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Metronidazole

Flagyl, Emgyl

RA
UR

Written by R. Author | Reviewed by Uche Ruth

Updated on June 16, 2024

Overview

Metronidazole (Flagyl) is an oral antibiotic for adults and children. It's used to treat certain bacterial and parasitic infections, including bacterial vaginosis (a type of vaginal bacteria infection) and trichomoniasis (a sexually transmitted infection, or STI). Metronidazole (Flagyl) is taken by mouth, typically 3 times a day. Some common side effects of this medication include diarrhea, nausea, and vomiting. Avoid drinking alcohol while taking and for 3 days after stopping metronidazole (Flagyl) because this combination can cause a serious reaction. Metronidazole (Flagyl) is available as a lower-cost generic.

What is Metronidazole (Flagyl) used for?

Treatment of certain bacterial and parasitic infections, including:

How Metronidazole (Flagyl) works

Metronidazole (Flagyl) is a nitroimidazole antibiotic. It kills bacteria and parasites by damaging their genetic material.

When does Metronidazole (Flagyl) start working?

Time passed since treatment started:

time to start working
Pros and cons of Metronidazole (Flagyl)
pros and cons of metrinidazole
Pharmacist tips for Metronidazole (Flagyl)

  • Don't drink alcohol or eat large amounts of food containing ethanol or propylene glycol while taking metronidazole (Flagyl) and for at least 3 days after. Doing so might cause belly cramps, nausea, vomiting, headaches, and flushing.
  • Metronidazole (Flagyl) might get into breastmilk. If you're nursing, you might want to consider feeding your baby previously stored breast milk or infant formula while taking metronidazole (Flagyl) and for 2 days after treatment ends.
  • If you're taking metronidazole (Flagyl) for trich, make sure that your sexual partner(s) also get treated to avoid getting reinfected. Don't have sex until you or your sexual partner(s) have finished treatment and have no more symptoms.
  • Your primary care provider might also test you for human immunodeficiency virus (HIV) and other STIs if you tested positive for bacterial vaginosis (BV) or trich. Having BV or trich can raise your risk for other STIs as well.
  • Metronidazole (Flagyl) can interact with many medications. Let your care team know what medications you're taking, including over-the-counter products and supplements, before you start metronidazole (Flagyl). Talk with your care team if you have any questions about interactions with this medication.
  • Let your primary care provider know if you're taking blood thinners, like warfarin (Coumadin), before starting treatment. Metronidazole (Flagyl) can raise your risk of bleeding when taking these medications together.

More tips for the metronidazole (Flagyl) extended-release (ER) tablets

  • Don't chew the metronidazole (Flagyl) extended-release (ER) tablets because doing so can alter how the medication gets absorbed into your body.
  • Metronidazole (Flagyl) ER tablets should be taken on an empty stomach, an hour before or 2 hours after your meals.

What are the risks and warnings for Metronidazole (Flagyl)?

Metronidazole (Flagyl) can cause some serious health issues. This risk may be even higher for certain groups. If this worries you, talk to your doctor or pharmacist about other options.

Possible risk of cancer

  • Risk factor: Taking metronidazole (Flagyl) for a long time

It’s not clear whether or not taking metronidazole (Flagyl) can put you at risk for cancer. Studies show that rats who took this medication throughout their lifetime were at higher risk for tumors in the liver, lung, and different organs. If you're taking metronidazole (Flagyl), don't take this medication more often than what your primary care provider has directed.

Serious allergic reactions

Metronidazole (Flagyl) can sometimes cause rash and other severe skin reactions, such as Stevens-Johnson syndrome (SJS). If you experience these reactions, your care team will need to stop metronidazole (Flagyl) right away and switch you to a different medication. If you've a rash, blisters on your lips, skin peeling, skin pain, mouth sores, or high fever, get medical help immediately.

Brain and nerve problems

Rarely, people who take metronidazole (Flagyl) can develop serious brain and nerve problems, such as seizures, encephalopathy (a health condition that leads to brain damage) or meningitis (inflammation of the brain lining). Get medical attention right away if you experience convulsions, problems with speech, coordination, clear thinking, headache, nausea, vomiting, stiff neck, and sensitivity to light. These are symptoms of brain damage and your care team might ask you to stop taking metronidazole (Flagyl) to prevent further injury. Symptoms can develop within hours of treatment, but will also go away within days to weeks of stopping this medication.

