Flagyl (Metronidazole) - Oral


Flagyl (metronidazole) has caused cancer in mice and rats. This does not necessarily mean that it will have the same effects in humans, but to be safe, it should only be used for conditions approved by the Food and Drug Administration (FDA).

What Is Flagyl?

Flagyl (metronidazole) is a prescription antibiotic used to treat certain bacterial or parasitic infections. It belongs to a drug class called nitroimidazole antimicrobials and works by stopping bacteria growth.

Metronidazole is available as an immediate-release (IR) capsule and tablet or extended-release (ER) tablet to be taken by mouth. It is also available in other forms, such as topical cream for rosacea or vaginal gel to treat bacterial vaginal infections.

However, this article will focus on Flagyl for oral use.

Drug Facts

Generic Name: Metronidazole

Brand Name(s): Flagyl, Flagyl ER

Drug Availability: Prescription

Therapeutic Classification: Nitroimidazole antimicrobial

Available Generically: Yes

Controlled Substance: N/A

Administration Route: Oral

Active Ingredient: Metronidazole

Dosage Form(s): IR capsule, IR tablet, ER tablet

What Is Flagyl Used For?

The Food and Drug Administration (FDA) approved Flagyl to treat certain bacterial infections in different areas of the body. This includes reproductive and nervous system infections and skin, heart, bone, joints, lungs, and blood infections.

It can also treat both partners for a sexually transmitted infection (STI) called trichomoniasis, which is caused by a parasite.

The extended-release (ER) version of Flagyl is used to treat bacterial vaginosis (BV), which is caused by an overgrowth of bacteria in the vagina.

Flagyl does not treat viral infections like colds, flu, or COVID-19.

How to Take Flagyl

If you are prescribed Flagyl, read the prescription label and the information leaflet that comes with your prescription. Consult your healthcare provider if you have any questions.

Use Flagyl exactly as directed, and do not skip doses. However, you may need to take a second course if you do. Your healthcare provider will let you know if this is the case.

Other considerations to keep in mind while taking this medication include the following:

  • Do not drink alcohol or consume products that contain propylene glycol while taking Flagyl and for at least three days after you finish treatment. The combination can cause a serious reaction with nausea, vomiting, fast heartbeat, low blood pressure, headaches, and flushing.
  • If you are taking the ER tablet, swallow it whole. Do not crush, chew, or break the tablet.
  • If you are taking it to treat trichomoniasis, ask your healthcare provider if your partner needs to take Flagyl to prevent reinfection.
  • Flagyl does not treat a vaginal infection that is caused by yeast. However, while taking it, you may develop a vaginal yeast infection. Let your healthcare provider know if you have symptoms like vaginal itching or discharge while on this medication, or after you stop treatment.
  • Continue to take this medicine for the entire time prescribed, even if you start to feel better. Skipping doses or stopping the medication too early can make you resistant to it. This means that it might not work as well the next time you need it.
  • Flagyl should only be used for the prescribed infection. It does not treat viral infections like colds, flu, or COVID-19.
  • Never share this medicine with anyone else, even if they share the same symptoms.
  • Flagyl can affect certain blood tests. Tell your healthcare providers that you are taking it.


Store metronidazole at room temperature (between 68 F and 77 F), away from direct light, heat, and moisture. Do not keep it in the bathroom. Store the tablets and capsules out of reach and sight of children and pets. Keep the cap tightly closed when not in use.

Off-Label Uses

Healthcare providers may prescribe metronidazole for off-label uses, meaning for conditions not specifically indicated by the FDA.

Off-label uses may include:

How Long Does Flagyl Take to Work?

Flagyl begins to absorb in the body about one or two hours after it is taken. Therefore, it may take a few days until you start to feel better.

It is important to take Flagyl for the entire time prescribed to ensure the infection is clear and does not return. In some cases, a second course may be needed.

What Are the Side Effects of Flagyl?

This is not a complete list of side effects, and others may occur. A healthcare provider can advise you on side effects. If you experience other effects, contact your pharmacist or a healthcare provider. You may report side effects to the FDA at fda.gov/medwatch or 1-800-FDA-1088.

Like other medications, Flagyl can cause side effects. Tell your healthcare provider about any side effects you experience while taking this medication.

