Flagyl contains metronidazole, an antibiotic effective against anaerobic bacteria and certain parasites. It treats dental infections, bacterial vaginosis, trichomoniasis, amoebiasis, giardiasis and abdominal infections. Alcohol must be avoided during the course and for 48 hours afterwards.
What is Flagyl?
Flagyl contains metronidazole, dispensed as 200 mg and 400 mg tablets, a suspension, a gel and an injection. It is a Schedule 4 prescription medicine, also sold as Metronidazole, Anaerobyl and Flagyl S.
Metronidazole is a nitroimidazole, and its mechanism explains exactly what it covers. The medicine enters a cell as an inactive compound and must be chemically reduced to become active. That reduction only happens efficiently in environments without oxygen.
So metronidazole is activated inside anaerobic bacteria and certain parasites — organisms that live where there is little or no oxygen — and remains inert in ordinary human tissue and in oxygen-tolerant bacteria. Once activated, it damages the organism’s DNA and kills it.
That is why metronidazole treats such an apparently unrelated list: infected gums, the deep parts of an abscess, the vagina’s altered flora, the gut, the abdominal cavity. All are low-oxygen environments.
It also explains a limit: metronidazole is useless against most ordinary bacterial infections — a typical chest or urinary infection is caused by organisms that live perfectly well with oxygen. This is why it is so often prescribed alongside another antibiotic such as amoxicillin, rather than instead of one.
What is Flagyl commonly used for?
Bacterial infections:
- Dental infections and abscesses, gum infection and pericoronitis — very commonly with amoxicillin.
- Abdominal and pelvic infections, including after bowel surgery, appendicitis and diverticulitis.
- Bacterial vaginosis.
- Pelvic inflammatory disease, as part of combination treatment.
- Infected pressure sores and diabetic foot infections, where anaerobes are involved.
- ***Clostridioides difficile* infection**, in some circumstances.
- ***Helicobacter pylori* eradication**, with a proton pump inhibitor and another antibiotic.
Parasitic infections:
- Trichomoniasis, a sexually transmitted infection — where partners must be treated at the same time, or reinfection follows immediately.
- Amoebiasis (amoebic dysentery), and amoebic liver abscess.
- Giardiasis, a common cause of persistent diarrhoea after travel.
For a dental abscess, the usual caveat applies: an antibiotic controls the spreading infection, but the definitive treatment is dental — drainage or treating the tooth. Antibiotics alone rarely settle an abscess permanently.
How Flagyl fits into this medicine category
| Group | Examples | Common targets |
|---|---|---|
| Nitroimidazoles | Metronidazole (Flagyl), tinidazole | Anaerobes, trichomonas, amoeba, giardia |
| Broad-spectrum penicillins | Amoxicillin | Chest, ear, throat, dental, urinary |
| Macrolides | Azithromycin, erythromycin | Chest, some STIs; penicillin allergy |
| Narrow-spectrum penicillins | Flucloxacillin | Skin — staphylococci |
| Sulfonamide combinations | Co-trimoxazole | Urinary, chest, PCP |
Metronidazole covers what most of the others do not, which is why it so often appears as a second antibiotic rather than a lone one. See the antibiotics and antimicrobials hub.
What to check before using Flagyl
Do not drink alcohol during the course or for at least 48 hours after finishing. This is the single most important thing to know about metronidazole. It can produce a disulfiram-like reaction — flushing, throbbing headache, nausea, vomiting, palpitations and marked distress. Even small amounts can trigger it.
Check hidden sources too: some cough mixtures, mouthwashes, tonics and sauces contain alcohol. If you have been prescribed metronidazole for a week and have an occasion planned, tell your prescriber — timing can sometimes be adjusted.
Take it with food to reduce stomach upset, and finish the whole course.
Tell your prescriber or pharmacist if:
- You have liver disease.
- You have a blood disorder, or are on a prolonged course.
- You have a neurological condition, or existing numbness or tingling in the hands or feet.
- You are pregnant or breastfeeding.
- You take warfarin — metronidazole substantially increases its effect and the risk of bleeding; lithium; phenytoin; phenobarbital; busulfan; or disulfiram.
- You are being treated for trichomoniasis, in which case sexual partners must be treated simultaneously and sex avoided until both have completed treatment.
Follow the dispensing label. This page does not set doses, and the amount differs considerably between indications.
Possible cautions and when to ask a pharmacist or doctor
A metallic taste is very common and settles after the course. It is harmless, though it can make food unappealing.
Nausea, loss of appetite and stomach upset are frequent — taking it with food helps.
Dark or reddish-brown urine occurs in some people. It is a harmless effect of the medicine’s breakdown products, not blood.
Nerve effects with prolonged use. Peripheral neuropathy — numbness, tingling or burning in the hands or feet — is associated with long or repeated courses. Report it promptly, as it can persist.
Neurological symptoms, though rare, are recognised: confusion, unsteadiness, slurred speech or seizures need urgent attention.
Diarrhoea is common. Severe, watery or bloody diarrhoea, particularly with fever, needs medical attention.
For trichomoniasis, treat partners together. This is the most common reason the infection returns immediately after a successful course.
Seek urgent medical attention for numbness, tingling or weakness in the hands or feet; confusion, unsteadiness or slurred speech; a seizure; a rash that blisters or peels; facial or throat swelling; severe or bloody diarrhoea; unusual bruising or bleeding; or yellowing of the skin or eyes.
Related medicine guides
Frequently asked questions
What is Flagyl 200 mg used for? Infections caused by anaerobic bacteria and certain parasites — dental infections, bacterial vaginosis, trichomoniasis, amoebiasis, giardiasis and abdominal infections.
Can I drink alcohol while taking it? No. Alcohol during the course or within 48 hours of finishing can cause flushing, severe headache, vomiting and palpitations.
Why has it been prescribed with another antibiotic? Metronidazole covers anaerobic organisms only. Many infections involve both anaerobic and oxygen-tolerant bacteria, so two antibiotics cover the full range.
Why does everything taste metallic? A metallic taste is a very common and harmless effect that resolves after the course.
My urine has gone dark. Is that a problem? Usually not. Metronidazole can turn urine dark or reddish-brown through its breakdown products. Mention it if you are unsure.
Does my partner need treatment too? For trichomoniasis, yes — both partners must be treated at the same time or reinfection is immediate. For other uses, generally not.