Metronidazole (By mouth)


Metronidazole (met-roe-NYE-da-zole)

Treats bacterial infections. Belongs to a class of drugs called antibiotics.

Brand Name(s)

Helidac Therapy, Flagyl 375, Flagyl, Flagyl ER

There may be other brand names for this medicine.

When This Medicine Should Not Be Used

You should not use this medicine if you have had an allergic reaction to metronidazole, or to other similar medicines such as tinidazole (Tindamax®). You should not use this medicine if you are pregnant, especially during the first 3 months of pregnancy.

How to Use This Medicine

Capsule, Tablet, Long Acting Tablet

  • Your doctor will tell you how much of this medicine to use and how often. Do not use more medicine or use it more often than your doctor tells you to.
  • You may take the capsule or tablet with food or milk to avoid stomach upset.
  • It is best to take the extended-release tablet on an empty stomach, one hour before or two hours after a meal.
  • Swallow the extended-release tablet whole. Do not crush, break, or chew it.
  • Keep using this medicine for the full treatment time, even if you feel better after the first few doses. Your infection may not clear up if you stop using the medicine too soon.

If a dose is missed:

  • If you miss a dose or forget to use your medicine, use it as soon as you can. If it is almost time for your next dose, wait until then to use the medicine and skip the missed dose. Do not use extra medicine to make up for a missed dose.

How to Store and Dispose of This Medicine

  • Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light.
  • Ask your pharmacist, doctor, or health caregiver about the best way to dispose of any outdated medicine or medicine no longer needed.
  • Keep all medicine away from children and never share your medicine with anyone.

Drugs and Foods to Avoid

Ask your doctor or pharmacist before using any other medicine, including over-the-counter medicines, vitamins, and herbal products.

  • If you have taken a medicine called disulfiram (Antabuse®) within the last 2 weeks, do not take metronidazole until you talk to your doctor. Disulfiram is used to help people who have a drinking problem. If these 2 medicines are taken close together, serious unwanted effects may occur.
  • Make sure your doctor knows if you are taking a blood thinner (such as warfarin, Coumadin®), cimetidine (Tagamet®), lithium (Eskalith®), phenobarbital (Donnatal®, Luminal®), or phenytoin (Dilantin®).
  • Do not drink alcohol or take medicine containing alcohol (such as cold or cough medicines). Wait at least 3 days after stopping metronidazole before drinking alcohol.

Warnings While Using This Medicine

  • Using this medicine while you are pregnant can harm your unborn baby, especially during the first 3 months of pregnancy. Use an effective form of birth control to keep from getting pregnant. If you think you have become pregnant while using the medicine, tell your doctor right away.
  • Make sure your doctor knows if you are breastfeeding, or if you have liver disease, leukopenia (low white blood cells), oral thrush, a vaginal yeast infection, epilepsy (seizures), optic neuropathy (eye disease with vision changes), or peripheral neuropathy (nerve disease with pain, numbness, or tingling).
  • Check with your doctor right away if you have dizziness, problems with muscle control or coordination, shakiness or an unsteady walk, slurred speech, or trouble with speaking. These may be symptoms of a serious brain condition called encephalopathy.
  • Call your doctor right away if you have confusion, drowsiness, fever, a general feeling of illness, a headache, loss of appetite, nausea, a stiff neck or back, or vomiting. These could be symptoms of a serious condition called aseptic meningitis.
  • Check with your doctor right away if you are having burning, numbness, tingling, or painful sensations in the arms, hands, legs, or feet. These could be symptoms of a condition called peripheral neuropathy.
  • If you are using this medicine for trichomoniasis (an infection of the sex organs in men or women), your doctor may want to treat your sexual partner at the same time you are being treated, even if he or she has no symptoms. Also, it may be desirable to use a condom (rubber) during sexual intercourse. These measures will help to keep you from getting the infection back again from your partner. If you have any questions about this, check with your doctor.
  • If your symptoms do not improve or if they get worse, call your doctor.
  • Your doctor will need to check your progress at regular visits while you are using this medicine. Be sure to keep all appointments. Blood tests may be needed to check for unwanted effects.
  • This medicine may turn your urine a reddish-brown color. This is normal.

Possible Side Effects While Using This Medicine

Call your doctor right away if you notice any of these side effects:

  • Allergic reaction: Itching or hives, swelling in your face or hands, swelling or tingling in your mouth or throat, chest tightness, trouble breathing
  • Agitation, confusion, depression, or irritability.
  • Blistering, peeling, or red skin rash.
  • Blurred or decreased vision.
  • Difficult or painful urination.
  • Feeling of constant movement of self or surroundings.
  • Fever, chills, cough, runny or stuffy nose, sore throat, and body aches.
  • Joint pain.
  • Numbness, tingling, burning pain, or weakness in your hands, arms, legs, or feet.
  • Seizures.
  • Shakiness or problems with coordination or walking.
  • Stiff neck or back.
  • Trouble with speaking.
  • Unusual bleeding, bruising, or weakness.
  • Vaginal swelling, itching, or discharge.
  • Warmth or redness in your face, neck, arms, or upper chest.

If you notice these less serious side effects, talk with your doctor:

  • Dizziness or lightheadedness.
  • Dry mouth.
  • Headache.
  • Loss of appetite.
  • Mild diarrhea, constipation, nausea, vomiting, or stomach pain.
  • Mild skin rash or itching.
  • Pain during sex or when urinating.
  • Problems having sex.
  • Sores, ulcers, or white patches in the mouth.
  • Trouble sleeping.
  • Unusual or unpleasant taste in your mouth.

If you notice other side effects that you think are caused by this medicine, tell your doctor

Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088

Version Info

  • Last Reviewed on 06/12/2013

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission ( URAC's accreditation program is an independent audit to verify that A.D.A.M. follows rigorous standards of quality and accountability. A.D.A.M. is among the first to achieve this important distinction for online health information and services. Learn more about A.D.A.M.'s editorial policy, editorial process and privacy policy. A.D.A.M. is also a founding member of Hi-Ethics and subscribes to the principles of the Health on the Net Foundation (

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- 2013 A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

This page was last updated: June 18, 2013

Average rating (1)