What Cannot Be Taken with Metronidazole? A Guide to Interactions

October 10, 2025 •

3 min read

Approximately 270 million antibiotics were prescribed for outpatients in a single year, with metronidazole being a common choice. Understanding what cannot be taken with metronidazole is crucial for safety and efficacy, as several substances can cause severe reactions.

Quick Summary

A detailed overview of substances to avoid while taking metronidazole. This includes absolute contraindications like alcohol and significant interactions with prescription drugs such as warfarin, lithium, and busulfan.

Key Points

Alcohol is Forbidden: Absolutely avoid alcohol and products with propylene glycol while taking metronidazole and for at least 3 days after, to prevent a severe disulfiram-like reaction.
Blood Thinner Risk: Metronidazole significantly increases the effect of warfarin, raising the risk of dangerous bleeding. Close monitoring is required.
Lithium Toxicity: The medication can cause lithium levels to become dangerously high in patients using it as a mood stabilizer, requiring careful monitoring.
Chemotherapy Hazard: Combining metronidazole with busulfan can lead to severe toxicity and should generally be avoided.
Psychotic Reactions with Disulfiram: Do not take metronidazole if you have used disulfiram within the last two weeks to avoid psychosis.
Heart Rhythm Changes: Metronidazole can affect heart rhythms, a risk that is amplified when taken with other QT-prolonging drugs.
Inform Your Doctor: Always provide a full list of your medications and supplements to your healthcare provider to screen for potential interactions before starting metronidazole.

Absolute Contraindications: Alcohol and Propylene Glycol

Combining metronidazole with alcohol or products containing propylene glycol is strongly discouraged. This combination can cause a disulfiram-like reaction, characterized by unpleasant and potentially severe symptoms. While the exact mechanism is debated, it is traditionally thought to involve the buildup of acetaldehyde due to the inhibition of aldehyde dehydrogenase.

Symptoms can include severe nausea and vomiting, flushing, headache, rapid heartbeat, and chest pain. More severe, though rare, reactions can involve heart issues, seizures, and even death. To avoid these risks, all forms of alcohol and products containing propylene glycol should be avoided during treatment and for at least three days after the last dose.

Significant Prescription Drug Interactions

Metronidazole can interact with several prescription drugs, affecting their metabolism and potentially increasing the risk of toxicity.

Warfarin (Blood Thinner)

Concurrent use with warfarin can increase its blood-thinning effects and the risk of bleeding. This is because metronidazole can inhibit warfarin metabolism. Patients on this combination need close monitoring of their INR and potential adjustment of warfarin dosage. Signs of bleeding should be reported to a healthcare provider.

Lithium (Mood Stabilizer)

Metronidazole can decrease the kidneys' ability to remove lithium, potentially leading to elevated lithium levels and toxicity. Due to lithium's narrow therapeutic range, even small increases can be dangerous. Symptoms of toxicity include tremors, confusion, and muscle weakness. Close monitoring of lithium levels is necessary if these medications are used together.

Busulfan (Chemotherapy Agent)

Taking metronidazole can significantly raise busulfan concentrations, increasing the risk of serious toxicity, such as liver problems and bone marrow suppression. This combination should generally be avoided unless the benefits outweigh the risks, and busulfan levels must be closely monitored if co-administered.

Other Notable Interactions

Disulfiram: Avoid metronidazole within two weeks of taking disulfiram to prevent psychotic reactions.
Drugs Affecting Heart Rhythm: Metronidazole can contribute to QT prolongation, increasing the risk of heart rhythm problems when combined with other drugs that have this effect.
Cimetidine: This medication can increase metronidazole levels by slowing its breakdown.
Antiepileptic Drugs (Phenytoin, Phenobarbital): These drugs can reduce metronidazole's effectiveness by speeding up its metabolism.

Metronidazole Interaction Comparison Table


Interacting Substance	Risk Level	Potential Effect	Management Recommendation
Alcohol / Propylene Glycol	High	Disulfiram-like reaction (nausea, vomiting, flushing, headache, palpitations).	Avoid completely during and for at least 3 days after treatment.
Warfarin	High	Increased risk of bleeding due to enhanced anticoagulant effect.	Frequent INR monitoring and potential warfarin dose adjustment.
Lithium	High	Increased risk of lithium toxicity (tremor, confusion, seizures).	Close monitoring of serum lithium levels; consider lithium dose reduction.
Busulfan	High	Increased plasma levels of busulfan, leading to severe toxicity.	Avoid combination if possible. If necessary, monitor busulfan levels closely and adjust dose.
Disulfiram	High	Psychotic reactions (confusion, hallucinations).	Do not use within 2 weeks of each other.
QT-Prolonging Drugs	Moderate	Increased risk of dangerous heart rhythm problems.	Use with caution and under medical supervision, especially in patients with heart conditions.
Cimetidine	Moderate	Increased metronidazole levels and side effects.	Monitor for side effects; consider alternative heartburn medication.
Phenytoin / Phenobarbital	Moderate	Decreased effectiveness of metronidazole.	Healthcare provider may need to adjust metronidazole dosage or choose an alternative.

Conclusion

While metronidazole is an important antibiotic, it is crucial to be aware of potential interactions. Avoiding alcohol and products containing propylene glycol is essential. Always inform your healthcare provider of all medications, including prescriptions, over-the-counter drugs, and supplements, to prevent harmful interactions and ensure safe treatment. Adhering to your doctor's and pharmacist's instructions is vital.

For more detailed information on specific drug interactions, you can consult resources like the Drugs.com Interaction Checker.

Frequently Asked Questions

You should wait at least 72 hours (3 days) after your last dose of metronidazole before consuming any alcohol to avoid a potential disulfiram-like reaction.

Even a small amount of alcohol can trigger symptoms like severe nausea, vomiting, flushing, and headache. While the reaction severity varies, it's best to avoid alcohol entirely. If you experience severe symptoms like chest pain or difficulty breathing, seek medical advice.

There is no significant reported interaction between metronidazole and acetaminophen (Tylenol). However, always consult your doctor or pharmacist before combining medications.

Some studies have suggested that antibiotics like metronidazole could potentially make oral birth control pills less effective, though other studies have not shown this. To be safe, your doctor may recommend using a backup method of contraception during treatment.

You must avoid any foods that contain alcohol or propylene glycol. Propylene glycol is a food additive found in items like some salad dressings, baked goods, and fast foods. Always check food labels.

It is an adverse reaction caused by the accumulation of acetaldehyde in the blood after consuming alcohol with certain medications. Symptoms include nausea, vomiting, flushing, rapid heartbeat, and headache.

This combination should be managed with extreme caution. Metronidazole can increase lithium levels, leading to toxicity. If it's medically necessary, your doctor must monitor your lithium blood levels very closely.