Reviewed on 8/5/2021

What Is Metronidazole and How Does It Work?

Metronidazole is used to treat a certain type of vaginal infection (bacterial vaginosis) in women who are not pregnant. Metronidazole is an antibiotic that works by stopping the growth of bacteria.

Metronidazole is available under the following different brand names: Flagyl, Flagyl ER, and Flagyl IV RTU.

What Are the Dosages of Metronidazole?

Dosage of Metronidazole:

Adult and Pediatric Dosage Forms and Strengths


  • 375mg


  • 250 mg
  • 500 mg

Tablet, extended-release

  • 750 mg

Infusion solution

  • 500 mg/100mL

Dosage Considerations – Should be Given as Follows:

Anaerobic Bacterial Infections

  • Loading dose: 15 mg/kg intravenously (IV); not to exceed 4 g/day
  • Maintenance dose: 7.5 mg/kg oral/intravenous (IV) (over 1 hour) every 6 hours x 7-10 days (or 2-3 weeks if severe)

Sexually Transmitted Disease

Bacterial Vaginosis

  • Non-pregnant women
    • 500 mg orally twice daily x 7 days, OR
    • 2 g orally once/day single dose, OR
    • Extended-release: 750 mg orally once/day x 7 days
  • Pregnant women
    • 500 mg orally twice daily x 7 days, OR
    • 250 mg orally three times/day x 7 days

Colorectal Surgical Infection

  • Prophylaxis; start after mechanical bowel preparation the afternoon and evening before surgery
  • 1 g orally every 6-8 hours for 3 doses
  • 15 mg/kg IV over 30-60 min; complete approximately 1 hour before surgery; may administer 7.5 mg/kg IV over 30-60 min at 6 and 12 hours after initial dose for maintenance; discontinue within 12 hours after surgery


  • Adult: 250 mg orally every 8 hours for 7 days; alternatively, 375 mg orally every 12 hours for 7 days
  • 2 g orally once daily single dose; alternatively, 1g orally every 12 hours for 2 doses
  • Pediatric: less than 45 kg body weight: 15 mg/kg/day IV/oral divided every 8 hours for 7 days; not to exceed 2 g/day


  • Adult: 500-750 mg orally every 8 hours for 5-10 days
  • Pediatric: 35-50 mg/kg orally divided once every 8 hours for 10 days

Giardiasis (Off-label)

  • Adult: 500 mg orally every 12 hours for 5-7 days
  • Pediatric: 15 mg/kg/day IV/oral divided once every 8 hours for 5 days

Gardnerella Infection

  • Immediate release: 500 mg orally every 12 hours
  • Extended-release: 750 mg orally once daily for 7 days; take on empty stomach

Helicobacter Pylori Infection (Off-label)

  • Adult: 250-500 mg orally four times daily in combination with tetracycline (500 mg) and bismuth subsalicylate (525 mg) x 14 days
  • Pediatric: With amoxicillin and bismuth subsalicylate: 15-20 mg/kg/day oral divided once every 12 hours for 4 weeks

Nongonococcal Urethritis (Off-label)

Pelvic Inflammatory Disease (Off-label)

Pouchitis (Orphan)

Orphan indication sponsor

  • Formac Pharmaceuticals, NV; Gaston Geenslaan 1; Belgium

Crohn's Disease (Orphan)

  • Topical treatment of active perianal Crohn's disease
  • Orphan indication sponsor
    • Braintree Laboratories, Inc; 60 Columbian Street West; PO Box 850929; Braintree, MA 02185-0929

Perioral Dermatitis (Orphan)

  • Orphan indication sponsor
    • Galderma Laboratories, Inc; P.O. Box 331329; Fort Worth, TX 76163

Neonatal (younger than 28 Days) Anaerobic Infection

Less than 1.2 kg

  • 7.5 mg/kg IV/oral every 48 hours

Younger than 7 days

  • 1.2-2 kg: 7.5 mg/kg IV/oral once/day
  • greater than 2 kg: 15 mg/kg/day IV/oral divided every 12 hours

Older than 7 days

  • 1.2-2 kg: 15 mg/kg/day IV/oral divided every 12 hours
  • every 12 hours 2 kg: 30 mg/kg/day IV/oral divided every 12 hours

Infants and Children

  • 30 mg/kg/day oral/IV divided every 6 hours; not to exceed 4 g/day

Clostridium Difficile Colitis - Pediatric


Fungal Skin Infections: Types, Symptoms, and Treatments See Slideshow

What Are Side Effects Associated with Using Metronidazole?

