Flagyl (Metronidazole)

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What is Flagyl (metronidazole)?

Flagyl is a prescription antibiotic used to treat symptoms of bacterial and parasite infections of the respiratory tract, spinal cord, skin, lungs, vagina, brain, heart, intestinal tract, joints, teeth, pelvis, bloodstream, and liver.

Flagyl is the brand name for the generic medication metronidazole and is available as a capsule, tablet, suspension, or powder.

This drug is also prescribed to treat trichomoniasis, a sexually transmitted disease caused by a parasite. Generally, both sexual partners are given this medication simultaneously, even if they don’t have symptoms.

You can use this medicine alone or when taking other medications.

As a nitroimidazole antibiotic, it works by affecting the DNA of parasites or bacteria that cause infections, stopping their growth.

What is Flagyl used to treat?

You can take Flagyl when prescribed by your doctor to treat several different infections caused by specific parasites and bacteria.

Some examples are:

  • Brain infections

  • Inflammatory bowel disease (IBD)

  • Soft tissue infection

  • Sexually transmitted diseases (STDs)

  • Dientamoeba fragilis

  • Heart infections

  • Intra-abdominal infection

  • Surgical prophylaxis

  • Giardia (and other parasites)

  • Wound infection

  • Amebiasis or infections caused by intestinal parasites

  • Oral and dental infections

  • Nongonococcal urethritis

  • Skin and structure infection

  • Other bacterial infections that cause meningitis, diarrhea, abscesses, or sepsis

  • Joint infection

  • Pseudomembranous colitis

  • Liver abscess

  • Osteomyelitis

Note that this drug won’t work for the common flu, cold, and other viral infections.¹

How do you take Flagyl?

Flagyl dosage² will differ depending on the infection you need to treat. It’s essential to take this drug exactly as prescribed and complete the entire course of antibiotics, even if you already feel better.

If you're taking the oral suspension (liquid) form of Flagyl, shake it thoroughly. Measure the dose with a measuring device.

The metronidazole injection is administered as an infusion into a vein. The doctor will give you this if you have a severe infection or cannot take the drug orally.

If you’re taking the tablet form, swallow it whole — don’t break, crush, or chew.

Drink eight ounces of water after taking this drug. You can take this medication with or without food.

Here is the typical Flagyl dosage:


Adults: 500–750mg by mouth three times a day, for five to ten days.

Children: 35–50mg/kg/day by mouth split into three doses, for ten days.


The usual dose is 250mg by mouth three times a day, for seven days.

Trichomoniasis (one day, female)

The usual dose is 2,000mg by mouth either as a single or split dose. For split dosing, take 1,000mg in the morning and 1,000mg at night.

Bacterial (anaerobic) infections

The usual dose is 500mg by mouth taken every six hours, for seven to ten days. Don’t take more than 4,000mg within 24 hours. It’s worth noting that some types of infections might call for extended treatment.

Your dose may be different if you’re currently suffering from kidney or liver problems.

Seeing results

You may notice your infection-related symptoms easing after taking Flagyl for two days.³ If you don’t see any results or your symptoms get worse, speak to your doctor.

Potential side effects of Flagyl

This drug may cause side effects, such as:

  • Trouble sleeping

  • Dizziness

  • Weakness

  • Diarrhea

  • Irritability

  • Headache

  • Pain or difficulty urinating

  • Ulcers or blisters in your mouth

  • Depression

  • Swollen or red gums

  • Trouble swallowing

  • Lightheadedness

Speak to your doctor if your side effects worsen or continue.

Severe side effects that need urgent medical attention include:

  • Seizure

  • Pain behind your eyes

  • Trouble speaking

  • Numbness

  • Fever

  • Seeing flashes of light

  • Tingling sensation

  • Muscle weakness

  • Increased sensitivity to light

  • Burning pain in your hands

  • Confusion

  • Neck stiffness

  • Vision problems

  • Trouble with coordination 

Long-term use of Flagyl

Flagyl is usually prescribed for treatment over seven to ten days,⁴ and shouldn’t be taken for any longer unless recommended by your doctor. Some infections require longer treatment, such as infections of the bone and joint, lower respiratory tract, and endocardium (inner layer of the heart).

Missed doses

Take a missed dose of Flagyl as soon as possible, but skip it if it’s nearly time to take your next dose.

