What Is Metronidazole? Uses, How It Works and What It Treats
A plain guide to the antibiotic behind most BV treatment — what it targets, what it does not, and the forms it comes in.
Part of the Bacterial Vaginosis condition guide.
Key fact: Metronidazole is a specialist rather than an all-rounder — it is highly effective against anaerobic bacteria and certain parasites, and essentially useless against everything else.
Get Metronidazole Online
Access Doctor provides metronidazole following a GPhC-regulated online consultation with our pharmacist independent prescribers.
Order Metronidazole →What metronidazole is
Metronidazole is an antibiotic belonging to the nitroimidazole class. It has been in clinical use since the 1960s, appears on the World Health Organization's list of essential medicines, and remains the first-line treatment for several common infections including bacterial vaginosis.
It is chemically unrelated to penicillins, which matters practically: a penicillin allergy does not prevent you from taking metronidazole. It is prescription-only in the UK in every form, including the vaginal gel.
| Detail | |
|---|---|
| Drug class | Nitroimidazole antibiotic |
| Active against | Anaerobic bacteria and certain protozoal parasites |
| Not active against | Aerobic bacteria, viruses, fungi and yeasts |
| Common UK brand (vaginal gel) | Zidoval |
| Typical BV dose | 400mg twice daily for 5–7 days |
| Availability | Prescription only |
How it works
Metronidazole is what pharmacologists call a prodrug: it is inactive as swallowed and only becomes active inside the target organism. That mechanism is what makes it so selective.
1
It enters the bacterial cell
Metronidazole diffuses freely into cells, including the anaerobic bacteria overgrowing in bacterial vaginosis.
2
Low-oxygen conditions activate it
Inside an anaerobic organism, the drug is chemically reduced into a highly reactive form. This reaction only happens efficiently in low-oxygen environments — which is precisely why aerobic bacteria and human cells are largely unaffected.
3
The activated drug damages bacterial DNA
The reactive form binds to and breaks the organism's DNA strands, halting replication and repair.
4
The organism dies
Metronidazole is bactericidal rather than merely inhibitory — it kills the target organism rather than just stopping it multiplying.
Why this matters for you. The selectivity is the reason metronidazole works so well on BV and does nothing at all for a chest infection or thrush. It is not that it is weak — it is that it is aimed at a specific category of organism.
What it treats
Metronidazole is used across several specialties, which is why you may have been prescribed it before in a completely different context.
- Bacterial vaginosis — first-line treatment in UK guidance, as tablets or vaginal gel.
- Trichomoniasis — a sexually transmitted parasitic infection that can look similar to BV.
- Dental and gum infections — dental abscesses and severe gum disease, often alongside another antibiotic.
- Pelvic inflammatory disease — as part of a combination regimen.
- Abdominal and surgical infections — where anaerobic organisms from the gut are involved.
- Clostridioides difficile infection — in certain cases.
- Infected leg ulcers and pressure sores — where anaerobic infection produces odour.
- Rosacea — as a topical gel or cream, working through a different anti-inflammatory mechanism.
- Amoebiasis and giardiasis — parasitic infections usually acquired abroad.
What it does not treat
This is worth being explicit about, because leftover metronidazole is often taken speculatively for the wrong problem.
Thrush
Caused by Candida yeast. Needs an antifungal such as clotrimazole or fluconazole. Metronidazole has no effect on yeast at all.
Colds and flu
Viral infections. No antibiotic of any kind works on them.
Most urinary tract infections
Usually caused by aerobic bacteria such as E. coli, which metronidazole does not target.
Most chest infections
Typically involve aerobic organisms requiring a different class of antibiotic.
Do not use a leftover course. Part-used antibiotic courses saved from a previous prescription are a common cause of delayed diagnosis and contribute to antibiotic resistance. If symptoms have returned, get assessed rather than self-prescribing.
The forms it comes in
| Form | Typical use | Notes |
|---|---|---|
| 400mg tablets | BV, dental infections, trichomoniasis | The standard oral form for adults |
| 200mg tablets | Lower-dose regimens | Also used where dose adjustment is needed |
| Oral suspension | Children, difficulty swallowing | Liquid formulation |
| Zidoval 0.75% vaginal gel | BV | Five nights, or twice weekly for suppression |
| Topical gel or cream | Rosacea, malodorous wounds | Different mechanism from the antibacterial use |
| Suppositories | Hospital and specialist settings | Where the oral route is unavailable |
| Intravenous infusion | Serious infections in hospital | Not used in community practice |
Its role in BV specifically
Bacterial vaginosis is an overgrowth of anaerobic bacteria displacing the protective lactobacilli that keep vaginal pH low. Because those organisms are anaerobes, they sit squarely in metronidazole's range — which is why it has remained first-line for decades.
