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Flagyl (metronidazole) Side Effects, Pregnancy Use & Dosage

What is metronidazole (Flagyl, Flagyl ER), and how does it work?

Flagyl is an antibiotic effective against anaerobic bacteria and certain parasites. Anaerobic bacteria are single-celled, living organisms that thrive in environments in which there is little oxygen (anaerobic environments). Anaerobic bacteria can cause disease in the abdomen (bacterial peritonitis), liver (liver abscess), and pelvis (abscess of the ovaries and the Fallopian tubes). Giardia lamblia and ameba are intestinal parasites that can cause abdominal pain and diarrhea in infected individuals. Trichomonas is a vaginal parasite that causes inflammation of the vagina (vaginitis). Metronidazole selectively blocks some of the functions within the bacterial cells and the parasites resulting in their death.

What brand names are available for metronidazole?

  • Flagyl and Flagyl ER are the available brand names for metronidazole in the US.
    • Metro IV, Metromidol, Protostat, and Satric are brands names for metronidazole that have been discontinued in the US.

    Is this drug available as a generic?


    Do I need a prescription for this drug?


    Why is metronidazole prescribed to patients?

    • Flagyl is used to treat parasitic infections including Giardia infections of the small intestine, amebic liver abscess, and amebic dysentery (infection of the colon causing bloody diarrhea), bacterial vaginosis, trichomonas vaginal infections, and carriers of trichomonas (both sexual partners) who do not have symptoms of infection.
    • Flagyl is also used alone or in combination with other antibiotics in treating abscesses in the liver, pelvis, abdomen, and brain caused by susceptible anaerobic bacteria.
    • Flagyl is also used in treating infection of the colon caused by a bacterium called C. difficile. Many commonly-used antibiotics can alter the type of bacteria that inhabit the colon. C. difficile is an anaerobic bacterium that can infect the colon when the normal types of bacteria in the colon are inhibited by common antibiotics. This leads to inflammation of the colon (pseudomembranous colitis) with severe diarrhea and abdominal pain.)
    • Flagyl also is used in combination with other drugs to treat Helicobacter pylori (H. pylori) that causes stomach or intestinal ulcers.
    • Flagyl topical gel is used for treating acne rosacea.
    • Flagyl vaginal gel is used for treating bacterial vaginosis.


    Bowel regularity means a bowel movement every day.
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    What are the side effects of metronidazole?

    Flagyl is a useful antibiotic and is generally well tolerated with appropriate use.

    The most common and minor side effects include:

    Side effects that are uncomfortable, but may become serious include:

    Serious side effects of Flagyl are rare and the drug should be stopped if these symptoms appear:

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    What is the dosage for metronidazole, and how do I take it?

    • Metronidazole may be taken orally with or without food.
    • In the hospital, metronidazole can be administered intravenously to treat serious infections.
    • The liver is primarily responsible for eliminating metronidazole from the body, and doses may need to be reduced in patients with liver disease and abnormal liver function.

    Various metronidazole regimens are used. Some examples are listed below.

    • Amebic dysentery: 750 mg orally 3 times daily for 5-10 days
    • Amebic liver abscess: 500-750 mg orally three times daily for 5-10 days
    • Anaerobic infections: 7.5 mg/kg orally or by injection every 6 hours for 7 to 10 days not to exceed 4 grams daily.
    • Bacterial vaginosis: 750 mg (extended release tablets) once daily for 7 days or 500 mg twice daily for 7 days or 2 g single dose or one applicator-full of 0.75% vaginal gel, once or twice daily for 5 days.
    • Clostridium difficile infection: 250-500 mg orally 4 times daily or 500-750 orally 3 times daily
    • Giardia: 250 mg orally three times daily for 5 days Helicobacter pylori: 800-1500 mg orally daily for several days in combination with other drugs.
    • Pelvic inflammatory disease (PID): 500 mg orally twice daily for 14 days in combination with other drugs.
    • Trichomoniasis: 2 g single dose or 1 g twice
    • Rosacea: apply topical gel 0.75-1% once daily

    Which drugs or supplements interact with this medication?

    • Alcohol should be avoided because metronidazole and alcohol together can cause severe nausea, vomiting, cramps, flushing, and headache.
    • Metronidazole can increase the blood thinning effects of warfarin (Coumadin, Jantoven) and increase the risk of bleeding probably by reducing the breakdown of warfarin.
    • Cimetidine (Tagamet) increases blood levels of metronidazole while cholestyramine (Questran, Questran Light) reduces blood levels of metronidazole by reducing its absorption.
    • Metronidazole should not be combined with amprenavir (Agenerase) for treating human immunodeficiency disease (infection with HIV) because amprenavir contains propylene glycol.
    • Metronidazole blocks the breakdown of propylene glycol in the liver leading to accumulation of propylene glycol in blood. Accumulation of propylene glycol could cause seizures, increased heart rate, and lead to kidney failure.
    • Metronidazole increases the blood levels of carbamazepine (Tegretol, Tegretol XR, Equetro, Carbatrol), lithium (Eskalith, Lithobid) and cyclosporine though unknown mechanisms. Serious reactions may occur if these drugs are taken with metronidazole.

    Is this drug safe to use during pregnancy or while breastfeeding?

    • Metronidazole is not used in early pregnancy because of potential adverse effects on the fetus.
    • Metronidazole is excreted in breast milk. Females who are nursing, because of potential adverse effects on the newborn, should not use metronidazole.

    What else should I know about Flagyl (metronidazole)?

    What preparations of Flagyl (metronidazole) are available?
    • Tablets: 250 and 500 mg.
    • Tablets, extended release: 750 mg.
    • Capsule: 375 mg.
    • Cream: 0.75% and 1%.
    • Lotion: 0.75%.
    • Gel: 0.75% and 1%.
    • Injection: 5 mg/ml
    How should I keep Flagyl (metronidazole) stored?
    • Metronidazole should be stored at room temperature and protected from light.
    When was metronidazole approved by the FDA?
    • The FDA approved metronidazole tablets in July 1963.

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