Common Side Effects

The most common side effects of Flagyl are:

  • Gasotrintestinal (GI) effects, such as nausea, vomiting, diarrhea, constipation, upper abdominal pain, stomach cramps, or appetite loss
  • Headache
  • Metallic taste in the mouth
  • Skin rash
  • Dizziness
  • Fungal (yeast) infection
  • Red, swollen, or hairy tongue (caused by an overgrowth of Candida)

Severe Side Effects

Call your healthcare provider immediately if you have serious side effects. Call 911 if your symptoms feel life-threatening or if you think you’re having a medical emergency. Serious side effects and their symptoms can include the following:

  • Hypersensitivity reaction or anaphylaxis: Symptoms can include rash, hives, swelling around the lips, tongue, and face, and difficulty breathing. An allergic reaction may require emergency medical attention. 
  • Severe skin reaction: Be alert to peeling or blistering skin, red or purple rash, burning eyes, sore throat, or fever. This reaction can be life-threatening and requires emergency medical attention. 
  • Seizures
  • Aseptic meningitis: Be alert to symptoms that may include headache, fever, vomiting, and stiff neck. Call your healthcare provider right away if symptoms occur.
  • Peripheral neuropathy (nerve damage): Tell your healthcare provider right away if you have numbness, tingling, or burning in the hands or feet.
  • Optic neuropathy (damage to the optic nerve of the eye): Call your healthcare provider right away if you have vision changes or eye pain.
  • Encephalopathy: Flagyl may cause problems with the brain. Symptoms can include personality changes, confusion, seizures, and twitching. Call your healthcare provider immediately if symptoms occur.
  • Leukopenia: A decrease in white blood cells, which can interfere with the body's ability to fight infection.
  • Thrombocytopenia (low platelet levels): Can increase the risk of bruising and bleeding
  • QT prolongation (a heart rhythm disorder): This condition can have no symptoms or may cause seizures and fainting.

People with a rare genetic disorder called Cockayne syndrome are at higher risk of having severe liver problems from Flagyl, which can cause death. Stop taking Flagyl and call your healthcare provider right away if you have:

  • Nausea
  • Stomach pain
  • Dark urine
  • Clay-colored stools
  • Yellowing of the skin or whites of the eyes

Long-Term Side Effects

While many people tolerate Flagyl well (and only take it for a short time), long-term or delayed side effects are possible, especially when taking Flagyl for an extended period. Some long-term side effects can be mild, such as:

  • Acne
  • Appetite loss
  • Back pain
  • Flu
  • Inflamed gums
  • Joint/muscle pain
  • Menstrual cramps
  • Vaginal discharge
  • Weakness

Moderate long-term side effects can include:

Severe long-term side effects may include:

  • Aseptic meningitis
  • Liver problems or liver failure
  • Seizures
  • Severe skin reactions

Report Side Effects

Flagyl may cause other side effects. Call your healthcare provider if you have any unusual problems while taking this medication.

If you experience a serious side effect, you or your healthcare provider may send a report to the FDA's MedWatch Adverse Event Reporting Program or by phone (800-332-1088).

Dosage: How Much Flagyl Should I Take?

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The dose of this medicine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of this medicine. If your dose is different, do not change it unless your doctor tells you to do so.

The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.

  • For bacterial vaginal infections:
    • For vaginal dosage form (cream):
      • Adults—One applicatorful (500 milligrams [mg]), inserted into the vagina. Use the medicine 1 or 2 times a day for 10 or 20 days.
      • Children—Use and dose must be determined by your doctor.
    • For vaginal dosage form (gel):
      • Nuvessa™:
        • Adults and children 12 years of age and older—One applicatorful (5 grams [g]) inserted into the vagina once (at bedtime). Each applicatorful of Nuvessa™ contains 65 mg of metronidazole.
        • Children younger than 12 years of age—Use and dose must be determined by your doctor.
      • Vandazole®:
        • Adults—One applicatorful (5 grams [g]) inserted into the vagina once a day (at bedtime) for 5 days. Each applicatorful of Vandazole® contains 37.5 mg of metronidazole.
        • Children—Use and dose must be determined by your doctor.
    • For vaginal dosage form (tablets):
      • Adults—One 500 milligram (mg) tablet, inserted high into the vagina. Use the medicine once a day in the evening for 10 or 20 days.
      • Children—Use and dose must be determined by your doctor.


Due to the possible effects of this medication, there may be changes to how it is used. Therefore, users need to be aware of the following when taking Flagyl.

Older Adults

You may need to use caution when taking Flagyl if you are 65 years or older. Your healthcare provider will monitor you closely for side effects.

Liver or Kidney Problems

People with severe liver problems will need a lower dose of Flagyl. People with mild or moderate liver problems (or kidney problems) can generally take the regular dose but will need to be monitored closely.


People who are pregnant or planning to become pregnant should consult their healthcare provider regarding the use of Flagyl. Pregnant people with trichomoniasis should not take Flagyl during the first trimester of pregnancy.


The prescribing information states that Flagyl should not be used while breastfeeding. A suggested alternate approach would be to pump and discard milk while on the medication and for 24 hours after the last dose and to feed the baby formula or previously stored breast milk.