Two routes are used. Oral tablets treat the whole body and are the NICE CKS first-line option. Zidoval vaginal gel delivers the same drug locally with far fewer systemic side effects. Cure rates are broadly similar; the choice is usually about tolerability and convenience, which is set out in BV treatment options compared.
400mg
Standard oral dose, taken twice daily
5–7 days
Standard course length for BV
70–80%
Typical cure rate for a first episode
The alcohol rule applies to every form that reaches the bloodstream — including the vaginal gel. Avoid alcohol during the course and for 48 hours after the last dose. Full detail is in how to take metronidazole.
Getting metronidazole in the UK
Metronidazole cannot be bought over the counter in any form. It requires a prescription from a GP, a sexual health clinic, or a registered online prescriber following a clinical assessment.
Access Doctor is a GPhC-registered online pharmacy. You complete a consultation, a pharmacist independent prescriber reviews it against your medical history and current medicines, and treatment is dispensed only if it is clinically appropriate. If it is not appropriate — if you are pregnant, if the symptoms do not fit BV, or if an interaction makes it unsafe — we will tell you rather than supply it.
Seek in-person assessment if you have pelvic or abdominal pain, fever, pain during sex, or bleeding between periods or after sex. These are not features of uncomplicated BV. In a medical emergency, call 999.
Order Metronidazole for BV Treatment
Complete a short online consultation and our GPhC-registered pharmacist independent prescribers will check suitability before dispensing.
Order Metronidazole →Frequently Asked Questions
What is metronidazole used for?
Metronidazole treats infections caused by anaerobic bacteria and certain parasites. In the UK it is commonly used for bacterial vaginosis, trichomoniasis, dental and gum infections, some abdominal and pelvic infections, infected leg ulcers and pressure sores, and Clostridioides difficile infection.
Is metronidazole a strong antibiotic?
It is highly effective against the organisms it targets, but it is narrow rather than broad in range. It works on anaerobic bacteria and some parasites, and has essentially no effect on the aerobic bacteria that cause many common infections, which is why it is not a general-purpose antibiotic.
Is metronidazole a penicillin?
No. Metronidazole belongs to the nitroimidazole class and is chemically unrelated to penicillins. A penicillin allergy does not prevent you from taking metronidazole.
What forms does metronidazole come in?
In UK practice it is available as 200mg and 400mg tablets, an oral suspension, a vaginal gel branded Zidoval, a topical gel or cream for skin conditions such as rosacea, suppositories and an intravenous infusion for hospital use.
Does metronidazole treat thrush?
No. Thrush is caused by Candida yeast and needs an antifungal such as clotrimazole or fluconazole. Metronidazole is an antibacterial and has no effect on yeast, which is why it is important to know which condition you have before treating.
Is metronidazole available over the counter in the UK?
No. Metronidazole is a prescription-only medicine in the UK in all its forms, including the vaginal gel. It can be supplied following an online consultation with a registered prescriber.
How quickly does metronidazole start working?
It begins acting on bacteria within hours of the first dose, but noticeable symptom improvement usually takes two to three days. The full course should always be completed regardless of how quickly symptoms settle.
Completing the treatment
Metronidazole is available from Access Doctor following an online consultation with a pharmacist independent prescriber, in tablet form or as Zidoval vaginal gel.
Bacterial vaginosis · Rx
Metronidazole 400mg Tablets
First-line oral antibiotic for BV, taken twice daily for five to seven days.
View product →Bacterial vaginosis · Rx
Zidoval Vaginal Gel
Metronidazole 0.75% gel applied nightly for five nights.
View product →Bacterial vaginosis · Rx
Dalacin 2% Cream
Clindamycin vaginal cream for when metronidazole is unsuitable.
View product →Self-test · Pharmacy
Canestest Self-Test
Checks vaginal pH at home to help distinguish BV from thrush before you treat.
View product →References
- British National Formulary. Metronidazole. bnf.nice.org.uk
- Electronic Medicines Compendium. Metronidazole 400mg Tablets — Summary of Product Characteristics. medicines.org.uk
- National Institute for Health and Care Excellence. Clinical Knowledge Summaries: Bacterial vaginosis. 2025. cks.nice.org.uk
- NHS. Metronidazole. nhs.uk
- World Health Organization. Model List of Essential Medicines. who.int
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.