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.
Get medical help first. Contact NHS 111 (NHS 24 on 111 in Scotland) or your GP the same day if you have pain in your lower tummy or pelvis, a high temperature or you feel unwell. If you’re pregnant, speak to your midwife about any change in discharge. Visit a sexual health clinic if you have bleeding between periods or after sex, sores or blisters, or a partner who has been diagnosed with an STI. Online treatment is not suitable in these situations. If your symptoms started after sex with a new partner, we can still treat BV, but please also get a free STI test.
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 BV Treatment Online
Access Doctor’s GPhC-registered pharmacist independent prescribers assess BV symptoms online and, where treatment is suitable, supply it from our UK pharmacy. Approved before 3pm, Monday to Friday? Dispatched the same day in plain packaging, by Royal Mail Tracked 48, Tracked 24 or Special Delivery guaranteed by 1pm next working day: you choose.
Start BV Consultation →Adults only online, and not in pregnancy or breastfeeding. Access Doctor’s online BV service is for adults aged 18 and over who aren’t pregnant or breastfeeding. This is a restriction of our online service only: if you are pregnant or breastfeeding, your GP, midwife or a local sexual health clinic can assess and treat BV free of charge, and metronidazole can be prescribed where clinically appropriate. Under 18? NHS sexual health clinics see young people confidentially.
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 a day 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 for something completely different. A dentist giving it for an abscess is a common one.
- 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 — where other treatments are unsuitable.
- 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
Course length for BV
48 hrs
No alcohol after the last tablet
The alcohol rule applies to the tablets: avoid alcohol during the course and for 48 hours after the last dose. With the vaginal gel, a reaction is unlikely but can’t be ruled out, so the same caution is sensible. 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 or breastfeeding, 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.
Get BV Treatment Online
Complete a short online consultation and our GPhC-registered pharmacist independent prescribers will check suitability before dispensing. Approved before 3pm, Monday to Friday? Dispatched the same day in plain packaging, by Royal Mail Tracked 48, Tracked 24 or Special Delivery guaranteed by 1pm next working day: you choose.
Start BV Consultation →UK safety and getting treatment
Every antibiotic treatment for BV in the UK, whether tablets, gel or cream, is a prescription-only medicine. Before anything is prescribed, a GPhC-registered pharmacist independent prescriber reviews your consultation, including your allergies and the other medicines you take. Always read the patient information leaflet in the pack. Never share antibiotics with anyone else, and don’t keep leftover tablets for next time: return unused medicine to a pharmacy.
If you think you’ve had a side effect from a medicine, tell your pharmacist or doctor. You can also report it to the MHRA through the Yellow Card scheme, which helps keep medicines safe for everyone.
You can get BV treatment free from your GP or a sexual health clinic, and many clinics offer walk-in appointments and STI testing at the same visit. Our private online service is an alternative for adults aged 18 and over who aren’t pregnant or breastfeeding. You can see all our BV treatment online options on one page.
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 where other treatments are unsuitable.
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.
What is the difference between metronidazole 400mg and 500mg?
They are the same medicine in different strengths. In the UK, 400mg is the usual tablet for BV, taken twice a day for 5–7 days. 500mg tablets are used for some other infections and are more common in some other countries. Always follow the dose on your label.
Completing the treatment
Metronidazole is available from Access Doctor following an online consultation with a pharmacist independent prescriber, as a 5–7 day course of 400mg tablets.
Bacterial vaginosis · Rx
Metronidazole 400mg Tablets
The usual first-choice BV treatment: one tablet twice a day for 7 days, after an online consultation.
View →Bacterial vaginosis
BV Treatment Online
All our BV options in one place, including when to see your GP or a sexual health clinic instead.
View →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.