Anti-infective
Metronidazole
An antibiotic for anaerobes and some parasites — Treats anaerobic bacterial and certain parasitic infections — with a strict no-alcohol rule.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Metronidazole treats infections caused by anaerobic bacteria (which thrive without oxygen) and certain parasites. It is widely used in dental, abdominal, pelvic and gynaecological infections.
How it works
Inside the target organisms it is activated and damages their DNA, killing bacteria and parasites that live in low-oxygen environments while sparing the body's ordinary aerobic bacteria.
In practice
In practice the counselling point everyone remembers is alcohol: metronidazole can cause an unpleasant flushing, nausea and vomiting reaction with alcohol, so patients are told to avoid it during the course and for a short time after. Clinically it is the go-to for anaerobic infections (dental, abdominal, pelvic), for bacterial vaginosis and for some gut parasites. A metallic taste is common, prolonged courses can rarely affect the nerves, and it interacts with warfarin by enhancing its effect.
Examples
Practical use
How to take it & use it well
- Take metronidazole as prescribed and complete the full course, even if symptoms improve, to fully clear the infection.
- Taking tablets with or after food can reduce stomach upset and nausea.
- Do NOT drink any alcohol during treatment and for a couple of days after finishing, as the combination can cause flushing, a pounding heartbeat, nausea and vomiting.
- Also avoid hidden alcohol in products such as some mouthwashes, cough syrups and certain foods sauces while on treatment.
- Space your doses evenly through the day and take it for as long as your clinician advises.
- Tell your clinician if you notice a persistent metallic taste, numbness or tingling, as some effects should be reviewed.
Common uses
- Anaerobic infections (dental, intra-abdominal, pelvic)
- Bacterial vaginosis
- Certain parasitic infections (e.g. giardiasis)
- Part of some H. pylori regimens
Monitoring
- Symptom response
- INR if taken with warfarin
- Neurological symptoms with prolonged use
Weighing it up
Advantages & disadvantages
Advantages
- It is highly effective against many anaerobic bacteria and certain parasites.
- It treats a range of infections, including some dental, gut, skin and gynaecological infections.
- It is available as tablets, liquid and other forms to suit different needs.
- Courses are often relatively short for many common infections.
Disadvantages
- Alcohol must be strictly avoided during and shortly after treatment due to an unpleasant reaction.
- It commonly causes a metallic taste, nausea and a furred tongue.
- Prolonged or repeated courses can rarely cause nerve symptoms such as numbness or tingling.
- It can interact with blood thinners, increasing bleeding risk.
Key safety principles
What to watch for
- Avoid alcohol during and shortly after the course (flushing, nausea, vomiting).
- Enhances the effect of warfarin — increased bleeding risk.
- Metallic taste is common; prolonged courses can rarely cause nerve effects.
Key interactions
What to avoid or check alongside
- Alcohol, including hidden alcohol in some medicines and mouthwashes, can trigger flushing, palpitations, nausea and vomiting.
- Warfarin's blood-thinning effect is enhanced by metronidazole, raising the risk of bleeding.
- Lithium levels can rise, increasing the risk of lithium toxicity.
- Phenytoin and some other medicines can have their levels affected when combined.
- Combining with other medicines that can affect the nerves may add to the risk of tingling or numbness with longer use.
Patient & carer advice
- Do not drink any alcohol while taking it or for a couple of days after
- A metallic taste is common and harmless
- Tell us if you take warfarin or develop tingling in the hands or feet
Answers
Metronidazole: frequently asked questions
Why can't I drink alcohol with metronidazole?
Metronidazole and alcohol together can cause an unpleasant reaction with flushing, a pounding heartbeat, headache, nausea and vomiting. Avoid all alcohol during the course and for a couple of days after finishing, including hidden alcohol in some mouthwashes and medicines.
Why do I have a metallic taste in my mouth?
A metallic or altered taste is a common and harmless side effect of metronidazole that settles after the course ends. Staying hydrated and good oral care can help. Mention it to your pharmacist if it is troubling you.
Does metronidazole interact with warfarin?
Yes. Metronidazole can enhance warfarin's blood-thinning effect, raising the risk of bleeding. If you take warfarin, your clinician may check your INR more closely during and after the metronidazole course and adjust as needed.
Should I finish the whole course?
Yes, complete the full course as prescribed even if you feel better, to fully clear the infection and reduce the chance of it returning or of resistance developing. Contact your clinician if symptoms persist or worsen.
What should I do if I feel tingling or numbness?
Numbness or tingling in the hands or feet can rarely occur, especially with longer or repeated courses. Report these symptoms to your clinician promptly, as the treatment may need reviewing to prevent the problem worsening.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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