Metronidazole: Uses, Side Effects, 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 known for its effectiveness against anaerobic bacteria and certain parasites. Anaerobic bacteria are microorganisms that thrive in low-oxygen environments and can cause various health issues, such as peritonitis, liver abscesses, and pelvic abscesses. Additionally, it is used to combat parasites like Giardia lamblia, amebas, and the vaginal parasite Trichomonas.

Metronidazole acts by selectively inhibiting crucial functions within bacterial cells and parasites, ultimately leading to their demise.

Common Uses:

  1. Parasitic Infections: Metronidazole is employed to treat parasitic infections like Giardia infections, amebic liver abscess, and amebic dysentery.
  2. Bacterial Vaginosis: Effective in treating bacterial vaginosis and related conditions.
  3. Anaerobic Bacterial Infections: It’s used alone or in combination with other antibiotics to treat infections caused by susceptible anaerobic bacteria, including abscesses in the liver, pelvis, abdomen, and brain.
  4. C. difficile Infection: Metronidazole plays a role in treating infections of the colon caused by the bacterium C. difficile.
  5. Helicobacter pylori (H. pylori): Used in combination with other drugs to address H. pylori infections, which can lead to stomach or intestinal ulcers.
  6. Topical Applications: Metronidazole in topical gel form is applied for treating acne rosacea and bacterial vaginosis.

Side Effects of Metronidazole

While generally well-tolerated, metronidazole may cause:

  • Nausea
  • Headaches
  • Loss of appetite
  • Metallic taste
  • Rash (rare)
  • Abdominal cramps
  • Vomiting
  • Diarrhea
  • Dry mouth
  • Dark-colored urine
  • Weight loss
  • Dizziness
  • Constipation
  • Furry tongue
  • Nasal congestion
  • Flushing
  • Vaginal dryness

Serious but rare side effects include brain disease, fevers, mouth sores, and peripheral neuropathy. If any of these symptoms occur, discontinue the medication immediately.


The dosage varies depending on the condition being treated, ranging from oral tablets to topical applications. Dosages should always be followed as per a healthcare provider’s instructions.

Drug Interactions

  • Alcohol: Avoid alcohol while taking metronidazole to prevent severe side effects.
  • Warfarin: Metronidazole can increase the blood-thinning effects of warfarin, potentially leading to bleeding issues.
  • Cimetidine: It raises metronidazole’s blood levels, while cholestyramine reduces its absorption.
  • Amprenavir: Combining metronidazole with amprenavir can be dangerous, as amprenavir contains propylene glycol, which can lead to seizures, increased heart rate, and kidney failure when metronidazole is present.
  • Other Drugs: Metronidazole can interact with medications like carbamazepine, lithium, and cyclosporine, potentially causing serious reactions.

Pregnancy and Breastfeeding

Metronidazole is generally avoided in early pregnancy due to potential adverse effects on the fetus. It’s also excreted in breast milk, making it inadvisable for nursing mothers.


Metronidazole should be stored at room temperature and protected from light.

FDA Approval

Metronidazole tablets were approved by the FDA in July 1963.


Metronidazole is a versatile antibiotic used to treat a variety of parasitic and bacterial infections. However, it’s essential to use it under medical guidance and be aware of potential side effects and drug interactions. Always follow your healthcare provider’s recommendations when using this medication.

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