Antibiotic Uses, Side Effects, Pregnancy, Dosage


generic name: metronidazole

Brand names: Flagyl, Flagyl ER, Metro IV (discontinued), Metromidol (discontinued), Protostat (discontinued) and Satric (discontinued)

drug class: nitroimidazoles


What is metronidazole and what is it used for?

Metronidazole is an antibiotic active against anaerobic bacteria and certain parasites. Anaerobic bacteria are single-celled, living organisms that thrive in environments where oxygen is scarce (anaerobic environments). Anaerobic bacteria can cause diseases of the abdomen (bacterial peritonitis), liver (liver abscess) and pelvis (abscess of the ovaries and fallopian tubes).


Giardia lamblia and amoebas 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.


  • Metronidazole is used to treat parasitic infections, including Giardia infections of the small intestine, amoebic liver abscess and amoebic dysentery (an infection of the large intestine that causes bloody diarrhea), bacterial vaginosis, trichomonas vaginal infections, and carriers of trichomonas (both sexual partners) who do this have no symptoms of infection.
  • Metronidazole is also used alone or in combination with other antibiotics to treat abscesses in the liver, pelvis, abdomen, and brain caused by susceptible anaerobic bacteria.
  • Metronidazole is also used to treat infections of the colon caused by a bacterium called C. difficile. Many commonly used antibiotics can change 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 conventional antibiotics. This leads to inflammation of the colon (pseudomembranous colitis) with severe diarrhea and abdominal pain.)
  • Metronidazole is also used in combination with other medicines to treat Helicobacter pylori (H. pylori), which causes stomach or intestinal ulcers.
  • Metronidazole topical gel is used to treat acne rosacea.
  • Metronidazole vaginal gel is used to treat bacterial vaginosis.
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What side effects can metronidazole have?

Metronidazole is a useful antibiotic and is generally well tolerated when used appropriately.


The most common and minor side effects are:

Side effects that are uncomfortable but can become serious include:

Serious side effects of metronidazole are rare and the drug should be discontinued if these symptoms occur:



Bowel movement means bowel movements every day.
See answer

What is the dosage for Metronidazole?

  • Metronidazole can be taken orally with or without food.
  • In the hospital, metronidazole can be given intravenously to treat severe infections.
  • The liver is primarily responsible for the elimination of metronidazole from the body and doses may need to be reduced in patients with liver disease and liver dysfunction.

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

  • Amoebic dysentery: 750 mg po 3 times a day for 5-10 days
  • Amoebic liver abscess: 500-750 mg po three times a day for 5-10 days
  • Anaerobic infections: 7.5 mg/kg orally or by injection every 6 hours for 7 to 10 days, no more than 4 grams daily.
  • Bacterial vaginosis: 750 mg (prolonged-release tablets) once a day for 7 days or 500 mg twice a day for 7 days or 2 g single dose or an applicator full of 0.75% vaginal gel, once or twice a day for 5 days.
  • Clostridium difficile infection: 250-500 mg po 4 times a day or 500-750 mg po 3 times a day
  • Giardia: 250 mg po three times a day for 5 days Helicobacter pylori: 800-1500 mg po a day for several days in combination with other drugs.
  • Pelvic inflammatory disease (PID): 500 mg po twice a day 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
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What drugs interact with this drug?

  • Alcohol should be avoided as metronidazole and alcohol together can cause severe nausea, vomiting, cramps, hot flashes and headaches.
  • Metronidazole may potentiate the blood-thinning effects of warfarin and increase the risk of bleeding, probably by reducing warfarin’s breakdown.
  • Cimetidine (Tagamet) increases blood levels of metronidazole, while cholestyramine decreases blood levels of metronidazole by decreasing its absorption.
  • Metronidazole should not be combined with amprenavir (Agenerase) for the treatment of 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 the blood. Propylene glycol buildup can cause seizures, increased heart rate, and kidney failure.
  • Metronidazole increases blood levels of carbamazepine (Tegretol, Tegretol XR, Equetro, Carbatrol), lithium (Eskalith, Lithobid), and cyclosporine by unknown mechanisms. Serious reactions can occur when these medicines are taken with metronidazole.

pregnancy and breast feeding period

  • Metronidazole is not used in early pregnancy because of possible side effects on the fetus.
  • Metronidazole is excreted in breast milk. Breastfeeding women should not use metronidazole because of possible side effects on the newborn.

What else should I know about metronidazole?

Do I need a prescription for metronidazole?
What preparations of metronidazole are there?
  • 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 store metronidazole?
  • 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.


Bacterial Infections 101: Types, Symptoms, and Treatments
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Medically checked on 06/23/2022



FDA Prescribing Information


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