In addition, people who take metronidazole (Flagyl) might also experience different nerve problems. Let your primary care provider know if you experience numbness in the hands and feet, pain when moving the eye, or changes in vision. Your primary care provider will discuss with you whether metronidazole (Flagyl) is a good option for you.

Liver and kidney problems

  • Risk factors: Current or history of liver problems | Current or history of kidney problems

People with liver or kidney problems who take metronidazole (Flagyl) might have a higher amount of the medication in the blood. This can lead to higher risk of side effects, such as nausea, headache, and seizures. Depending on how severe your liver or kidney problems are, your care team will help you determine the lowest possible dose of metronidazole (Flagyl) that is safe for you. If you experience symptoms of liver problems (feeling sleepy, upper right stomach pain, yellowing of the skin or eyes, dark urine) or kidney problems (urinating more or less than usual, feel nauseous or weak, have a fever, rash, joint pain), talk with your care team right away.

Risk of fungal infection

Taking metronidazole (Flagyl) can raise your risk of getting a fungal infection, including a vaginal yeast infection. It's important to let your care team know if your symptoms don't go away or get worse after starting treatment. Your primary care provider might prescribe an antifungal medication for you if symptoms develop. Please talk with your care team if you notice your vagina is red and itchy, you have thick white vaginal fluid, or if you've fever or chills.

Risk of drug resistance

  • Risk factors: Taking metronidazole (Flagyl) more often or differently than prescribed

Taking metronidazole (Flagyl) more often or differently than prescribed can put you at risk for infections that are harder to treat in the future. To make sure that this medication will work well to fight future infections, follow your care team's instruction on when and how long to take metronidazole (Flagyl).

Lower white blood cell counts

  • Risk factors: Current or history of blood problems | Taking medications that lower white blood cell counts

Though extremely rare, some people developed lower white blood cell count while taking metronidazole (Flagyl). This can put you at higher risk for infections. Your primary care provider will typically require lab work before and while you're taking this medication to make sure it's safe for you. Let your care team know if you've symptoms of low white blood cells, such as fever, chills, weakness, fast heart beat, sores in the mouth and throat.

Interaction with other medications

Metronidazole (Flagyl) can interfere with how your body gets rid of other medications. For example, taking certain medications like disulfiram with metronidazole (Flagyl) can cause delusions and hallucinations. These medications should be avoided together.

When metronidazole (Flagyl) is taken with lithium, it can raise the amount of lithium in your body. This can lead to higher risk of lithium toxicity and side effects, such as confusion, tremors, and uncontrolled eye movements.

Metronidazole (Flagyl) can also raise the risk of bleeding when it's taken with certain blood thinners, such as warfarin (Coumadin). Make sure to get your routine blood tests done on time during treatment.

Strengths

Capsule

  • 375mg

Tablet

  • 250mg
  • 500mg

Dosages

The typical dosing for metronidazole (Flagyl) can differ for each person depending on what type of infection you have. Your dose might also differ if you have kidney or liver problems.

Adults

Immediate-release tablets

  • Trichomoniasis (1-day course): The typical dosing is 2 grams by mouth, taken as a single dose or split into 2 doses and taken in one day.
  • Trichomoniasis (7-day course): The typical dosing is 250 mg by mouth 3 times daily for a week.
  • Amebiasis: The typical dosing ranges from 500 mg to 750 mg by mouth 3 times daily for 5-10 days.
  • Bacterial infections: Dosing for certain bacterial infections will depend on your weight and the severity of the infection. The typical dosing is 7.5 mg/kg of body weight by mouth every 6 hours for 7-10 days. The maximum recommended dose is 4 grams daily.

Immediate-release capsules

  • Trichomoniasis: The typical dosing is 375 mg by mouth 2 times daily for a week.
  • Amebiasis: The typical dosing is 750 mg by mouth 3 times daily for 5-10 days.
  • Bacterial infections: Dosing for certain bacterial infections will depend on your weight and the severity of the infection. The typical dosing is 7.5 mg/kg of body weight by mouth every 6 hours for 7-10 days. The maximum recommended dose is 4 grams daily.

