Metronidazole – Uses, Mechanism & Side Effects

What is Metronidazole?

Metronidazole is a synthetic antibiotic belonging to the nitroimidazole class. It is effective against a wide range of anaerobic bacteria (bacteria that thrive without oxygen) and certain protozoa (single-celled parasites). Metronidazole has been a cornerstone of infectious disease treatment for decades and is widely used around the world.

Indications

Metronidazole is used to treat a variety of infectious conditions, including:

• Trichomoniasis: a sexually transmitted infection caused by the parasite Trichomonas vaginalis
• Bacterial vaginosis: a common bacterial imbalance of the vaginal flora
• Amoebiasis: an intestinal infection caused by Entamoeba histolytica
• Giardiasis: an intestinal parasitic disease caused by Giardia lamblia
• Anaerobic bacterial infections: such as abscesses, wound infections, and peritonitis (inflammation of the abdominal lining)
• Helicobacter pylori eradication: in combination with other antibiotics for the treatment of peptic ulcers

Mechanism of Action

Metronidazole is a prodrug, meaning it is converted into its active form inside the body. Within anaerobic microorganisms, specific enzymes called nitroreductases activate the drug. The activated form then binds to the DNA of the pathogen, disrupting its structure and preventing replication and survival. Because this activation process requires anaerobic conditions, metronidazole largely spares the normal aerobic flora of the body.

Forms and Dosage

Metronidazole is available in several forms:

• Tablets/capsules for oral administration
• Intravenous infusion for severe or systemic infections
• Vaginal suppositories and gel for local treatment
• Cream and gel for topical application, e.g. in rosacea

Dosage depends on the type and severity of the infection as well as body weight. A typical adult dose is 400–500 mg two to three times daily for 5–10 days, but this may vary by indication. The treating physician determines the appropriate dosage for each individual patient.

Side Effects

Metronidazole is generally well tolerated, but may cause the following side effects:

• Common: nausea, vomiting, diarrhea, metallic taste in the mouth, headache
• Occasional: dizziness, dry mouth, burning sensation of the tongue
• Rare: tingling or numbness in the limbs (peripheral neuropathy), central nervous system disturbances, allergic reactions

Important: Alcohol must be avoided during treatment with metronidazole, as it can trigger a severe reaction similar to the disulfiram reaction, with symptoms such as nausea, vomiting, rapid heartbeat, and flushing.

Contraindications and Drug Interactions

Metronidazole should not be used in patients with:

• known hypersensitivity to nitroimidazoles
• concurrent use of alcohol or alcohol-containing products
• severe liver disease (dose adjustment required)

Notable drug interactions include warfarin (an anticoagulant) and lithium. Use during pregnancy — especially during the first trimester — should only be considered after careful medical evaluation.

References

1. World Health Organization (WHO): Model List of Essential Medicines, 23rd Edition (2023). Available at: https://www.who.int/publications/i/item/WHO-MHP-HPS-EML-2023.02
2. Brunton L.L., Hilal-Dandan R., Knollmann B.C.: Goodman & Gilman's The Pharmacological Basis of Therapeutics. 13th Edition. McGraw-Hill, New York, 2018.
3. Centers for Disease Control and Prevention (CDC): Sexually Transmitted Infections Treatment Guidelines (2021). Available at: https://www.cdc.gov/std/treatment-guidelines/