A4. Examples include cyclosporine, tacrolimus, certain statins, and some antifungals. Ask your doctor or pharmacist to check your full medication list.

Certain antidepressants (SSRIs, SNRIs): Some antidepressants can prolong the QT interval (a heart rhythm measurement). Metronidazole can also cause QT prolongation. Combining them increases the risk of a rare but serious arrhythmia called torsades de pointes.

Cholestyramine (LoCholest, Prevalite): Can reduce absorption of metronidazole in the gut. If you take cholestyramine, take metronidazole at least 1 hour before or 4 hours after.

Conditions That Increase Risk of Side Effects

Certain health conditions can amplify metronidazole's side effects or risks:

Cockayne syndrome: Metronidazole is contraindicated in this rare genetic disorder. It can cause life-threatening liver injury in these patients.

Severe liver disease: Metronidazole is primarily metabolized in the liver. In severe hepatic impairment, the drug can accumulate to toxic levels. Dose reduction may be necessary.

Long QT syndrome: Pre-existing QT prolongation is a risk factor for arrhythmia with metronidazole. Discuss with your doctor if you have a known heart rhythm abnormality.

Blood disorders: Metronidazole may worsen leukopenia (low white blood cells) in susceptible patients.

What to Tell Your Doctor Before Starting Metronidazole

Bring a complete list of your medications — prescription, OTC, vitamins, and supplements. Specifically mention:

Any blood thinners (especially warfarin/Coumadin)

Disulfiram (Antabuse) — within the past 2 weeks

Lithium (for bipolar disorder)

Anti-seizure medications (phenytoin, phenobarbital)

Antidepressants or antipsychotics

Chemotherapy drugs (busulfan in particular)

Any heart rhythm medications

Whether you have liver disease, kidney disease, or Cockayne syndrome

The Bottom Line

Metronidazole has significant interactions — particularly with alcohol, disulfiram, and warfarin — that require attention. The alcohol restriction is non-negotiable and applies for at least 3 days after your last dose. For everything else, a thorough medication review with your prescriber or pharmacist before starting metronidazole is the safest approach. See also our guide on

metronidazole side effects for a full overview of what to watch for during treatment.