Common Side effects of Metronidazole include:

Side effects of metronidazole from postmarketing reports include:

This document does not contain all possible side effects and others may occur. Check with your physician for additional information about side effects.

What Other Drugs Interact with Metronidazole?

If your doctor has directed you to use this medication, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider or pharmacist first.

Severe Interactions of metronidazole include:

Metronidazole has serious interactions with at least 39 different drugs.

Metronidazole has moderate interactions with at least 117 different drugs.

Metronidazole has mild interactions with at least 84 different drugs.

This information does not contain all possible interactions or adverse effects. Therefore, before using this product, tell your doctor or pharmacist of all the products you use. Keep a list of all your medications with you, and share this information with your doctor and pharmacist. Check with your health care professional or doctor for additional medical advice, or if you have health questions, concerns or for more information about this medicine.

What Are Warnings and Precautions for Metronidazole?


  • Animal data have shown possible carcinogenic effect.
  • This medication contains metronidazole. Do not take Flagyl, Flagyl ER, or Flagyl IV RTU if you are allergic to metronidazole or any ingredients contained in this drug.

Keep out of reach of children. In case of overdose, get medical help or contact a Poison Control Center immediately.


  • Hypersensitivity to metronidazole or other nitroimidazoles (although cautious desensitization has been applied)
  • Pregnancy, 1st trimester (controversial)
  • Use of disulfiram within past 2 weeks; use of alcohol during therapy or within 3 days of discontinuing therapy

Effects of Drug Abuse

  • No information available

Short-Term Effects

  • See "What Are Side Effects Associated with Using Metronidazole?"

Long-Term Effects

  • Encephalopathcy, seizures, aseptic meningitis, and neuropathies reported with increase in dose and chronic therapy
  • Superinfection may occur with prolonged use
  • See "What Are Side Effects Associated with Using Metronidazole?"


  • Encephalopathy, seizures, aseptic meningitis, and neuropathies reported with increase in dose and chronic therapy
  • Superinfection may occur with prolonged use
  • Severe hepatic impairment; administer lower doses with caution
  • Use with caution; potential accumulation in end stage renal disease; supplemental doses may be needed in patients receiving hemodialysis
  • Use with caution in history of blood dyscrasias, heart failure, hepatic failure, H. pylori infection, renal impairment
  • Avoid alcohol while taking medication and for at least 1 day after discontinuation
  • Antiandrogen: May cause gynecomastia
  • Known or previously unrecognized candidiasis may present more prominent symptoms during therapy with Metronidazole Injection and requires treatment with a candicidal agent

Pregnancy and Lactation

  • Metronidazole may be acceptable for use during pregnancy. Either animal studies show no risk but human studies not available or animal studies showed minor risks and human studies done and showed no risk. There are published data from case-control studies, cohort studies, and 2 meta-analyses that included more than 5000 pregnant women who used metronidazole systemically during pregnancy
  • Many studies included first trimester exposures. One study showed an increased risk of cleft lip, with or without cleft palate, in infants exposed to metronidazole in utero; however, these findings were not confirmed
  • In addition, more than 10 randomized, placebo-controlled clinical trials that together enrolled over 5000 pregnant women assessed the possible effect of systemic antibiotic treatment (including with metronidazole) for bacterial vaginosis on the incidence of preterm delivery; most studies did not show an increased risk of congenital anomalies or other adverse fetal outcomes following metronidazole exposure during pregnancy
  • Three studies conducted to assess the risk of infant cancer following systemic metronidazole exposure during pregnancy did not show an increased risk; however, the ability of these studies to detect such a signal was limited.
  • Metronidazole is excreted in human milk; it is not recommended for use while breastfeeding
  • Following oral administration, concentrations in human milk are similar to concentrations in plasma
  • Potential for tumorigenicity shown in animal studies; a decision should be made whether to discontinue nursing or to discontinue metronidazole; breastfeeding women may choose to pump and discard milk for the duration of therapy and for 24 hours after therapy ends and to feed her infant stored human milk or formula


Bowel regularity means a bowel movement every day. See Answer
Medscape. Metronidazole.

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