Metronidazole Side Effects and Warnings
What is common, what is serious, what interacts — and practical ways to make a course easier to complete.
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: Most metronidazole side effects are mild, appear in the first day or two, and settle once the course finishes — but numbness or tingling in the hands or feet is a specific signal to stop and seek advice.
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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. If that isn’t you, your GP, midwife or a local sexual health clinic can assess and treat BV free of charge. Under 18? NHS sexual health clinics see young people confidentially.
The side effects people notice most
Metronidazole is a well-established antibiotic with a predictable side effect profile. Most people complete a course with nothing worse than an unpleasant taste and some queasiness in the first few days. The product information doesn’t give exact frequencies, but these are the ones people report most.
- Nausea — the most common complaint, usually mild and much improved by taking doses with food.
- Metallic or bitter taste — persistent and distinctive; a recognised effect of the drug rather than a warning sign.
- Reduced appetite — often accompanies the taste change and resolves with it.
- Headache — usually mild and manageable with paracetamol.
- Dry or furred mouth — helped by regular fluids and teeth cleaning.
- Mild abdominal discomfort or loose stools — common with most antibiotics.
- Dark or reddish-brown urine — harmless and caused by a breakdown product of the drug.
Worth knowing: side effects often appear before any improvement in your symptoms. Feeling slightly worse on day one while the discharge and odour are unchanged is the normal sequence, not a sign the antibiotic is failing.
Managing nausea and metallic taste
These two are the usual reasons people struggle to finish a course, and both respond well to simple measures. A bit of planning really does help.
Always take with food
Take each dose with or immediately after a meal. Metronidazole on an empty stomach is markedly harder to tolerate.
Full glass of water
Swallow the tablet whole with plenty of water and stay well hydrated throughout the course.
Space doses evenly
Roughly twelve hours apart — for example 8am and 8pm — keeps levels steadier and reduces peaks.
Blunt the taste
Sugar-free mints or gum, citrus drinks, and brushing your teeth after meals all help. It resolves within a few days of finishing.
If nausea is severe enough that you are considering abandoning the course, contact your prescriber rather than stopping. Switching to a vaginal preparation — Zidoval gel or Dalacin cream — achieves similar cure rates with far less systemic upset. Both are currently out of stock with us, but your GP or a sexual health clinic can prescribe them.
Serious side effects to act on
These are uncommon, but each has a clear action attached.
Stop taking metronidazole and seek medical advice if you develop numbness, tingling or pins and needles in your hands or feet; severe, persistent or bloody diarrhoea; unexplained bruising, bleeding or a sore throat with fever; yellowing of the skin or eyes; seizures; confusion or problems with balance and coordination; or a rash with swelling of the face or throat and difficulty breathing. In an emergency, call 999.
| Sign | What it may indicate | Action |
|---|---|---|
| Numbness or tingling in hands or feet | Peripheral neuropathy — more likely with long or repeated courses | Stop and contact a prescriber promptly |
| Severe or bloody diarrhoea | Antibiotic-associated colitis | Stop and seek medical advice the same day |
| Unexplained bruising, bleeding or fever | Effect on blood cell counts | Seek medical advice |
| Yellow skin or eyes, dark urine with pale stools | Liver involvement | Seek medical advice promptly |
| Seizure, confusion or unsteadiness | Central nervous system effects | Seek urgent medical attention |
| Rash with facial or throat swelling | Allergic reaction | Call 999 |
Peripheral neuropathy deserves particular attention if you are having repeated or extended courses for recurrent BV, since risk relates to cumulative exposure. Report any new numbness or tingling rather than waiting to see whether it settles.
Alcohol: the short version
Do not drink alcohol during a metronidazole course or for 48 hours after the last dose. The reaction can include flushing, throbbing headache, severe nausea, vomiting and palpitations. With the vaginal gel, a reaction is unlikely but can’t be ruled out, so avoid alcohol with that too. Full detail on the rule, the timings and hidden sources of alcohol is in our guide to how to take metronidazole.
If avoiding alcohol for the length of a course is impractical for you, say so at consultation. Dalacin 2% cream contains clindamycin and carries no alcohol restriction. It’s currently out of stock with us, but your GP or a sexual health clinic can prescribe it.
Interactions and who should take care
Metronidazole has a small number of clinically important interactions. Declare every medicine you take, including anything bought over the counter, when you complete a consultation.
| Medicine | Interaction | What it means |
|---|---|---|
| Warfarin and other anticoagulants | Effect enhanced | Increased bleeding risk; INR monitoring usually needed |
| Lithium | Levels can rise | Risk of lithium toxicity; monitoring or an alternative antibiotic |
| Phenytoin and phenobarbital | Levels affected | May alter seizure control — specialist advice needed |
| Ciclosporin | Levels can rise | Monitoring required |
| Fluorouracil | Levels can rise | Higher risk of side effects; your prescriber will check |
| Disulfiram | Confusion, psychotic reactions | Should not be taken within two weeks of each other |
| Busulfan | Levels can rise | Combination generally avoided |
Metronidazole must not be taken if you are allergic to it or to a similar antibiotic such as tinidazole, or if you have Cockayne syndrome, a rare genetic condition in which it can cause severe liver damage.