Extended-release tablets

  • Bacterial vaginosis: The typical dosing is 750 mg by mouth once a day for a week.

Children

IR tablets and capsules

  • Amebiasis: Your child's pediatrician will determine the dose based on your child's body weight. The typical dose is 35-50 mg/kg of body weight by mouth daily, split into 3 doses and taken throughout the day for 10 days.

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Interactions

Metronidazole (Flagyl) may interact with certain medications or supplements. Always let your doctor and pharmacist know about any other medications or supplements (including prescribed and over-the-counter medications, vitamins, and dietary or herbal supplements) that you are currently taking. The list below does not include all possible drug interactions with Metronidazole (Flagyl). Please note that only the generic name of each medication is listed below.

Using this medicine with any of the following medicines is not recommended. Your doctor may decide not to treat you with this medication or change some of the other medicines you take.

Using this medicine with any of the following medicines is usually not recommended, but may be required in some cases. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.

Using this medicine with any of the following medicines may cause an increased risk of certain side effects, but using both drugs may be the best treatment for you. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.

Side-effects

The following side effects may get better over time as your body gets used to the medication. Let your healthcare provider know immediately if you continue to experience these symptoms or if they worsen over time.

Common Side Effects

Please note: These are common side effects reported by people who took extended-release metronidazole (Flagyl)

  • Headache (18%)
  • Inflammation of the vagina (15%)
  • Nausea (10%)
  • Metallic taste (9%)
  • Flu-like symptoms (6%)

Other Side Effects

  • Itchy groin
  • Belly pain
  • Dizziness
  • Diarrhea
  • Upper respiratory infection
  • Sinus infection
  • Stuffy nose
  • Sore throat
  • Dry mouth
  • Urinary tract infection

Serious Side Effects

Contact your healthcare provider immediately if you experience any of the following.

  • Brain and nerve problems: seizures, numbness in the feet and hands, pain when moving the eye, headache, dizziness, vertigo, impaired coordination, slurred speech, easily irritable, depression, weakness, trouble sleeping
  • Severe allergic or skin reaction: rash, blistering or peeling of skin, hives, painful sores in your mouth or around your eyes, fever, trouble breathing, swollen lymph nodes
  • Kidney problems: feeling sick or weak, urinating less than usual, confusion, blood in urine, swelling in the legs, ankles, or feet
  • Liver problems: pain in the right side of the stomach, yellowing of the whites of the eyes or skin, dark-colored urine, light-colored stool

When do Metronidazole (Flagyl) possible side effects begin and end?

Time passed since treatment started:

time passed

The following side effects have also been reported

Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur:

More common

  • Agitation
  • Back pain
  • Blindness
  • Blurred vision
  • Burning, numbness, tingling, or painful sensations in the hands or feet
  • Changes in speech patterns
  • Confusion
  • Decreased vision
  • Depression
  • Dizziness
  • Drowsiness
  • Eye pain
  • Fever
  • Headache
  • Irritability
  • Lack of coordination
  • Nausea
  • Seeing or hearing things that are not there
  • Seizures
  • Shakiness and unsteady walk
  • Slurred speech
  • Stiff neck or back
  • Trouble speaking
  • Unsteadiness, trembling, or other problems with muscle control or coordination
  • Unusual tiredness or weakness
  • Vomiting
  • Weakness in the arms, hands, legs, or feet

Less common

  • Black, tarry stools
  • Blood in the urine
  • Body aches or pain
  • Burning while urinating
  • Chills
  • Clumsiness or unsteadiness
  • Difficulty with breathing
  • Ear congestion
  • Fainting
  • Feeling of pelvic pressure
  • Frequent or painful urination
  • Irregular heartbeat
  • Loss of voice
  • Nasal congestion
  • Pinpoint red spots on the skin
  • Runny nose
  • Skin rash, hives, redness, itching
  • Sneezing
  • Stomach and back pain (severe)
  • Unusual bleeding or bruising
  • Vaginal irritation, discharge, or dryness not present before taking the medicine