You should not take two doses at the same time.


Overdose symptoms of this drug commonly include tingling sensations, numbness, nausea, problems with muscle movement, feeling unsteady, or vomiting.

Seek medical help immediately if you think you or someone else has taken too much Flagyl.

What to discuss with your doctor before taking Flagyl

Before you take Flagyl, you must make your doctor aware of the following things if they apply to you:

  • You’re currently undergoing dialysis

  • You have a fungal infection anywhere in your body

  • You have or previously had a liver condition

  • You have a nervous system disease

  • You have or previously had an intestinal or stomach disease (such as Crohn’s disease)

  • You’re allergic to metronidazole and its other contents or inactive ingredients

  • You have a blood cell disorder

Talk to your doctor about your medical history. You should also tell them if you are taking other medications (prescription and non-prescription), nutritional supplements, or herbal remedies as they could interact with Flagyl and cause complications.

Stopping Flagyl

You should continue taking Flagyl until your prescription finishes, even if your symptoms ease. Stopping Flagyl too early may cause your infection to return or worsen, and it may decrease the effectiveness of antibiotics in the future.

Flagyl and pregnancy

Flagyl is a pregnancy category B⁵ drug, meaning animal studies have not shown the medication is harmful to a developing fetus. However, not enough research has been done to assess the safety of metronidazole for pregnant humans.

The medication is commonly prescribed during pregnancy to treat bacterial vaginosis (BV) and Trichomonas vaginalis infections. It is not thought to increase the risk of pregnancy complications or preterm birth.

Tell your doctor if you’re pregnant before taking Flagyl. They will talk to you about the potential risks and benefits and help you make a decision.

Flagyl and breastfeeding

Flagyl enters breast milk in small quantities and is unlikely to harm your baby. However, some babies may develop diarrhea.

Get medical help straight away if you develop any concerns about your nursing baby. If your baby develops oral thrush or diarrhea, get your doctor’s advice. They may recommend a different medicine if they feel Flagyl is unsafe for you to take while breastfeeding.

Interactions with other drugs

It can be unsafe to take specific medications simultaneously. Some drugs reduce the effectiveness of Flagyl or increase its levels in your blood, potentially worsening side effects.

Drugs that can interact with Flagyl include:

  • Busulfan or other cancer medicines

  • Antidepressants

  • Blood pressure or heart medications

  • Blood thinners (such as warfarin)

  • Asthma medication

  • Drugs to treat HIV, malaria, or other infections

  • Lithium or other antipsychotic drugs

You must tell your doctor if you are taking other medications before taking Flagyl.

Allergy information

Get urgent medical assistance if you develop signs of an allergic reaction while taking Flagyl.

Symptoms of an allergic reaction include:

  • Itching

  • Vaginal dryness

  • Difficulty breathing

  • Severe skin reaction

  • Dry mouth

  • Joint pain

  • Stuffy nose

  • Skin rash with peeling and blistering

  • Fever

  • Burning eyes

  • Swelling in the throat or face

  • Tingling sensation

Seek urgent medical help if you think you are having an allergic reaction to Flagyl.

Clinical trial history

The efficacy of metronidazole for treating trichomoniasis was assessed in a clinical trial⁶ where various dosage regimens were given to 97 patients diagnosed with the condition.

After the first examination, all 97 patients (including 76 who received oral metronidazole only) were found to have no trichomoniasis. Of the 65 patients who followed up, only eight cases showed the parasite had returned.

Side effects were experienced by 20% of participants, and they were generally mild and temporary. The side effects were not serious enough to stop treatment before the infection was successfully treated.

Tips and advice for taking Flagyl

Here are some tips for taking Flagyl safely and effectively:

  • Follow your doctor or pharmacist’s instructions carefully, and try not to miss a dose of Flagyl.

  • If you do miss a dose, take it as soon as possible. If it’s nearly time for the next dose, skip the missed dose and resume your usual dosing schedule.

  • You should keep taking Flagyl until your prescription finishes to prevent the infection recurring.

  • Discuss your medical history and all the current medications you’re taking with your doctor to ensure Flagyl is a safe option for you.

  • Store Flagyl in a closed container at room temperature. It should be kept out of direct sunlight.

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Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.