Metronidazole

What is Metronidazole?

Metronidazole is an antibiotic and antiprotozoal medicine that is the first-line treatment for bacterial vaginosis (BV) and trichomoniasis in Europe, as recommended by the WHO and European STI treatment guidelines. It is also used as part of combination regimens for pelvic inflammatory disease (PID) and for various other anaerobic bacterial infections.

Metronidazole is listed on the WHO Model List of Essential Medicines and has been in clinical use since the 1960s. It is available in oral tablet, vaginal gel, and topical cream formulations, allowing flexible treatment approaches depending on the condition and patient preference.

How does Metronidazole work?

Metronidazole is a nitroimidazole antibiotic that works through a unique mechanism targeted at anaerobic organisms:

1. Intracellular activation: Metronidazole enters bacterial and protozoal cells. In anaerobic organisms (which lack oxygen), the nitro group of metronidazole is reduced by intracellular electron transport proteins (ferredoxins) to form a reactive intermediate.
2. DNA damage: The reactive intermediate binds to and damages bacterial and protozoal DNA, causing strand breaks and preventing normal function.
3. Selective toxicity: This reduction and activation process occurs primarily in anaerobic and microaerophilic organisms. Human aerobic cells do not reduce metronidazole in the same way, giving the drug its selective activity against anaerobes and protozoa with minimal harm to normal human cells.

This mechanism explains why metronidazole is highly effective against:

• Anaerobic bacteria: Including Gardnerella vaginalis, Prevotella spp., Mobiluncus spp. (key organisms in bacterial vaginosis)
• Protozoa: Trichomonas vaginalis (trichomoniasis), Giardia lamblia, Entamoeba histolytica

Who is Metronidazole for?

Primary indications:

• Bacterial vaginosis (BV): First-line treatment — oral metronidazole 400–500 mg twice daily for 5–7 days, or metronidazole vaginal gel 0.75% once daily for 5 days
• Trichomoniasis: Single oral dose of 2 g (or 400–500 mg twice daily for 5–7 days) — partner treatment is essential as trichomoniasis is sexually transmitted
• Pelvic inflammatory disease (PID): Used in combination with other antibiotics as part of multi-drug regimens
• Dental and oral anaerobic infections
• Abdominal and pelvic anaerobic infections
• Clostridioides difficile infection (oral)

Metronidazole is not effective against:

• Viral infections (herpes, HPV, HIV)
• Aerobic bacteria (standard bacterial infections — use appropriate antibiotics)
• Fungal infections (use antifungals such as clotrimazole or fluconazole)
• Chlamydia or gonorrhoea (different antibiotics are required)

Dosage

Oral metronidazole should be taken with food to reduce gastrointestinal side effects. Alcohol must be completely avoided during treatment and for 48 hours after completion.

Side effects

Very common side effects

• Nausea
• Metallic taste in the mouth (very characteristic of metronidazole)

Common side effects

• Vomiting
• Abdominal discomfort or cramping
• Diarrhoea
• Loss of appetite
• Headache
• Dizziness

Less common side effects

• Dark or brownish urine (harmless discolouration from metronidazole metabolites)
• Dry mouth
• Tongue discomfort, furring of the tongue
• Peripheral neuropathy: Tingling, numbness, or weakness in the hands or feet — report to a doctor if this occurs; treatment may need to be stopped

Rare but serious side effects

• Disulfiram-like reaction with alcohol: Severe flushing, nausea, vomiting, tachycardia — alcohol must be strictly avoided
• Seizures and encephalopathy: Rare; more likely with high doses or prolonged treatment
• Leucopenia: Reduced white blood cells — usually reversible
• Serious allergic reactions: Including angioedema and anaphylaxis

Metronidazole vs alternatives for bacterial vaginosis

Metronidazole vs vaginal metronidazole gel: Both are effective for BV. Vaginal gel delivers medicine directly to the infection site with lower systemic exposure, potentially causing fewer gastrointestinal side effects but does not justify disregarding the product’s alcohol warning. The oral route treats the systemic bacterial load and may be more appropriate in some clinical situations.

Online assessment for Metronidazole

Metronidazole is a prescription-only medicine (POM) in most EU member states. It cannot legally be dispensed without a valid prescription from a licensed medical professional.

Prescrivia operates as a technology intermediary: we do not prescribe medicines, employ doctors, or sell medicines. Our platform connects patients with independent doctors who can conduct confidential online health assessments.

Availability of an online assessment, prescribing, dispensing and delivery depends on the medicine, country and applicable law. Spain prohibits distance sale of prescription medicines; a catalogue entry or cross-border prescription does not establish a right to delivery. An in-person examination or testing may be required.

The process, where legally and clinically available:

1. Complete a health assessment: Describe your symptoms (discharge, odour, itching), relevant sexual history, and any current medications.
2. Doctor review: An independent doctor reviews your assessment. If metronidazole is clinically appropriate, they may issue a prescription.
3. Pharmacy fulfilment: The prescription is sent to a pharmacy partner to assess dispensing and any supply under the applicable country rules.

Important: Prescrivia does not guarantee that a prescription will be issued. All prescribing decisions are made independently by qualified medical professionals.

Important safety information

Do not take Metronidazole if you:

• Are pregnant without medical assessment of the risks and benefits; this applies throughout pregnancy
• Are breastfeeding (if a single high dose is used, breastfeeding should be interrupted for 12–24 hours)
• Are allergic to metronidazole or other nitroimidazole medicines (tinidazole, ornidazole)

Tell your doctor before taking Metronidazole if you:

• Have liver disease (dose reduction or increased monitoring may be required)
• Have a history of neurological conditions or seizures
• Are taking warfarin (metronidazole significantly increases anticoagulant effect — INR monitoring is required)
• Are taking lithium (metronidazole can raise lithium levels)
• Are taking disulfiram (do not use together — risk of psychotic reactions)

Recurrent BV: Bacterial vaginosis frequently recurs after treatment. If you experience multiple episodes, a doctor should investigate contributing factors such as sexual practices, hygiene, hormonal factors, and the potential role of suppressive vaginal gel regimens.

Trichomoniasis — treat partners: Trichomoniasis is a sexually transmitted infection and partners must be treated simultaneously to prevent reinfection, regardless of whether they have symptoms.

Sources

• Product information / clinical reference 1
• Product information / clinical reference 2

Medical information on this page is based on the following sources:

• World Health Organization (WHO). WHO Guidelines for the Treatment of Trichomonas vaginalis. 2016. who.int
• European STI Guidelines Editorial Board. 2018 European guideline on the management of vaginal discharge. Journal of the European Academy of Dermatology and Venereology.
• World Health Organization (WHO). WHO Model List of Essential Medicines. 22nd edition, 2021.

The content-edit date does not certify a clinical review. It is provided for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for advice relevant to your individual health circumstances.