Metronidazole is an antimicrobial used against anaerobic bacteria and certain parasites. In South Africa it is commonly prescribed for dental infections, bacterial vaginosis, trichomoniasis, some gut infections and abscesses. It has one caution that stands out: avoid alcohol during treatment and for the period stated on your label.

What is Metronidazole?

Metronidazole is a nitroimidazole antimicrobial. It is often called an antibiotic, which is true but incomplete — it works against both anaerobic bacteria and certain single-celled parasites, which is why it appears in treatments for infections that ordinary antibiotics do not touch.

Anaerobic bacteria are those that grow where there is little or no oxygen: deep in gum tissue, in abscesses, in the bowel, and in the vagina when the normal balance is disturbed. Metronidazole is taken up by these organisms and damages their DNA. Because the process depends on the low-oxygen environment, it has little effect on the bacteria that need oxygen — which is why it is often prescribed alongside a second antibiotic when both types may be involved.

In South Africa it is dispensed as plain metronidazole and under brand names including Anaerobyl, Flagyl and Bemetrazole. It comes as tablets, a suspension, a vaginal gel or pessary, a topical preparation and an infusion.

What is Metronidazole commonly used for?

Metronidazole is prescribed for infections involving anaerobic organisms or susceptible parasites:

Dental infections and abscesses, often with a penicillin such as amoxicillin.
Bacterial vaginosis, one of its most common uses.
Trichomoniasis, a sexually transmitted infection, where partners are usually treated together.
Pelvic inflammatory disease, as part of a combination.
Abdominal and pelvic infections, including after surgery.
Skin and soft tissue infections where anaerobes are involved.
Giardiasis and amoebiasis, parasitic gut infections.
Clostridioides difficile infection, in some circumstances.
Leg ulcers and pressure sores with anaerobic infection, sometimes topically.

Note what is absent. It is not used for ordinary urinary tract infections, chest infections, strep throat or most everyday bacterial infections — those involve organisms it does not cover. And like every antimicrobial, it does nothing against viruses, so it will not help a cold or flu.

How Metronidazole fits into this medicine category

Within antibiotics and antimicrobials, metronidazole occupies a distinct niche, and understanding it explains why it is so often prescribed alongside something else rather than alone.

Penicillins such as amoxicillin cover common chest, ear, throat and dental organisms. Macrolides such as azithromycin cover chest and throat infections and some STIs, and are an option in penicillin allergy. Quinolones such as ciprofloxacin cover urinary and some gut infections. Antifungals treat fungal infections, which no antibacterial will touch.

Metronidazole covers what most of those miss: anaerobes and certain parasites. A dental abscess may involve both aerobic and anaerobic bacteria, which is why a dentist may prescribe two medicines rather than one — not a belt-and-braces measure but two different coverages.

The whole group is on the antibiotics, antifungals and antimicrobials hub.

What to check before using Metronidazole

Check the alcohol instruction on your label, and follow it. This is the caution metronidazole is known for, and it is covered in detail below.

Speak to your prescriber or pharmacist if:

You have liver disease. Metronidazole is processed by the liver.
You have a blood disorder, or have had one.
You have a neurological condition, or develop numbness or tingling during treatment.
You are pregnant, planning pregnancy or breastfeeding. It is used in pregnancy in some circumstances, but that decision belongs with your doctor.
You take warfarin. Metronidazole increases its effect and raises bleeding risk; monitoring is usually adjusted.
You take lithium, phenytoin or phenobarbitone.
You are being treated for trichomoniasis — sexual partners generally need treatment at the same time, or the infection simply passes back.

Take it with or after food to reduce nausea, and complete the course as prescribed. Follow the dispensing label for amounts; this page does not set doses.

Possible cautions and when to ask a pharmacist or doctor

Avoid alcohol during treatment and for the period your label states — commonly 48 hours after finishing, and longer for some related medicines. Combining them can cause a disulfiram-like reaction: flushing, throbbing headache, nausea, vomiting, abdominal cramps and palpitations. It is unpleasant rather than usually dangerous, but it is well documented and easily avoided. Remember that alcohol appears in some cough mixtures, tonics and mouthwashes.

Expect a metallic taste, and possibly nausea, a furred tongue or a dry mouth. These are common and settle after the course.

Your urine may darken. This is harmless.

Report numbness or tingling in the hands or feet, which can occur with prolonged use and should be assessed.

Finish the course. Stopping early leaves the hardier organisms behind and contributes to resistance.

Do not take someone else’s, and do not keep leftovers for next time. Metronidazole covers a narrow set of organisms, and using it for the wrong infection means an untreated infection plus unnecessary side effects.

Seek medical attention for severe or persistent diarrhoea during or after the course, a rash or swelling of the face or lips, yellowing of the eyes or skin, unusual bruising or bleeding, seizures, or persistent numbness. If symptoms worsen after two or three days of treatment, go back to the prescriber rather than waiting it out.

Related medicine guides

Frequently asked questions

What is metronidazole 400 mg used for? Infections involving anaerobic bacteria or certain parasites — dental infections, bacterial vaginosis, trichomoniasis, some gut and pelvic infections, and abscesses.

Can I drink alcohol while taking it? No. Avoid it during treatment and for the period stated on your label. The reaction — flushing, nausea, vomiting, palpitations — is well documented.

Is metronidazole used for urinary tract infections? Not usually. UTIs are generally caused by organisms metronidazole does not cover, so a different antibiotic is normally chosen.

Why do I have a metallic taste? It is a common and harmless effect of the medicine, and it goes once the course finishes.

Does my partner need treating too? For trichomoniasis, yes — partners are treated together, otherwise the infection is passed back. Your prescriber will advise for other infections.

Can I stop once I feel better? No. Finish the course as prescribed. Stopping early leaves the more resistant organisms behind.

What Is Metronidazole Used For?