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Metronidazole, Oral Tablet

Highlights

  1. Metronidazole oral tablets are available as both a generic and brand-name drugs. The immediate-release tablet is also available as a generic drug, but the extended-release tablet is not. Brand name(s): Flagyl (immediate-release), Flagyl ER (extended-release)
  2. Metronidazole comes in the form of a capsule and tablet you take by mouth. It also comes as a cream, gel, and lotion you apply to your skin, as well as a vaginal gel. It’s also available as a self-injectable drug.
  3. Metronidazole oral tablets are used to treat infections caused by bacteria or parasites.
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Important warnings

Important warnings

FDA warning: Cancer risk
  • Metronidazole oral tablets and capsules have a black box warning. This is the most serious warning from the Food and Drug Administration (FDA). A black box warning indicates drug effects that may be dangerous. 
  • Cancer was found in some animals during testing with metronidazole. There may be a similar risk in humans. Because of this risk, metronidazole should only be used to treat conditions as approved by the FDA.

Nervous system effects

  • Metronidazole oral tablets may cause certain serious conditions that affect the nervous system. These conditions include:
    • encephalopathy (abnormal brain function)
    • meningitis (inflammation in the brain)
    • seizures
    • peripheral neuropathy (nerve damage in the hands and feet)
  • Call your doctor right away if you have symptoms such as:
    • memory loss
    • trouble focusing
    • fever
    • stiff neck
    • pain or numbness in your hands or feet

About

What is metronidazole?

The metronidazole immediate-release tablet and extended-release tablet are prescription drugs. They’re both taken by mouth. These tablets are available as the brand-name drugs Flagyl (immediate-release) and Flagyl ER (extended-release).

These drugs are also available as generic drugs. Generic drugs usually cost less. In some cases, they may not be available in every strength or form as the brand-name version. 

Why it's used

Metronidazole immediate-release oral tablets are used to treat many infections caused by bacteria or parasites. These include infections that occur in the gastrointestinal tract or reproductive system. Metronidazole extended-release oral tablets are used to treat vaginal infections in women.

Metronidazole may be used as part of a combination therapy. This means you may need to take it with other medications.

What’s the difference?
Immediate-release drugs are released into the body right away. Extended-release drugs are released into the body slowly over time.

How it works

Metronidazole belongs to a class of drugs called nitroimidazole antimicrobials. A class of drugs is a group of medications that work in a similar way. These drugs are often used to treat similar conditions. 

The metronidazole tablets work by killing bacteria. This relieves the infection.

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Drug class

Drug class

Metronidazole oral tablet belongs to a class of drugs called nitroimidazole antimicrobials. Antimicrobials are drugs used to treat infections. Nitroimidazole antimicrobials treat infections caused by bacteria and other organisms called protozoa. They’re typically used to treat infections such as bacterial vaginosis, amebiasis, giardiasis, and trichomoniasis.

Metronidazole is the most commonly used type of nitroimidazole antimicrobial. The only other type available in the United States is tinidazole.

Side effects

Metronidazole side effects

Metronidazole oral tablet doesn’t cause drowsiness, but it can cause other side effects.

More common side effects

The more common side effects that can occur include:

  • headache
  • loss of appetite
  • nausea or vomiting
  • diarrhea
  • heartburn
  • cramping in your stomach area
  • constipation
  • metallic taste

If these effects are mild, they may go away within a few days or a couple of weeks. If they’re more severe or don’t go away, talk to your doctor or pharmacist.

Side effects for men?
For the most part, the side effects of metronidazole for men are the same as they are for women. The only real differences in side effects affect women. For instance, metronidazole increases the risk of yeast infections, which occur much more often in women. Also, metronidazole can cause vaginal irritation and discharge.

Serious side effects

Call your doctor right away 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 can include:

  • nervous system effects, including seizures and encephalopathy (abnormal brain function). Symptoms can include:
    • convulsions (sudden movements caused by tightening of your muscles)
    • dizziness
    • headache
    • confusion
    • ataxia (loss of control of body movements)

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs affect each person differently, we cannot guarantee that this information includes all possible side effects. This information is not a substitute for medical advice. Always discuss possible side effects with a healthcare provider who knows your medical history.

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Interactions

Metronidazole may interact with other medications

Metronidazole oral tablet can interact with other medications, vitamins, or herbs you may be taking. An interaction is when a substance changes the way a drug works. This can be harmful or prevent the drug from working well.

To help avoid interactions, your doctor should manage all of your medications carefully. Be sure to tell your doctor about all medications, vitamins, or herbs you’re taking. To find out how this drug might interact with something else you’re taking, talk to your doctor or pharmacist.