Extra caution also applies if you have liver disease, a blood disorder, a history of seizures, or are pregnant or breastfeeding. Metronidazole passes into breast milk and can affect its taste. Our online service doesn’t treat BV while you’re breastfeeding, so speak to your GP or a sexual health clinic, who can advise on the safest option for you and your baby.
Pregnancy. Symptomatic BV in pregnancy is generally treated, but always under the care of a clinician who knows you are pregnant. Our online service doesn’t treat BV in pregnancy, so speak to your midwife, GP or a sexual health clinic.
Side effects of the vaginal preparations
Zidoval gel contains the same antibiotic but very little of it reaches the bloodstream, which changes the side effect picture considerably.
| Metronidazole tablets | Zidoval gel | |
|---|---|---|
| Nausea | More likely | Less likely |
| Metallic taste | More likely | Occasional and mild |
| Local irritation | Not applicable | Common |
| Change in discharge | Less likely | Common |
| Thrush after the course | Common | Common |
| Alcohol restriction | Yes, until 48 hours after | Advised, to be safe |
| Peripheral neuropathy risk | Low, rises with repeated courses | Very low |
This is the main reason vaginal preparations are preferred for women who have already had several oral courses. See BV treatment options compared for the full decision framework.
Thrush after a course
Developing thrush during or shortly after metronidazole is common, because suppressing bacteria can allow Candida yeast to overgrow. The symptoms are distinct from BV: thick white discharge with prominent itching and soreness, rather than thin discharge with a fishy odour.
It is treated with an antifungal, not with another antibiotic course. Our guides to telling BV and thrush apart and how long BV takes to clear up cover the distinction and what to do next.
Get BV Treatment Online
Complete a short online consultation, declaring your current medicines, and our GPhC-registered pharmacist independent prescribers will check suitability before supply. 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 are the most common side effects of metronidazole?
Nausea, a persistent metallic taste, reduced appetite, headache and a furred or dry mouth are the ones people notice most. They are usually mild, tend to be worse in the first couple of days, and settle after the course finishes.
How can I stop metronidazole making me feel sick?
Take each dose with or immediately after food rather than on an empty stomach, swallow the tablet with a full glass of water, and space doses about twelve hours apart. Eating smaller meals more often and avoiding rich or greasy food during the course also helps.
Why does metronidazole cause a metallic taste?
It is a recognised effect of the drug itself rather than a sign of anything going wrong. It usually appears within a day or two of starting and resolves within a few days of finishing the course. Sugar-free mints, citrus drinks and regular teeth cleaning make it more tolerable.
Does metronidazole interact with warfarin?
Yes. Metronidazole enhances the effect of warfarin and can increase the risk of bleeding, so INR monitoring is usually needed. Tell your prescriber before starting if you take warfarin or any other anticoagulant.
When should I stop taking metronidazole and seek help?
Stop and seek medical advice if you develop numbness or tingling in the hands or feet, severe or bloody diarrhoea, unexplained bruising or bleeding, yellowing of the skin or eyes, seizures, or a rash with facial swelling or breathing difficulty. In an emergency, call 999.
Can metronidazole cause thrush?
Yes. By suppressing bacteria it can allow Candida yeast to overgrow, so thrush during or shortly after a course is common. It needs an antifungal rather than a repeat of the antibiotic course.
Do the side effects mean the antibiotic is not working?
No. Nausea and metallic taste often appear before any symptom improvement, which is disconcerting but not a sign of failure. Improvement in BV symptoms typically begins around day two or three.
Does metronidazole make your urine dark?
It can. Metronidazole occasionally turns urine darker or reddish-brown, which is harmless and goes back to normal after the course. If you also have yellowing of your skin or eyes, or pale stools, get medical advice, as that can point to a liver problem.
Can metronidazole make you dizzy or drowsy?
It can, though it isn’t common. Dizziness, drowsiness and headache have all been reported. If you feel dizzy or drowsy, don’t drive or use machinery until it passes, and get medical advice if you also have confusion, unsteadiness or problems with your vision.
Completing the treatment
Metronidazole is available from Access Doctor following an online consultation with a pharmacist independent prescriber. Declaring your full medicine list matters here more than in most consultations — the warfarin and lithium interactions are the ones we check for first.
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
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View →References
- Electronic Medicines Compendium. Metronidazole 400mg Tablets — Summary of Product Characteristics. medicines.org.uk
- British National Formulary. Metronidazole. bnf.nice.org.uk
- National Institute for Health and Care Excellence. Clinical Knowledge Summaries: Bacterial vaginosis. 2025. cks.nice.org.uk
- NHS. Metronidazole. nhs.uk
- Medicines and Healthcare products Regulatory Agency. Yellow Card scheme. yellowcard.mhra.gov.uk
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.