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.
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.
Get Metronidazole Online
Access Doctor provides metronidazole following a GPhC-regulated online consultation with our pharmacist independent prescribers.
Order Metronidazole →Common side effects
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.
- 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 account for the great majority of stopped courses, and both respond well to simple measures.
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.
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. This applies to the vaginal gel as well as the tablets. 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 at all.
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 | Toxicity increased | Usually avoided together |
| Disulfiram | Confusion, psychotic reactions | Should not be taken within two weeks of each other |
| Busulfan | Levels can rise | Combination generally avoided |
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, so a vaginal preparation is often preferred while breastfeeding — but that should be a prescriber decision, not a self-selected one.
Pregnancy. Symptomatic BV in pregnancy is generally treated, but always under the care of a clinician who knows you are pregnant. Do not order treatment online or self-treat while pregnant.
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 | Common | Uncommon |
| Metallic taste | Common | Occasional and mild |
| Local irritation | Not applicable | Common |
| Change in discharge | Uncommon | Common |
| Thrush after the course | Common | Common |
| Alcohol restriction | Yes | Yes |
| 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.
Order Metronidazole for BV Treatment
Complete a short online consultation, declaring your current medicines, and our GPhC-registered pharmacist independent prescribers will check suitability before supply.
Order Metronidazole →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 most frequently reported. 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.
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
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 — much less nausea than the tablets.
View product →Bacterial vaginosis · Rx
Dalacin 2% Cream
Clindamycin vaginal cream, a seven-night course with no alcohol restriction.
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
- 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.