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Flagyl

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

Medically reviewed by Sanjai Sinha, MD. Last updated on Oct 20, 2021.

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.

Warnings

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:

  • you drank alcohol in the past 3 days;

  • you consumed foods or medicines that contain propylene glycol in the past 3 days; or

  • you took disulfiram (Antabuse) within the past 14 days.

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:

  • liver disease;

  • kidney disease (or if you are on dialysis);

  • a heart rhythm disorder;

  • a stomach or intestinal disease such as Crohn's disease;

  • a blood cell disorder such as anemia (lack of red blood cells) or low white blood cell (WBC) counts;

  • a fungal infection anywhere in your body; or

  • a nerve disorder.

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:

  • new or worsening symptoms of infection;

  • painful or difficult urination;

  • confusion;

  • a light-headed feeling (like you might pass out);

  • vaginal itching or discharge; or

  • blisters or ulcers in your mouth, red or swollen gums, trouble swallowing.

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):

  • numbness, tingling, or burning pain in your hands or feet;

  • vision problems, pain behind your eyes, seeing flashes of light;

  • muscle weakness, problems with speech or coordination;

  • trouble speaking or understanding what is said to you;

  • a seizure; or

  • fever, neck stiffness, and increased sensitivity to light.

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:

  • depression, trouble sleeping, feeling irritable;

  • headache, dizziness, weakness;

  • nausea, vomiting, loss of appetite, stomach pain;

  • diarrhea, constipation;

  • unpleasant metallic taste;

  • rash, itching;

  • vaginal itching or discharge, pain during sex;

  • mouth sores; or

  • swollen, red, or "hairy" tongue.

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:

  • an antidepressant;

  • asthma medication;

  • busulfan or other cancer medicine;

  • heart or blood pressure medication;

  • lithium or other antipsychotic medicine;

  • medicine to treat malaria, HIV, or other infection; or

  • a blood thinner - warfarin, Coumadin, Jantoven.

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

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.

However, there is controversy around this reaction because while some studies have shown serious problems for some people taking metronidazole, others have shown the combination does not cause any problems. Large clinical trials in humans have never been conducted to confirm this interaction.

The reaction has been referred to as a disulfiram-like reaction – disulfiram is a medication given to people to discourage alcohol consumption. When a person consumes alcohol, the body breaks it down in two steps. First, it breaks alcohol down into a compound called acetaldehyde. Acetaldehyde is responsible for the unwanted effects of alcohol such as nausea, vomiting, and flushing, and is toxic. Next, the body reduces acetaldehyde to acetate using an enzyme called aldehyde dehydrogenase. Acetate is easily oxidized by our body into carbon dioxide (CO2), which we then breathe out. Disulfiram blocks the effects of this enzyme which leads to acetaldehyde accumulation causing symptoms such as skin redness, palpitations, nausea, vomiting, headache and in severe cases a dangerous rapid heart rate or a sudden drop in blood pressure. It was thought that metronidazole blocked the enzyme aldehyde dehydrogenase as well, although this now seems to be incorrect.

Several studies that have investigated the reaction of metronidazole with alcohol have found evidence of the existence of this interaction to be absent or weak. It does seems that the concern attached to this reaction is overstated. It is possible that the reaction could just be a side effect of metronidazole or potentially only occur in a small sub-group of susceptible people, because the reaction does not appear to occur in everybody.

There needs to be more research done investigating this potential interaction, but because doctors are unable to say which people are more at risk of this interaction, it is best to err on the side of caution and avoid alcohol while taking metronidazole until more is known. 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

Metronidazole does not treat chlamydia and is not a recommended treatment for chlamydia, but it may be given if symptoms of chlamydia persist after finishing a course of first-line treatments for chlamydia such as doxycycline, azithromycin, or levofloxacin. When metronidazole is given as follow up treatment for persistent symptoms of chlamydia, usually in addition to other antibiotics such erythromycin, it is there to provide treatment for other possible bacterial causes that may cause similar symptoms to chlamydia.

Infections linked with sexual activity that are usually treated with metronidazole include bacterial vaginosis, trichomoniasis, and moderate-to-severe pelvic inflammatory disease. Continue reading

If you are taking oral metronidazole or using metronidazole gel for an infection that is linked with sexual activity, such as trichomoniasis, pelvic inflammatory disease, or bacterial vaginosis you should not have sex for 7 days after single-dose therapy (or until you have completed the 7-day treatment regimen) and your symptoms have resolved. This will help reduce the risk of reinfection. If your sexual partners are also being treated, then abstain from sexual intercourse until they have finished treatment. Continue reading

About 10% of women report a vaginal yeast infection (vaginal candidiasis) as a side effect of metronidazole treatment. This is because metronidazole not only kills the bacteria responsible for conditions such as bacterial vaginosis, but useful bacteria in the vaginal flora that help to keep other microbes in check. This can result in an overgrowth of the candida yeast. Symptoms of a yeast infection include itching and a thick, white odor-free discharge and it can be easily treated with medication such as fluconazole.

Oral yeast infections (oral thrush or oral candidiasis) have also been reported with metronidazole oral treatment. Symptoms may include creamy white lesions on the tongue, inner cheeks, or sometimes on the roof of your mouth, gums and tonsils; mouth redness, burning, or soreness and a furry or swollen tongue. Treatment is with an antifungal mouthwash such as nystatin or oral antifungal tablets or capsules. 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.