Specifically, the Food and Drug Administration (FDA) has approved metronidazole oral forms to treat various infections caused by bacteria or parasites. Examples of these infections include:
This article describes possible side effects of metronidazole oral forms. You’ll also find information on cost savings and coupon options for metronidazole.
Metronidazole oral forms are a generic version of the brand-name drug Flagyl. Metronidazole also comes in topical forms and an injectable form. But these forms aren’t described in this article. To learn more about these forms, talk with your doctor.
You can explore this overview article to learn more about metronidazole oral forms, including the drug’s uses, dosage, ways to save on cost, and more.
Boxed warnings
This drug has a boxed warning about the risk of cancer and about use for specific purposes. Boxed warnings are serious warnings from the FDA.
For details about risk of cancer, see the “Side effects of metronidazole in detail” section below. For more information about using the drug for specific purposes, see the “Warnings for metronidazole” section below.
More common side effects of metronidazole
Metronidazole oral forms may cause mild or serious side effects. Below are some of the more common side effects of this medication.
Commonly reported* side effects of metronidazole include:
- nausea
- headache
- appetite loss
- diarrhea
- metallic taste†
* For more information, see the prescribing information for metronidazole oral capsules and oral tablets.
† To learn more about this side effect, see the “Side effects of metronidazole in detail” section below.
Mild side effects of metronidazole
Metronidazole may cause certain mild side effects. Most often, mild side effects are temporary and can be easily managed. Tell your doctor or pharmacist if you have side effects that don’t go away or become bothersome.
Mild side effects that occurred in studies* of metronidazole include:
* For more information, see the prescribing information for metronidazole oral capsules and oral tablets.
† To learn more about this side effect, see the “Side effects of metronidazole in detail” section below.
Metronidazole oral forms may cause more mild side effects than those listed above. For details, see the prescribing information for metronidazole oral tablets and oral capsules.
Serious side effects of metronidazole
Metronidazole may cause certain serious side effects. If you have any serious side effects from metronidazole oral forms, call your doctor right away. But if you think you’re having a medical emergency or your side effects feel life threatening, take immediate action. Call 911 or a local emergency number, or go to the nearest emergency room.
After the Food and Drug Administration (FDA) approves a drug, it tracks and reviews side effects of the medication. Visit MedWatch’s website if you’d like to notify the FDA about a side effect you’ve had with metronidazole oral forms.
Serious side effects that occurred in studies* of metronidazole include:
* For more information, see the prescribing information for metronidazole oral capsules and oral tablets.
† Metronidazole oral tablets and oral capsules have a boxed warning for this side effect. A boxed warning is the most serious warning from the FDA. To learn more, see the “Side effects of metronidazole in detail” section below.
‡ To learn more about this side effect, see the “Side effects of metronidazole in detail” section below.
Common questions about side effects of metronidazole
Here are answers to some common questions about side effects of metronidazole oral forms. If you have other questions, talk with your doctor or pharmacist.
Can metronidazole cause changes in urine?
Yes, it might. Metronidazole can cause urine to appear darker than usual. But it’s extremely rare and thought to be harmless.
After you take a dose of metronidazole, your liver breaks down the medication. Your kidneys also help eliminate it from your body. It’s believed that a certain pigment is formed after metronidazole is broken down. This pigment causes darkened urine in very rare cases.
So far, it doesn’t seem that this side effect has caused symptoms or led to any medical problems.
That said, darkened urine can be a symptom of serious medical conditions, such as rhabdomyolysis. If you develop darkened urine, tell your doctor. They can help determine whether this is caused by metronidazole or another condition that may need to be treated.
How long do side effects of metronidazole usually last?
In most cases, metronidazole side effects last a few days at most. Mild side effects should go away on their own, especially if you only take the drug for a short time. Talk with your doctor if you develop side effects from metronidazole that don’t go away or disrupt your daily activities.
