7 Metronidazole Interactions You Should Know About

Metronidazole (Flagyl) is a prescription antibiotic that’s used to treat a variety of bacterial and parasitic infections. This includes bacterial vaginosis, sexually transmitted infections (STIs), skin infections, and more. It works by attacking the DNA of bacteria or protozoans (parasites) that are causing your infection. 

Although metronidazole is an effective and affordable option for treating infections like these, it has some important interactions to keep in mind. Drug interactions can change the way your medication works in your body or increase your risk of side effects. Here, we’ll highlight seven key interactions to be aware of prior to taking metronidazole.

1. Disulfiram

Disulfiram is a medication that treats alcohol use disorder. It works by stopping the breakdown of alcohol, making it unpleasant to drink. When you drink an alcoholic beverage while disulfiram is in your system, it causes nausea, vomiting, and other bothersome side effects. This is called a disulfiram-alcohol reaction. It helps discourage people living with alcohol use disorder from consuming alcohol.

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If you or a loved one take disulfiram, it’s also important to use caution if you’re given a prescription for metronidazole. When they’re taken together, the combination can result in psychosis. Psychosis symptoms can include hallucinations, paranoid delusions, and an altered mental state. 

As a result, metronidazole and disulfiram should not be taken together. Metronidazole should only be taken if at least 2 weeks have passed since your last dose of disulfiram.

2. Alcohol

While many of us enjoy an alcoholic beverage from time to time, it doesn’t always mix well with medications you may be taking. Metronidazole is no exception.

This interaction is similar to the reaction with disulfiram, mentioned above. The interaction between alcohol and metronidazole is called a “disulfiram-like reaction.” You’ll likely experience nausea, vomiting, and/or flushing if you drink alcohol while taking metronidazole.

To avoid having a sour and potentially dangerous evening, it’s important to avoid alcohol while taking metronidazole. You should wait at least 3 days after taking your last metronidazole dose before drinking alcohol.

3. Warfarin

Warfarin (Coumadin, Jantoven) is an anticoagulant (blood thinner) medication that prevents and treats blood clots. You or a loved one may be taking it for several reasons, such as preventing a blood clot after having a heart attack or stroke.

Sometimes, warfarin’s blood thinning effects can be too strong. What’s more, many factors can change your warfarin levels — including medications like metronidazole. Metronidazole can enhance warfarin’s effects and cause you to bleed more easily. This can be dangerous.

While you’re taking warfarin, your healthcare provider monitors blood tests to reduce your risk of bleeding. But you should also look out for signs of bleeding on your own. Possible symptoms include bleeding that takes a long time to stop, easy bruising, or dark stools. If any of these occur, you should immediately contact your care team.

4. Lithium

Lithium (Lithobid) is a medication that’s often prescribed by mental health specialists. It's a first-choice medication for bipolar disorder. It’s commonly referred to as a mood stabilizer.

Like warfarin, lithium levels are sensitive to small changes. Metronidazole can disrupt lithium’s balance in your body. It increases the levels of lithium in your blood. This is especially worrisome if you’re taking a higher dose of lithium. 

If you start a prescription for metronidazole, your healthcare provider may want to monitor your lithium levels more closely. If your levels become too high, they may adjust your lithium dose out of caution. When lithium levels are too high, you may experience symptoms of lithium toxicity. This includes symptoms like:

• Severe confusion or blacking out

• Vomiting that doesn’t stop

• Ringing in the ears (tinnitus) 

• Blurry vision

• Tremors or muscle spasms

• Drowsiness or lightheadedness

• Seizures

If you have symptoms of possible lithium toxicity, you should contact your care team as soon as you can.

5. Medications that affect your heart beat

Some medications can unintentionally affect your heart beat. Metronidazole is one such medication. This can pose a problem if you have arrhythmias or other heart problems. 

The risk of this side effect goes up when you take other medications that also affect your heart rhythm. Examples include medications like:

• Citalopram (Celexa)

• Haloperidol (Haldol)

• Methadone (Methadose)

• Levofloxacin

• Ziprasidone (Geodon)

Keep in mind, these are not all the medications that can have this kind of effect. This is why it’s important to give your healthcare provider and pharmacist a complete list of all the medications that you take. This helps them screen for interactions and make adjustments to your medication regimen.

6. Cimetidine

Heartburn can be unpleasant. If you have heartburn, you may have tried a medication called cimetidine (Tagamet HB). Cimetidine is a medication that reduces the amount of acid in your stomach. This helps treat conditions like acid reflux (heartburn) and stomach ulcers, but it can cause trouble when taken with metronidazole. 

When you take metronidazole, enzymes (proteins) in your liver are responsible for breaking it down. Some medications can slow down this process. This elevates the levels of metronidazole in your bloodstream, which increases the risk for side effects.

Cimetidine is one medication that can have this effect. Cimetidine blocks liver enzymes that are responsible for breaking down metronidazole. When taken together, cimetidine can cause levels of metronidazole to go up. This makes it more likely that you’ll experience side effects of metronidazole. 

Other medications can cause a similar effect. Some examples are: 

• Amiodarone (Pacerone)

• Clarithromycin (Biaxin XL)

• Diltiazem (Cardizem)

• Medications that contain cobicistat

7. Medications that speed up metronidazole’s clearance

Several medications can speed up your liver’s ability to break down metronidazole. If this happens, it can make metronidazole less effective. This can result in an undertreated infection. 

Some examples of these medications include:

• Phenytoin (Dilantin, Phenytek)

• Phenobarbital

• Rifampin (Rifadin)

• Carbamazepine (Tegretol)

This list is not exhaustive. You should refer to your local pharmacist to screen for these types of interactions.

When should I contact my healthcare provider about a metronidazole interaction?

Don’t hesitate to contact your care team if you’re concerned about a medication interaction at any point. However, it’s best to address interactions up front. It always helps to provide your care team with a complete list of medications, supplements, and vitamins that you are taking. This especially applies if you’re filling medications at multiple pharmacies. If this is the case, your pharmacist may not know about all the medications you’re taking.

Otherwise, if you experience any bothersome side effects after you start taking metronidazole, you may also want to contact your care team. They may be able to adjust your medication routine to help control metronidazole side effects.

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Written by:

Mandeep Sohal, PharmD

Mandeep Sohal, PharmD, has more than 7 years of experience in the pharmaceutical industry and is licensed in California. In his career, he’s had experience with specialty pharmacy, community pharmacy, pharmacy benefit management, and pharmaceutical manufacturers.

Edited by:

Joshua Murdock, PharmD, BCBBS

Joshua Murdock, PharmD, BCBBS, is a licensed pharmacist in Arizona, Colorado, and Rhode Island. He has worked in the pharmacy industry for more than 10 years and currently serves as a pharmacy editor for GoodRx.

Reviewed by:

Austin Ulrich, PharmD, BCACP

Austin Ulrich, PharmD, BCACP, is a board-certified ambulatory care clinical pharmacist. In his early career, he provided direct patient care in a variety of settings, including hospital and community pharmacies, and in a primary care clinic as a clinical pharmacist.

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