References UK clinical guidance from the NHS, the British HIV Association (BHIVA), and the British Association for Sexual Health and HIV (BASHH). Last updated: April 2026.
Metronidazole is an antibiotic used to treat several infections including trichomoniasis, bacterial vaginosis (BV), and some other bacterial and parasitic infections. It’s one of the most commonly prescribed antibiotics in sexual health clinics across the UK and is available as tablets, capsules, or a topical gel.
If you’ve been prescribed metronidazole or want to know whether it’s the right treatment for your infection, this guide covers how it works, the correct dosage, side effects to watch for, and important interactions.
Not sure what you have? Get tested first with a home STI kit
What Does Metronidazole Treat?
| Condition | Dosage | Duration |
|---|---|---|
| Trichomoniasis | 400mg twice daily OR 2g single dose | 5-7 days (or single dose) |
| Bacterial vaginosis (BV) | 400mg twice daily | 5-7 days |
| Giardiasis | 2g once daily | 3 days |
| Pelvic inflammatory disease (part of combination therapy) | 400mg twice daily | 14 days |
Metronidazole does NOT treat chlamydia, gonorrhoea, syphilis, herpes, or HIV. Each of these requires different medication. If you’re unsure which infection you have, get tested before starting any antibiotic.
How Does Metronidazole Work?
Metronidazole works by entering bacterial and parasitic cells and damaging their DNA, which kills the organism. It’s particularly effective against anaerobic bacteria (bacteria that thrive without oxygen) and protozoan parasites like Trichomonas vaginalis.
The drug is well absorbed when taken orally and reaches effective concentrations throughout the body, including the reproductive tract, which makes it suitable for treating genital infections.
How to Take Metronidazole
- Take with or after food to reduce stomach upset
- Swallow tablets whole with water
- Complete the full course even if symptoms improve before it’s finished
- Space doses evenly (e.g., one in the morning and one in the evening for twice-daily dosing)
- Do not drink alcohol during treatment and for at least 48 hours after finishing (see below)
Metronidazole and Alcohol: Why You Must Avoid It
This is the most important thing to know about metronidazole. You must not drink any alcohol while taking it and for at least 48 hours after your last dose.
Metronidazole interferes with how your body processes alcohol, causing a disulfiram-like reaction. Symptoms can include:
- Severe nausea and vomiting
- Flushing and hot flushes
- Rapid heartbeat
- Abdominal cramps
- Headache
This reaction can happen with even small amounts of alcohol, including alcohol in cooking, mouthwash, or cough medicines. Check labels carefully during treatment.
Common Side Effects
Most people tolerate metronidazole well, but side effects can include:
- Nausea (the most common side effect, reduced by taking with food)
- Metallic taste in the mouth
- Loss of appetite
- Headache
- Diarrhoea
- Dark urine (harmless, caused by the drug’s metabolites)
Less common but important side effects
- Dizziness or drowsiness
- Skin rash or itching
- Numbness or tingling in hands and feet (peripheral neuropathy, usually with prolonged use)
If you experience tingling in your extremities, contact your doctor. This side effect can occasionally be permanent if the drug is continued.
Who Should Not Take Metronidazole?
- People with a known allergy to metronidazole or other nitroimidazole drugs
- People taking certain blood-thinning medications (warfarin, as metronidazole increases its effect)
- People taking lithium (metronidazole can increase lithium levels)
- People taking disulfiram (Antabuse)
Pregnancy: Metronidazole can be used in pregnancy when the benefits outweigh the risks. BASHH guidelines recommend it for treating symptomatic BV and trichomoniasis during pregnancy, though the single 2g dose is generally avoided in favour of the 7-day course.
Breastfeeding: Metronidazole passes into breast milk. The 7-day course can usually be taken while breastfeeding, but the single 2g dose should be avoided. Consult your doctor.
How to Get Metronidazole in the UK
- Sexual health clinic: Free, often dispensed same day after diagnosis
- Your GP: Prescription after consultation
- Online prescription services: After an online consultation for confirmed infections
Metronidazole requires a prescription. It is not available over the counter in the UK.
Read more: What is trichomoniasis? | What is bacterial vaginosis? | What is PID?
Frequently Asked Questions
How quickly does metronidazole work?
You may notice symptoms improving within 2-3 days, but you must complete the full course. For trichomoniasis treated with a single 2g dose, the infection is usually cleared within a week.
Can I have sex while taking metronidazole?
Avoid sex until you and your partner have both completed treatment and symptoms have resolved. For trichomoniasis, your partner also needs treatment to prevent reinfection.
What if I miss a dose?
Take the missed dose as soon as you remember, unless it’s almost time for your next dose. Don’t take a double dose. Continue with the remaining course as normal.
Does metronidazole treat chlamydia?
No. Chlamydia requires azithromycin or doxycycline. Metronidazole treats trichomoniasis and bacterial vaginosis, not chlamydia or gonorrhoea.
Can I take metronidazole with the contraceptive pill?
Yes. Metronidazole does not affect hormonal contraception. However, if you experience vomiting or severe diarrhoea as a side effect, this could reduce the pill’s effectiveness. Use additional contraception if needed.
Related reading
- Using Antibiotics to Prevent STIs: DoxyPEP Explained
- Azithromycin: Uses in Sexual Health
- Best Antibiotics for STIs: A Practical Guide
- Ciprofloxacin for STIs: Uses, Dosage, and Side Effects
- Clindamycin: Uses in Sexual Health
Sources
- NHS — Sexual health conditions
- BASHH — UK clinical guidelines for STI management
- British HIV Association (BHIVA) — HIV treatment guidelines
- Terrence Higgins Trust — sexual health information
- UK Health Security Agency — STI surveillance data
Steve Page is a recognised expert on Sexually Transmitted Diseases (STDs) and STD treatments, having published numerous articles in peer-reviewed journals and presented his research at conferences around the world. He has an in-depth understanding of the latest medical research on STDs, and is an advocate for the development of new treatments and protocols to improve the health of those affected. In addition to his research, he has dedicated his career to understanding the causes and symptoms of STDs, as well as how to best treat those impacted.
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