Missed Dose

If you miss a dose of Flagyl, take it as soon as you remember. If it is almost time for the next dose, skip the missed dose. Do not take two doses together to try to make up for a missed dose. If you miss too many doses or stop taking it too early, you may need a second course of treatment.

Overdose: What Happens If I Take Too Much Flagyl?

Taking too much Flagyl can cause nausea, vomiting, numbness and tingling, and impaired coordination. Call your healthcare provider immediately if you suspect you've overdosed on metronidazole. You may need to be treated for your symptoms.

What Happens If I Overdose on Flagyl?

If you think you or someone else may have overdosed on Flagyl, call a healthcare provider or the Poison Control Center (800-222-1222).

If someone collapses or isn't breathing after taking Flagyl, call 911 immediately.


Drug Content Provided and Reviewed by IBM Micromedex®

It is important that your doctor check your progress after you finish using this medicine to make sure that the infection is cleared up.

If your symptoms do not improve within a few days after you start this medicine or if they become worse, check with your doctor.

You should not use this medicine if you are taking or have taken disulfiram (Antabuse®) within the last 2 weeks. Disulfiram is used to help people who have a drinking problem. If these 2 medicines are taken close together, serious unwanted effects may occur.

Drinking alcoholic beverages while using this medicine may cause stomach pain, nausea, vomiting, headache, or flushing or redness of the face. Alcohol-containing medicines (eg, elixirs, cough syrups, tonics) may also cause problems. The chance of these problems occurring may continue for at least a day after you stop using metronidazole. You should not drink alcoholic beverages (eg, ethanol or propylene glycol) or take other alcohol-containing medicines while you are using this medicine and for at least 24 hours after treatment.

Check with your doctor right away if you have dizziness, problems with muscle control or coordination, shakiness or an unsteady walk, slurred speech, or trouble with speaking. These may be symptoms of a serious brain condition called encephalopathy.

Call your doctor right away if you have confusion, drowsiness, fever, a general feeling of illness, a headache, loss of appetite, nausea, a stiff neck or back, or vomiting. These could be symptoms of a serious condition called aseptic meningitis.

Check with your doctor right away if you are having burning, numbness, tingling, or painful sensations in the arms, hands, legs, or feet. These could be symptoms of a condition called peripheral neuropathy.

This medicine may cause some people to become dizzy or lightheaded. Do not drive or do anything else that could be dangerous until you know how this medicine affects you. If these reactions are especially bothersome, check with your doctor.

Vaginal medicines usually leak out of the vagina during treatment. To keep the medicine from getting on your clothing, wear a mini-pad or sanitary napkin. Do not use tampons since they may soak up the medicine.

To help clear up your infection completely and to help make sure it does not return, good health habits are also required.

  • Wear cotton panties (or panties or pantyhose with cotton crotches) instead of synthetic (eg, nylon or rayon) panties.
  • Wear only freshly washed panties daily.

Do not have sexual intercourse while you are using this medicine. Having sexual intercourse may reduce the strength of the medicine. This may keep the medicine from working properly. Also, oils in the cream and vaginal tablets (but not the vaginal gel) may damage latex (rubber) contraceptive devices (eg, cervical caps, condoms, or diaphragms), causing them to leak, wear out sooner, or not work properly.

Avoid using douches or other vaginal products unless your doctor tells you to.

Many vaginal infections (eg, trichomoniasis) are spread by having sexual intercourse. You can give the infection to your sexual partner, and the infection could be given back to you. Your partner may also need to be treated for some infections. Until you are sure that the infection is completely cleared up after your treatment with this medicine, your partner should wear a condom during sexual intercourse. If you have any questions about this, check with your doctor.

Before you have any medical tests, tell the medical doctor in charge that you are using this medicine. The results of some tests may be affected by this medicine.

Do not take other medicines unless they have been discussed with your doctor. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements.

What Are Reasons I Shouldn't Take Flagyl?

Flagyl is not appropriate for everyone. You should not take this medication if you are allergic to metronidazole, similar drugs in the same class of drugs (such as tinidazole), or any inactive ingredients in Flagyl.

You should also not take this medication if you:

  • Consume alcohol (alcohol should be avoided during treatment and for at least three days after the last dose)
  • Have taken a drug called disulfiram in the past 14 days
  • Have a genetic disorder called Cockayne syndrome
  • Are breastfeeding

Flagyl may be prescribed with caution in some people only if the healthcare provider determines it is safe. This includes people:

What Other Medications May Interact With Flagyl?

