6 Metronidazole Side Effects You Should Know About

Metronidazole (Flagyl) is a popular medication that treats infections caused by bacteria and parasites. If you’re prescribed this medication, chances are that you already aren’t feeling your best. The last thing you want is extra side effects from something that’s supposed to make you feel better.

Unfortunately, all medications, including metronidazole, may cause side effects. But knowing what to expect and how to manage them can make all the difference. Here, we’ll discuss six oral metronidazole side effects that you should know about. 

1. Headache

If you’re going to experience any side effects while taking metronidazole, headaches are the most likely. They happen in up to 18% of people who take it. Thankfully, most of them are easily manageable.

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There are a few steps you can take to help make headaches go away. These include:

• Drinking plenty of water

• Getting enough sleep

• Trying an over-the-counter (OTC) pain medication like ibuprofen (Advil, Motrin) or naproxen (Aleve)

Sometimes, a long-lasting headache can be part of a larger issue, such as a brain or spinal cord (neurological) condition. If your headache persists despite your best efforts to treat it — or you have associated vision loss, confusion, or severe pain — get medical help right away.

2. Nausea

Nausea is another common side effect of metronidazole. It happens in up to 12% of people who take it.

Metronidazole doesn’t have to be taken with food, but doing so may help lessen stomach upset. These actions can also help treat nausea:

• Eat smaller, more frequent meals throughout the day rather than a few large meals.

• Try ginger or peppermint tea.

• Take an OTC nausea medication like bismuth subsalicylate (Pepto-Bismol), or ask your healthcare provider for a prescription antinausea medication.

It’s equally important to avoid alcohol or any alcohol-containing products (e.g., mouthwash, cough syrup) while you’re taking metronidazole. It’s a good idea to do this for 3 days after you finish taking it, too. Using alcohol with metronidazole can lead to an unpleasant reaction that includes nausea, vomiting, and flushing.

3. Diarrhea

Diarrhea is a common side effect caused by antibiotics. Metronidazole is no exception. It can happen in up to 4% of people who take it.

While diarrhea is not a fun side effect to experience, there are some things you can do to manage it at home. These include:

• Drinking enough fluids so you don’t get dehydrated

• Eating bland, low-fiber foods — part of the BRAT diet

• Taking OTC antidiarrheal medication such as bismuth subsalicylate (Pepto-Bismol)

Make sure you let your healthcare team know if your diarrhea doesn’t improve. This could be a sign of a more serious infection that may need treatment with different antibiotics.

4. Metallic taste

There’s a chance that metronidazole will cause a bitter, metallic taste in your mouth after taking it. This has been reported in up to 9% of people who take metronidazole. Thankfully, most metronidazole prescriptions don’t last longer than 2 weeks. And the strange taste should go away after stopping it.

In the meantime, you could try some dietary strategies to help limit the metallic taste. These include switching to plastic utensils, adding strong herbs or spices to foods that you take with metronidazole, or eating cold or frozen foods. Gargling with salt water or mouthwash might help, too.

Talk to your healthcare provider if the taste is unbearable — they may be able to prescribe an alternative.

5. Neurological effects

Overall, neurological side effects from metronidazole are rare. The most common type is mild to moderate peripheral neuropathy, or nerve damage. Peripheral neuropathy causes numbness, tingling, or a burning sensation in your fingers or toes. This is a larger concern in people who take metronidazole for longer than 4 weeks.

But keep in mind that metronidazole crosses the blood-brain barrier. It’s a natural filter in your body that shields your brain from unwanted substances. And, it’s your body’s way of protecting the brain from bacteria, viruses, and certain medications. If you take metronidazole long enough, it could build up and cause side effects related to the brain and spinal cord (central nervous system).

Encephalopathy is one of these rare but serious central nervous system (CNS) side effects. Encephalopathy is a word for anything that changes how the brain works. Specific symptoms include dizziness, difficulty with walking and balance, and trouble speaking. It typically only happens in a small number of people taking metronidazole for longer than 2 weeks.

Rarely, metronidazole can cause a condition called aseptic meningitis — brain inflammation that is not caused by infection. This can happen as early as a few hours after taking metronidazole. Aseptic meningitis usually causes headache, fever, and neck stiffness. 

While these side effects may sound frightening, all of them are typically reversible after stopping metronidazole.

6. Heart rhythm changes

Metronidazole can cause QT prolongation — an abnormal heartbeat that can put you at risk for heart rhythm changes. But it’s unlikely to do this on its own. The real concern is when metronidazole is combined with other medications that also affect your heart rhythm. Certain medication classes, like antipsychotics, antiarrhythmics, and some antibiotics, are common culprits.

You can have your pharmacist check your list of medications to see if anything might be a bad combination. Even if some of your medications can affect your heart rhythm, you can often still take them together. Your healthcare team may just need to watch you more closely or might suggest getting an electrocardiogram.

When should I contact my healthcare provider about metronidazole side effects?

Metronidazole usually only causes mild to moderate side effects. But, some side effects can be persistent or more serious. You should report any side effects like these to your healthcare provider:

• Dizziness

• Fever

• Flu-like symptoms

• Itching or mild rash

• Numbness, tingling, or burning sensation in your fingers or toes

• Persistent headache

• Taste changes that affect eating

• Uncontrolled nausea or vomiting

You should get medical help right away if you experience any of the following:

• Abnormal heartbeat

• New or sudden confusion

• Trouble with walking or balance

• Difficulty speaking

• Neck stiffness

• Seizures

• Severe and sudden headache

• Vision loss or changes

In general, don’t stop your medication without talking with your provider first. Stopping it too early could cause resistance to future infections.

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Written by:

Rachel Feaster, PharmD, BCOP, BCPS

Rachel Feaster has over 10 years of professional experience in ambulatory, inpatient, pharmacogenomics, and oncology care. She is board certified in oncology and pharmacotherapy.

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:

Tegan Smedley, PharmD, APh

Tegan Smedley, PharmD, APh, has 10 years of experience as a pharmacist. She has worked in a variety of settings, including retail, hospital, and ambulatory care.

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