Drugs you should not take with metronidazole

Disulfiram: Do not take disulfiram with metronidazole. Doing so can cause dangerous effects in your body. Using it with metronidazole can cause psychotic reactions. Symptoms can include:

  • confusion
  • hallucinations (seeing or hearing things that aren’t real)
  • delusions (believing things that aren’t real)

Do not take metronidazole if you’ve taken disulfiram in the last two weeks.

Interactions that increase your risk of side effects

Taking metronidazole with certain medications raises your risk of side effects. This is because the amount of either drug may be increased in your body. Examples include:

Lithium: Increased side effects of metronidazole can include a higher risk of QT prolongation (irregular heartbeat).

Warfarin or other blood thinners: Increased side effects of these drugs include a higher risk of bleeding.

Busulfan: If possible, you should avoid taking busulfan with metronidazole. If you do take these drugs together, your doctor may check the amount of busulfan in your body more often.

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs interact differently in each person, we cannot guarantee that this information includes all possible interactions. This information is not a substitute for medical advice. Always speak with your healthcare provider about possible interactions with all prescription drugs, vitamins, herbs and supplements, and over-the-counter drugs that you are taking.

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Use with birth control

Metronidazole and birth control

Metronidazole oral tablet is safe to use with hormonal birth control methods. While one type of antibiotic (rifampin) is known to cause problems with some birth control, it’s not an issue with metronidazole. No research shows that this drug can make you more likely to get pregnant if you’re using the pill or other types of hormonal birth control.

Of course, if you have concerns, be sure to ask your doctor. And you can always use a backup method of birth control, such as a condom, while you’re taking metronidazole. To learn more, check out this article, which gives more information about antibiotics and birth control.

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Warnings

Metronidazole warnings

Metronidazole oral tablet comes with several warnings.

Allergic reaction

Metronidazole can cause a severe allergic reaction or hypersensitivity. Symptoms can include:

  • hives (itchy welts)
  • red rash
  • peeling or sloughing of your skin
  • wheezing
  • fever
  • chills 

If you have an allergic reaction, call your doctor or local poison control center right away. If your symptoms are severe, call 911 or go to the nearest emergency room.

Don’t take this drug again if you’ve ever had an allergic reaction to it. In some cases, taking it again could lead to death.

Alcohol interaction

You should stop using drinks that contain alcohol at least three days before starting metronidazole. Also avoid alcohol for three more days after you stop treatment with this drug.

This is because alcohol can cause side effects when used with metronidazole. These include:

  • nausea
  • vomiting
  • abdominal cramps
  • headaches
  • flushing (sudden redness and warmth in your face)

If you drink alcohol, talk to your doctor about whether this drug is safe for you.

Warnings for certain groups

For people with liver disease: Your liver helps process this drug. If you have severe liver disease, your liver may process this drug more slowly. This would increase the amount of the drug in your body and raise your risk of side effects. Your doctor may lower your dosage of metronidazole or have you take it less often.

For people with kidney disease: Your kidneys help clear this drug from your body. If you have severe kidney disease, your kidneys may process this drug more slowly. This increases the amount of the drug in your body and raises your risk of side effects. Your doctor may lower your dose of metronidazole or have you take it less often.

For pregnant women: Metronidazole is a category B pregnancy drug. That means two things:

  • Studies of the drug in pregnant animals have not shown a risk to the fetus.
  • There aren’t enough studies done in pregnant women to show if the drug poses a risk.

Talk to your doctor if you’re pregnant or planning to become pregnant. Metronidazole should not be taken during the first trimester of pregnancy. For the second and third trimesters, this drug should be used only if the potential benefit justifies the potential risk.

For women who are breastfeeding: Metronidazole may pass into breast milk and cause side effects in a child who is breastfed. Talk to your doctor if you breastfeed your child. You may need to decide whether to stop breastfeeding or stop taking this medication.

For seniors: The kidneys of older adults may not work as well as they used to. This can cause your body to process drugs more slowly. As a result, more of a drug stays in your body for a longer time. This increases your risk of side effects. 

Dosage

How to take metronidazole

All possible dosages and drug forms may not be included here. Your dosage, drug form, and how often you take the drug will depend on: 

  • your age
  • the condition being treated
  • how severe your condition is
  • other medical conditions you have
  • how you react to the first dose

For bacterial and amoebic infections

Generic: Metronidazole

  • Form: immediate-release oral tablet
  • Strengths: 250 mg, 500 mg

Brand: Flagyl

  • Form: immediate-release oral tablet
  • Strengths: 250 mg, 500 mg

Adult dosage (ages 18–64 years)

Your dosage and length of treatment depend on your infection type.

