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Generic name: metronidazole (oral/injection) [ me-troe-NI-da-zole ]
Brand names: FIRST Metronidazole, Flagyl
Drug classes: Amebicides, Miscellaneous antibiotics

Medically reviewed by Melisa Puckey, BPharm. Last updated on Feb 29, 2024.

What is Flagyl?

Flagyl is an antibiotic that is used to treat bacterial infections of the vagina, stomach, liver, skin, joints, brain and spinal cord, lungs, heart, or bloodstream.

Flagyl is also used to treat trichomoniasis, a sexually transmitted disease caused by a parasite. Usually both sexual partners are treated at the same time, even if one has no symptoms.

Do not use Flagyl to treat any conditions that have not been checked by your doctor.


You should not use Flagyl if you recently drank alcohol, or have taken disulfiram (Antabuse) within the past 2 weeks.

Do not drink alcohol or consume foods or medicines that contain propylene glycol while you are taking Flagyl and for at least 3 days after you stop taking it.

Seizures and other nervous system abnormalities have been reported in patients treated with Flagyl. You should stop using this medicine immediately if you experience any neurological symptoms such as seizures, headaches, visual changes, weakness, numbness, or tingling.

This medicine will not treat a viral infection such as the common cold or flu.

In animal studies (mice and rats), this medicine caused certain types of cancers or tumors. It is not known whether these effects would occur in people using this medicine. Ask your doctor about your risk

Before taking this medicine

You should not use Flagyl if you are allergic to metronidazole, secnidazole, or tinidazole, or if:

May harm an unborn baby. Do not use metronidazole to treat trichomoniasis during the first trimester of pregnancy. Tell your doctor if you become pregnant.

Not all uses of metronidazole are approved for treating children and teenagers. Metronidazole is not approved to treat vaginal infections in girls who have not begun having menstrual periods.

To make sure you can safely take Flagyl, tell your doctor if you have ever had:

In animal studies, metronidazole caused certain types of tumors, some of which were cancerous. However, it is not known whether these effects would occur in humans. Ask your doctor about your risk.

You should not breastfeed within 24 hours after using Flagyl. If you use a breast pump during this time, throw out the milk and do not feed it to your baby.

How should I use Flagyl?

Take Flagyl exactly as prescribed by your doctor. Follow all directions on your prescription label and read all medication guides or instruction sheets.

Flagyl oral is taken by mouth.

Flagyl injection is given as an infusion into a vein. A healthcare provider will give you this injection if you are unable to take the medicine by mouth.

Shake the oral suspension (liquid). Measure a dose with the supplied measuring device (not a kitchen spoon).

Swallow the extended-release tablet whole and do not crush, chew, or break it.

If you are treating a vaginal infection, your sexual partner may also need to take Flagyl so you don't become reinfected.

Flagyl is usually given for up to 10 days in a row. You may need to repeat this dosage several weeks later.

Keep using this medicine even if your symptoms quickly improve. Skipping doses could make your infection resistant to medication. Flagyl will not treat a viral infection (flu or a common cold).

Flagyl will not treat a vaginal yeast infection. You may even develop a new vaginal yeast infection, which may need to be treated with antifungal medication. Tell your doctor if you have symptoms such as itching or discharge during or after treatment with Flagyl.

Do not share this medicine with another person, even if they have the same symptoms you have.

This medicine can affect the results of certain medical tests. Tell any doctor who treats you that you are using this medicine.

Store at room temperature away from moisture and heat.

What happens if I miss a dose?

Take the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Do not take two doses at one time.

What happens if I overdose?

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.

Overdose symptoms may include nausea, vomiting, numbness, tingling, or problems with balance or muscle movement.

What should I avoid while using Flagyl?

While taking Flagyl and for 3 days after your last dose: Do not drink alcohol or consume foods, medicines, or other products that contain alcohol or propylene glycol. You may have unpleasant effects such as headaches, nausea, vomiting, stomach cramps, and warmth or tingling under your skin.

