Anaerobyl 400 contains metronidazole 400 mg, a prescription antibiotic. It treats infections caused by anaerobic bacteria and certain parasites — dental abscesses, bacterial vaginosis, trichomoniasis, amoebiasis, giardiasis and some abdominal and pelvic infections. Do not drink alcohol while taking it.
What is Anaerobyl 400?
Anaerobyl is registered in South Africa as Schedule 4 — prescription-only:
| Product | Contains |
|---|---|
| Anaerobyl 200 | Metronidazole 200 mg |
| Anaerobyl 400 | Metronidazole 400 mg |
The name tells you what it does. Anaerobyl comes from anaerobic — bacteria that grow without oxygen. Other South African brands of the same ingredient include Flagyl, Metrazole, Bemetrazole and generic metronidazole. Being dispensed one rather than another is ordinary generic substitution.
Metronidazole is a nitroimidazole, and its selectivity comes from an unusual mechanism. The drug enters cells as an inactive compound. In organisms that survive without oxygen, the internal chemistry is strongly reducing — and that environment converts metronidazole into a reactive form that fragments the organism’s DNA.
Human cells and ordinary oxygen-using bacteria do not have that chemistry, so they do not activate the drug. This is why metronidazole is highly effective against anaerobes and several parasites, and has essentially no effect on the aerobic bacteria that cause most chest, urinary and skin infections.
That specificity explains why it is so often prescribed alongside another antibiotic — for a mixed infection, one covers the anaerobes and the other covers everything else.
What is Anaerobyl 400 commonly used for?
A doctor may prescribe it for:
- Dental infections and abscesses, where anaerobes from the mouth are typically involved. A very common use, alongside dental treatment.
- Bacterial vaginosis, an overgrowth of anaerobic bacteria in the vagina.
- Trichomoniasis, a sexually transmitted parasitic infection. Sexual partners usually need treating at the same time, or reinfection is likely.
- Pelvic inflammatory disease, usually with other antibiotics.
- Intra-abdominal infections, including appendicitis and diverticulitis, generally in combination.
- Amoebiasis and giardiasis, parasitic gut infections.
- Infected wounds, pressure sores and diabetic foot infections with anaerobic involvement.
- ***Clostridioides difficile* colitis**, in some cases.
- **As part of Helicobacter pylori eradication**, with a PPI and another antibiotic.
It has no effect on viruses, and it will not treat an ordinary chest, urinary or skin infection caused by aerobic bacteria.
How Anaerobyl 400 fits into this medicine category
| Group | Examples | Typical role |
|---|---|---|
| Nitroimidazoles | Metronidazole (Anaerobyl) | Anaerobic bacteria and some parasites |
| Penicillins | Amoxicillin, flucloxacillin | Common first-line choices |
| Penicillin + inhibitor | Amoxicillin/clavulanic acid | Broader cover, including some anaerobes |
| Cephalosporins | Cefalexin | Skin, throat, urinary infections |
| Macrolides | Azithromycin | Chest infections, penicillin allergy |
| Fluoroquinolones | Ciprofloxacin | Urinary, gut and bone infections; reserved |
Metronidazole occupies a niche the others largely do not cover. See the antibiotics and antimicrobials hub.
What to check before using Anaerobyl 400
Do not drink alcohol during the course, or for at least 48 hours afterwards. This is the most important instruction with metronidazole, and it is not a general precaution — it is a specific chemical interaction. Metronidazole blocks an enzyme that breaks down alcohol, allowing acetaldehyde to accumulate. The result is a disulfiram-like reaction: flushing, throbbing headache, nausea, vomiting, abdominal cramps, a racing heart and a drop in blood pressure. It is thoroughly unpleasant and can be severe.
Check hidden sources of alcohol too — some cough and cold syrups, tonics and mouthwashes contain it.
Complete the full course, even once you feel better.
Tell your doctor or pharmacist if you:
- Have liver disease, which usually means a lower dose.
- Have a blood disorder, or have had one on metronidazole before.
- Have a neurological condition, or existing nerve symptoms such as numbness or tingling.
- Have epilepsy.
- Are pregnant or breastfeeding. It is used in pregnancy in some situations, but must be discussed.
- Take warfarin — metronidazole increases its effect significantly, and bleeding risk rises.
- Take lithium, whose levels can rise to toxic concentrations.
- Take phenytoin, phenobarbital, ciclosporin, busulfan or disulfiram.
- Are being treated for a sexually transmitted infection, so partners can be treated too.
Take it with or after food to reduce stomach upset, and space doses evenly.
Possible cautions and when to ask a pharmacist or doctor
Common effects are nausea, a persistent metallic taste, loss of appetite, abdominal discomfort, headache, dry mouth and a furred tongue. The metallic taste is characteristic of metronidazole, harmless, and resolves after the course.
Metronidazole can turn urine dark or reddish-brown. This is harmless and expected — but it is worth knowing so it is not mistaken for blood.
Stop and seek medical attention if you develop:
- Numbness, tingling or burning in the hands or feet. Peripheral neuropathy is a recognised effect, more likely with prolonged or repeated courses, and can persist — report it early rather than finishing the course and hoping.
- A seizure.
- Unsteadiness, confusion, slurred speech or difficulty coordinating movement.
- Severe or bloody diarrhoea, during or after the course.
- Fever, sore throat or mouth ulcers, or unusual bruising or bleeding — possible blood problems.
- Yellowing of the eyes or skin.
- Swelling of the face, lips or tongue, or difficulty breathing.
- A severe rash, blistering or peeling skin.
- Visual disturbance or eye pain.
Contact your doctor if you are no better after a few days, if symptoms worsen, or if a treated infection returns — recurrence may mean a partner needs treating, or that a different approach is needed.
Related medicine guides
- Browse Antibiotics, Antifungals & Antimicrobials guides
- Read: Azrotrin used for
- Read: Metronidazole used for
Frequently asked questions
What are Anaerobyl 400 tablets used for? Infections caused by anaerobic bacteria and certain parasites — dental abscesses, bacterial vaginosis, trichomoniasis, amoebiasis, giardiasis and some abdominal and pelvic infections. Each tablet contains metronidazole 400 mg.
Why is it called Anaerobyl? The name comes from “anaerobic” — bacteria that grow without oxygen, which is what metronidazole targets.
Can I drink alcohol on it? No. Alcohol can cause flushing, severe headache, vomiting and a racing heart. Avoid it during the course and for at least 48 hours afterwards, including in syrups and mouthwashes.
Is it the same as Flagyl or Metrazole? The active ingredient is the same — metronidazole. The manufacturer differs.
Why does everything taste metallic? A metallic taste is characteristic of metronidazole. It is harmless and resolves after the course.
Why is my urine dark? Metronidazole can darken urine to a reddish-brown. This is harmless and expected.
Will it treat a chest or urinary infection? Usually not. It works on anaerobic bacteria and some parasites, not the aerobic bacteria behind most chest and urinary infections.
Does my partner need treatment? For trichomoniasis, yes — partners are usually treated at the same time to prevent reinfection. Ask your doctor.