In rare cases, the drug may cause long-term side effects. To learn more, view the later question “Does metronidazole cause long-term side effects?” or talk with your doctor.
Will metronidazole cause heart-related side effects?
This is rare, but possible. For example, metronidazole can cause a heart rhythm problem called long QT interval. In studies, this heart-related side effect was rarely reported* in people taking metronidazole. But it may occur.
Your risk for this side effect is increased if you:
- already have a problem with your heart rhythm
- take other medications that can cause long QT interval
Before taking metronidazole, let your doctor or pharmacist know about your health history and any medications you take. They can check for any interactions with metronidazole and determine whether it’s safe for you to take this drug.
* For more information, see the prescribing information for metronidazole oral capsules and oral tablets.
Does metronidazole cause long-term side effects?
Yes, this is possible, but it’s rare. Long-term side effects are those that start either:
- during treatment and continue for a long time, possibly after stopping treatment, or
- after you’ve taken the drug for a long time or after ending treatment
Long-term side effects possible with metronidazole oral tablets and oral capsules include the following. These can occur at any time while you’re taking metronidazole:
- Damage to the brain. This damage to the brain usually goes away once treatment is stopped. But it can take up to a few weeks to clear.
- Damage to certain nerves. This damage to certain nerves outside of the brain or spinal cord usually goes away once treatment is stopped. But its symptoms can rarely be long lasting.
- Damage to the optic nerve. This damage to the optic nerve may need to be treated or managed long term. But that depends on how severe it is.
Also, keep in mind that metronidazole has a boxed warning for the risk of cancer. Boxed warnings are serious warnings from the Food and Drug Administration (FDA). For details about this risk, see the “Side effects of metronidazole in detail” section below.
Talk with your doctor or pharmacist to learn more about your risk for long-term side effects with metronidazole.
Coupon for metronidazole
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Side effects of metronidazole in detail
Below you’ll find details on some of the side effects of metronidazole oral forms.
Risk of cancer
Metronidazole has a boxed warning about the risk of cancer. Boxed warnings are serious warnings from the Food and Drug Administration (FDA).
In animal studies* of metronidazole, tumors formed in certain animals, but not others. It’s not known if the drug raises this risk in people.
That said, the drug has been in use for a long time. Follow-up studies have not shown an increased risk for cancer in people taking metronidazole.
* For more information, see the prescribing information for metronidazole oral capsules and oral tablets.
Ways to manage
Due to this risk, doctors will only prescribe metronidazole to treat infections the drug is known to work on. They’ll recommend the lowest dose for the shortest possible time needed to treat the infection.
Talk with your doctor if you have more questions or any concerns about this risk with metronidazole.
Metallic taste
Having a metallic taste in the mouth is a common side effect of metronidazole oral tablets and oral capsules. This taste has been described as “sharp” and “unpleasant.”
It’s not known why metronidazole causes this side effect. It’s expected to be temporary, and it should go away on its own or after you finish treatment with the drug.
Ways to manage
There’s no known way to treat metallic taste caused by metronidazole, other than stopping treatment with the drug. One way to help manage this side effect during treatment is to have a small snack or drink that you enjoy. This may mask the metallic taste in your mouth.
Some people can complete treatment with metronidazole despite this side effect. But if the metallic taste is bothering you, let your doctor know. They can determine whether another treatment could be used for your infection instead of metronidazole. Do not stop taking metronidazole without first talking with your doctor.
Furry tongue
Furry tongue is a common side effect of metronidazole oral tablets and oral capsules. Other antibiotics can also cause furry tongue, so this isn’t unique to metronidazole.
This side effect causes a black or brown hairy appearance to develop on your tongue. In rare cases, other color discolorations have been seen. These include blue, green, and yellow.
Ways to manage
Maintaining healthy oral hygiene while taking metronidazole may help prevent furry tongue. It can also help to avoid tobacco use.
Although this side effect can be visually unappealing, it usually doesn’t need medical attention or treatment. It should go away within a week or so after you’ve stopped taking the drug. Brushing your tongue with a toothbrush can also help clear the discolored appearance.