Flagyl side effects

Get emergency medical help if you have signs of an allergic reaction to Flagyl (hives, itching, warmth or tingling; fever, joint pain; dry mouth, dry vagina; stuffy nose, difficult breathing, swelling in your face or throat) or a severe skin reaction (fever, sore throat, burning eyes, skin pain, red or purple skin rash with blistering and peeling).

Call your doctor at once if you have:

Stop taking the medicine and call your doctor right away if you have neurologic side effects (more likely to occur while taking metronidazole long term):

Metronidazole can cause life-threatening liver problems in people with Cockayne syndrome. If you have this condition, stop taking Flagyl and contact your doctor if you have signs of liver failure--nausea, stomach pain (upper right side), dark urine, clay-colored stools, or jaundice (yellowing of the skin or eyes).

Side effects may be more likely in older adults.

Common Flagyl side effects may include:

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

What other drugs will affect Flagyl?

Sometimes it is not safe to use certain medicines at the same time. Some drugs can affect your blood levels of other drugs you use, which may increase side effects or make the medicines less effective.

Tell your doctor about all your current medicines. Many drugs can affect metronidazole, especially:

This list is not complete and many other drugs may interact with metronidazole. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible drug interactions are listed here.

Popular FAQ

In some cases, Flagyl (metronidazole) might be used to treat certain types of infections due to bacteria or protozoa (parasites) that include diarrhea as a symptom. It is not used to treat most cases of acute diarrhea (viral gastroenteritis or food poisoning), which are usually self-limiting and do not typically need treatment with an antibiotic. Do not use Flagyl to treat diarrhea unless a doctor has specifically prescribed it for this use. Continue reading

The preferred oral treatments for uncomplicated UTI infections (acute cystitis) are: nitrofurantoin (Furadantin, Macrobid, Macrodantin); fosfomycin (Monurol); and sulfamethoxazole-trimethoprim (Bactrim DS, Septra DS). Continue reading

Metronidazole (brand: Flagyl) can leave a bitter, metallic taste in your mouth. This is a well-known side effect. For many, the bad taste lasts throughout treatment, too, not just when you're swallowing the medicine. There are ways to help mask this taste with certain foods. Continue reading

Yes, Flagyl (generic name: metronidazole) can change your urine to a dark or reddish-brown color during treatment, but this is not dangerous. Continue reading

Flagyl is well-known for causing diarrhea, as well as other stomach side effects like nausea, heartburn and stomach pain. Diarrhea is common and occurs in about 1% to 10% of patients. Flagyl may also cause an unpleasant metallic taste during therapy which can lead to nausea and a decreased appetite. Continue reading

Drinking alcohol while taking metronidazole is not recommended because the combination of metronidazole and alcohol can cause a reaction (often referred to as a disulfiram-like reaction) in some people. Symptoms may include flushing, headaches, nausea, vomiting, and stomach cramps. There has been one reported death associated with this reaction. The product information and health professionals recommend not to drink alcohol during metronidazole treatment and for 3 days after finishing the course. Continue reading

Metronidazole starts to work 1 to 2 hours after you take it, because it is quickly absorbed, and it reaches its maximum concentration after 20 minutes to 3 hours. But it may take a couple of days before you start to feel better or notice an improvement in your symptoms. It is important that you still finish the course of metronidazole that your doctor has prescribed, even if you feel better, because the infection may still be present, and it could flare up or recur if you haven’t finished the course. Continue reading

About 10% of women report a vaginal yeast infection (vaginal candidiasis) as a side effect of metronidazole treatment. Although metronidazole spares vaginal lactobacilli, which are naturally resistant to nitroimidazoles such as metronidazole, it does not restore a Lactobacillus-dominated microbiome and disruption in the normal vaginal microbiota can persist. Continue reading

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Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use Flagyl only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.