Tell your healthcare provider about all your medicines, including prescription and over-the-counter (OTC) medications and vitamins or supplements. Some examples of drugs or substances that interact with Flagyl include:

  • Disulfiram is a medication used to treat people who have alcohol use disorder. People taking this medicine in the past 14 days should not take Flagyl. The combination could cause a psychotic reaction.
  • Alcohol or foods/drinks that contain an additive called propylene glycol should not be combined with Flagyl. The combination could cause stomach cramps, nausea, vomiting, headaches, and flushing.
  • Flagyl can interact with blood thinners such as Jantoven (warfarin), increasing the chances of bleeding.
  • Flagyl may cause lithium toxicity in some instances. People who take lithium should discuss this possibility with their healthcare provider before taking Flagyl.
  • Flagyl may interfere with specific lab tests. If you are going for bloodwork, tell your healthcare provider that you are taking metronidazole.

Other drug interactions may occur. Consult your healthcare provider for a complete list of drug interactions.

What Medications Are Similar?

Alternative medications to metronidazole depend on the condition being treated. For example, Flagyl can be used for a sexually transmitted infection called trichomoniasis. Other medications used for trichomoniasis may include Tindamax (tinidazole) or Solosec (secnidazole).

Another common use for metronidazole is bacterial vaginosis. Alternatives for this indication include:

  • MetroGel Vaginal (metronidazole in a gel form inserted vaginally with an applicator)
  • Cleocin (clindamycin) vaginal cream
  • Tinidazole)
  • Secnidazole

This is a list of drugs also prescribed for certain types of infections. It is NOT a list of drugs recommended to take with metronidazole. Talk to your pharmacist or a healthcare provider if you have questions.

Frequently Asked Questions

  • What is Flagyl used for?

    Flagyl can be used to treat various bacterial infections, such as infections of the stomach, reproductive system, skin, heart, bone, joints, lung, blood, and nervous system. It can also be used to treat a parasitic sexually transmitted infection called trichomoniasis and bacterial vaginosis.

    Flagyl does not treat viral infections such as the common cold, flu, or COVID-19.

  • How does Flagyl work?

    Flagyl works by stopping the growth of bacteria. It works against various Gram-negative and Gram-positive anaerobic bacteria and protozoal parasites. Your healthcare provider will prescribe this only if your infection is caused by these types of bacteria.

  • What drugs may interact with Flagyl?

    Flagyl can interact with certain drugs, such as disulfiram, lithium, and blood thinners (e.g., warfarin). It is also important to note that Flagyl can have a serious reaction with alcohol or products that contain an additive called propylene glycol. Let your healthcare provider know what medications, vitamins, or herbal supplements you take before starting treatment.

  • How long does it take for Flagyl to work?

    You may start to feel better within a day or two of starting Flagyl. However, it is important to finish the entire course of medication to ensure the infection is completely cleared. In some cases, a second course may be necessary.

  • What are the side effects of Flagyl?

    Common side effects include stomach problems like pain or cramps, nausea, vomiting, and diarrhea or constipation. A metallic taste in the mouth can also occur, as well as a red, swollen, or hairy tongue (caused by an overgrowth of Candida). Serious side effects are also possible. Before starting treatment, discuss potential side effects with your healthcare provider, so you know what to expect.

  • How do I stop taking Flagyl?

    Your healthcare provider will tell you how long to take Flagyl. Do not skip doses or stop taking it before your treatment course is complete.

How Can I Stay Healthy While Taking Flagyl?

Before taking Flagyl, discuss your medical history and medication use with your healthcare provider.

During treatment, follow your healthcare provider’s instructions for use. Read the patient information leaflet that comes with your prescription. Don't hesitate to talk to your healthcare provider or pharmacist if you have any questions or concerns.

Remember that alcohol can cause a severe interaction with Flagyl. Avoid alcohol while taking it and for at least three days after the last dose of Flagyl. This includes alcoholic beverages (and even non-alcoholic beers, which may contain some alcohol). Also, look for hidden sources of alcohol in foods such as sauces or vanilla extract and other products such as aftershave, mouthwash, and cough syrups. Your pharmacist can help you check labels for alcohol content if you are unsure.

The same warning also applies to an additive called propylene glycol. Products that contain propylene glycol should be avoided while taking Flagyl and for at least three days after the last dose. Propylene glycol can be found in certain medications and many packaged foods, such as ice cream, soups, seasoning, and salad dressings.

Medical Disclaimer

Verywell Health's drug information is meant for educational purposes only and is not intended to replace medical advice, diagnosis, or treatment from a healthcare provider. Consult your healthcare provider before taking any new medication(s). IBM Watson Micromedex provides some of the drug content, as indicated on the page.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Karen Berger, PharmD
Karen Berger, PharmD, is a community pharmacist and medical writer/reviewer.