Rare

  • Bleeding gums
  • Bloating
  • Chest pain
  • Constipation
  • Cough
  • Dark urine
  • Fast heartbeat
  • Indigestion
  • Loss of appetite
  • Painful or difficult urination
  • Pains in the stomach, side, or abdomen, possibly radiating to the back
  • Sore throat
  • Sores, ulcers, or white spots on the lips or in the mouth
  • Swollen glands
  • Yellow eyes or skin

Incidence not known

  • Blistering, peeling, or loosening of the skin
  • Bloody or cloudy urine
  • Burning while urinating
  • Continuing diarrhea
  • Continuing stomach pain
  • Diarrhea
  • Feeling of warmth
  • Increased volume of pale, dilute urine
  • Joint or muscle pain
  • Loss of bladder control
  • Red skin lesions, often with a purple center
  • Red, irritated eyes
  • Redness of the face, neck, arms, and occasionally, upper chest
  • Redness of the skin

Some side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

More common

  • Dizziness or lightheadedness
  • Feeling of constant movement of self or surroundings
  • Heartburn
  • Sensation of spinning
  • Stomach cramps
  • Trouble sleeping
  • Weight loss

Rare

  • Change in taste sensation
  • Congestion
  • Dry mouth
  • Pain or tenderness around the eyes and cheekbones
  • Tender, swollen glands in the neck
  • Trouble with swallowing
  • Unpleasant or sharp metallic taste
  • Voice changes

Incidence not known

  • Decreased interest in sexual intercourse
  • Inability to have or keep an erection
  • Loss in sexual ability, desire, drive, or performance
  • Painful sexual intercourse

Common Concerns

Metronidazole (Flagyl) isn't FDA-approved to treat urinary tract infections. But it's used to treat bacterial vaginosis (BV), an infection that can cause similar symptoms as an UTI. If you experience symptoms such as pain or burning while urinating, talk to your primary care provider. They can help figure out whether you have an infection, what kind of infection it might be, and prescribe the right treatment for your condition.
People who took metronidazole (Flagyl) have reported candidiasis yeast infections, including in the throat (thrush) and vagina. But these side effects aren't common with oral metronidazole (Flagyl). However, vaginal yeast infection is one of the more common side effect of vaginal metronidazole gel. Talk with your healthcare team if you think you might have a yeast infection and they can talk with you about treatment options.
Metronidazole (Flagyl) shouldn't be taken with disulfiram, a medication for alcohol use disorder. Taking these medications together can cause severe changes in mental health, such as delusions and hallucinations. Make sure to avoid these medications together. There are other medications that interact with metronidazole (Flagyl), so be sure to review a list of your current medications with your prescriber.
Don't drink alcohol or eat foods containing large amounts of ethanol or propylene glycol (food additives), such as salad dressing, dried soup or cake mix, during treatment with and for at least 3 days after taking metronidazole (Flagyl). Doing so can lead to a serious interaction and cause symptoms such as nausea, flushing, and headache that last for 30 minutes to up to several hours.
You need to avoid drinking alcohol for at least 3 days after your last dose of metronidazole (Flagyl). This gives time for the medication to leave your system. But you might need to wait longer if you have liver problems. Ask your healthcare team if you're not sure when you can start to drink alcohol after completing treatment with metronidazole (Flagyl).
You might start to feel better a few days after you start taking metronidazole (Flagyl). Make sure to take metronidazole (Flagyl) exactly as prescribed. It's important to finish your treatment course, even if you're feeling better. Stopping the medication earlier than prescribed can make your infection come back and become harder to treat.
If you're pregnant, let your healthcare team know before starting metronidazole (Flagyl). Metronidazole (Flagyl) should be avoided during the first trimester of pregnancy because it might raise the risk of preterm birth if taken by mothers during this time. Talk with your healthcare team to learn more about the risks and benefits of taking metronidazole (Flagyl) during pregnancy.
Once you've tested positive for trich, it's important that both you and your sexual partner(s) get treated so you don't re-infect each other or other people. In many states, if you've tested positive for certain STIs, your prescriber can write a prescription to treat your partner(s) without having them come in for an appointment. This is called Expedited Partner Therapy (EPT). If you suspect that you or your partner(s) have trich, please call your primary care provider to discuss possible treatment options.

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