Bacterial infections:

  • Typical dosage: 500 mg four times per day
  • Maximum dosage: 4 g per day

Amoebic infections: 

  • Typical dosage: 750 mg three times per day for 5–10 days
  • Maximum dosage: 4 g per day

Trichomoniasis: 

  • Typical dosage: Either 2 grams (g) once per day, 1 g twice per day, or 250 milligrams three times per day. Treatment lasts for seven days.
  • Maximum dosage: 4 g per day.

Child dosage (ages 0–17 years)

Amoebic infections:

  • Typical dosage: 35–50 mg/kg of bodyweight per day given in three divided doses for 10 days
  • Maximum dosage: 2,250 mg per day

Trichomoniasis:

  • Typical dosage: 15 mg/kg of bodyweight per day given in three divided doses for seven days
  • Maximum dosage: 1,000 mg per day

Senior dosage (ages 65 years and older)

The kidneys of older adults may not work as well as they used to. This can cause your body to process drugs more slowly. As a result, more of a drug stays in your body for a longer time. This increases your risk of side effects. Your doctor may start you on a lowered dose or a different medication schedule. This can help keep levels of this drug from building up too much in your body.

For bacterial vaginosis

Brand: Flagyl ER

  • Form: extended-release oral tablet
  • Strength: 750 mg

Adult dosage (ages 18–64 years)

  • Typical dosage: 750 mg per day
  • Maximum dosage: 4 g per day

Senior dosage (ages 65 years and older)

The kidneys of older adults may not work as well as they used to. This can cause your body to process drugs more slowly. As a result, more of a drug stays in your body for a longer time. This increases your risk of side effects.

Your doctor may start you on a lowered dose or a different medication schedule. This can help keep levels of this drug from building up too much in your body.

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Take as directed

Take as directed

Metronidazole tablets are used for short-term treatment. They come with risks if you don’t take them as prescribed.

If you stop taking the drug suddenly or don’t take it at all

Your infection may not improve, and may get worse.

If you miss doses or don’t take the drug on schedule

Your medication may not work as well or may stop working completely. For this drug to work well, a certain amount needs to be in your body at all times.

If you take too much

You could have dangerous levels of the drug in your body. If you think you’ve taken too much of this drug, call your doctor or local poison control center. If your symptoms are severe, call 911 or go to the nearest emergency room right away.

When to call the doctor
Call your doctor right away if you have completed treatment with metronidazole and still have symptoms of your infection. You should also call if you become pregnant while taking this drug.

What to do if you miss a dose

Take your dose as soon as you remember. But if you remember just a few hours before your next scheduled dose, take only one dose. Never try to catch up by taking two doses at once. This could result in dangerous side effects.

How to tell if the drug is working

Your symptoms of infection should improve.

Overdose

In case of overdose

If you take too much metronidazole, you could have dangerous levels of the drug in your body. Symptoms of an overdose can include increased side effects, such as:

  • headache
  • loss of appetite
  • nausea or vomiting
  • diarrhea
  • heartburn
  • cramping in your stomach area
  • constipation
  • metallic taste

If you think you’ve taken too much of this drug, call your doctor or local poison control center. If your symptoms are severe, call 911 or go to the nearest emergency room right away.  

Important considerations

Important considerations for taking metronidazole

General

  • You can take the immediate-release tablets with or without food. Taking them with food may help reduce upset stomach.
  • Do not take the extended-release tablets with food. You should take them at least 1 hour before a meal or 2 hours after a meal.
  • You can cut or crush the immediate-release tablets. However, do not cut or crush the extended-release tablets.
  • Take metronidazole at the time(s) recommended by your doctor.

Storage

  • Store this drug at a temperature below 86°F (30°C).
  • Keep it away from light.
  • Don’t store this medication in moist or damp areas, such as bathrooms.

Refills

A prescription for this medication may or may not be refillable. Ask your doctor or pharmacist. You or your pharmacy may have to contact your doctor for a new prescription if you need this medication refilled.

Travel

When traveling with your medication, follow these tips:

  • Carry your medication with you.
  • When flying, never put it into a checked bag. Keep it in your carry-on bag.
  • Don’t worry about airport X-ray machines. They can’t hurt your medication.
  • You may need to show airport staff the pharmacy label for your medication, so carry the original prescription-labeled box with you.
  • Don’t put this medication in your car’s glove compartment or leave it in the car. Be sure to avoid doing this when the weather is very hot or very cold.

Alternatives

There are other drugs available to treat your condition. Some may be better suited for you than others. Talk to your doctor about other drug options that may work for you.

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