If you develop furry tongue while taking metronidazole, tell your doctor. They’ll suggest ways to help manage and treat furry tongue if the problem persists. Do not stop taking metronidazole without first talking with your doctor.
Allergic reaction
For some people, metronidazole oral forms can cause an allergic reaction. This side effect was rare in studies of metronidazole oral tablets and oral capsules. But it can still occur.
Symptoms can be mild or serious, and they may include:
- itchiness
- skin rash
- swelling under the skin, usually affecting eyelids, lips, hands, or feet
- swelling of the mouth, tongue, or throat, which can make it hard to breathe
- flushing (temporary warmth, redness, or deepening of skin color)
Ways to manage
For mild allergic reaction symptoms, such as a mild rash, call your doctor right away. They may recommend treatments to help manage your symptoms. They’ll also let you know if you should keep taking the medication.
For severe allergic reaction symptoms, such as swelling or trouble breathing, call 911 or your local emergency number right away. These symptoms require immediate medical care because they can become life threatening. If you’ve had a serious allergic reaction to metronidazole oral forms, your doctor may recommend taking a different medication instead.
Warnings for metronidazole
Below are some factors you may want to discuss with your doctor before taking metronidazole oral forms.
Boxed warnings
This drug has a boxed warning about the risk of cancer and about use for specific purposes. Boxed warnings are serious warnings from the Food and Drug Administration (FDA).
Doctors will only prescribe metronidazole for infections the drug is known to work on. They’ll usually avoid prescribing the drug for just any infection. Always take this medication as prescribed by your doctor.
For more details about risk of cancer, see the “Side effects of metronidazole in detail” section above. If you have questions about these boxed warnings, talk with your doctor.
Other warnings
It may not be safe for you to take metronidazole oral forms if you have specific health conditions. These are sometimes called drug-condition interactions. Other things may also affect whether metronidazole is a safe treatment option for you.
Before taking metronidazole oral forms, tell your doctor about your health and medications you take. Things to consider include:
Metronidazole and pregnancy
Doctors usually avoid prescribing metronidazole during the first pregnancy trimester. (The first trimester refers to the time between conception and week 12 of pregnancy).
If you’re pregnant and past the first trimester, your doctor will discuss the risks and benefits of metronidazole use. There aren’t enough studies* of metronidazole to know for sure whether the drug is safe to use during this time.
Talk with your doctor before taking this drug during pregnancy.
* For more information, see the prescribing information for metronidazole oral capsules and oral tablets.
Metronidazole and breastfeeding
It’s recommended that you avoid breastfeeding while taking metronidazole. Or you could take a different medication if you plan to breastfeed. Metronidazole passes into breast milk, so it could affect a child who is breastfed.
Talk with your doctor to learn more about alternatives to either metronidazole or to breastfeeding.
Metronidazole and alcohol
You should not drink any alcohol while taking metronidazole or for at least 3 days after your last dose. Metronidazole oral forms do interact with alcohol.
Consuming alcohol while taking metronidazole can cause symptoms that may be extremely uncomfortable, including:
- belly cramps
- nausea and vomiting
- headaches
- flushing (temporary warmth, redness, or deepening of skin color)
If you drink alcohol, talk with your doctor about when it’ll be safe to consume again after you finish treatment with metronidazole.
What to ask your doctor
This article describes common and serious side effects of metronidazole oral forms. If your doctor prescribes this medication, they can discuss possible side effects with you. Let your doctor know if you have concerns or questions about treatment with this medication.
Here’s a list of some possible questions you may want to ask your doctor:
- Does my health history increase my risk for side effects from metronidazole?
- Will any of the medications I take interact with metronidazole and cause side effects?
- How long should I avoid consuming alcohol after I finish taking metronidazole?
- What should I do if I become pregnant while taking metronidazole?
Disclaimer: Optum Perks has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.
