Nidasall contains metronidazole 400 mg, a prescription antibiotic effective against anaerobic bacteria and certain parasites. It is used for dental infections, bacterial vaginosis, trichomoniasis, gut parasites and some abdominal and pelvic infections. Alcohol must be avoided during and shortly after the course.
What is Nidasall?
Nidasall is registered in South Africa as a Schedule 4 tablet containing metronidazole 400 mg — prescription-only.
The same ingredient is sold under several South African names, including Flagyl, Anaerobyl, Metrazole, Bio-Metronidazole and generic metronidazole. Being dispensed one rather than another is ordinary generic substitution.
Metronidazole is a nitroimidazole, and its selectivity comes from a genuinely elegant mechanism. It enters cells as an inactive compound and stays inactive in normal, oxygen-rich conditions. Inside organisms that live without oxygen, the strongly reducing internal chemistry converts it into a highly reactive form that fragments the organism’s DNA, killing it.
Human cells and oxygen-using bacteria never activate it. That is the source of both its effectiveness and its limitation:
- Highly effective against anaerobic bacteria and several protozoal parasites.
- Essentially useless against the aerobic bacteria responsible for most chest, urinary and skin infections.
This is why metronidazole is so often prescribed alongside another antibiotic rather than instead of one. In a mixed infection, one antibiotic covers the aerobes while metronidazole covers the anaerobes.
What is Nidasall commonly used for?
A doctor may prescribe it for:
- Dental infections and abscesses, where mouth anaerobes are typically involved.
- Bacterial vaginosis, an overgrowth of anaerobic bacteria in the vagina.
- Trichomoniasis, a sexually transmitted parasitic infection where partners are normally treated at the same time, or reinfection is likely.
- Pelvic inflammatory disease, usually with other antibiotics.
- Intra-abdominal infections, including appendicitis and diverticulitis.
- Amoebiasis and giardiasis, parasitic gut infections.
- Infected wounds, leg ulcers, 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, so it will not help a cold, flu or most sore throats.
How Nidasall fits into this medicine category
| Group | Examples | Typical role |
|---|---|---|
| Nitroimidazoles | Metronidazole (Nidasall) | Anaerobic bacteria and some parasites |
| Penicillins | Amoxicillin, flucloxacillin | Common first-line choices |
| Penicillin + inhibitor | Co-amoxiclav | 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 Nidasall
Do not drink alcohol during the course or for at least 48 hours afterwards. This is the single most important instruction. Metronidazole blocks an enzyme in alcohol metabolism, allowing acetaldehyde to build up and producing a disulfiram-like reaction: flushing, throbbing headache, nausea, vomiting, cramps, a racing heart and falling blood pressure. It is severe enough that people often mistake it for a serious allergic reaction.
Check hidden sources of alcohol in cough syrups, tonics and mouthwashes.
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 a blood problem on metronidazole before.
- Have a neurological condition, or existing numbness or tingling.
- Have epilepsy.
- Are pregnant or breastfeeding. It is used in pregnancy in some circumstances, but must be discussed.
- Take warfarin — metronidazole markedly increases its effect and bleeding risk.
- 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 through the day.
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 — harmless and expected, but 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 it can persist — report it early rather than finishing the course and hoping.
- A seizure.
- Unsteadiness, confusion, slurred speech or poor coordination.
- Severe or bloody diarrhoea, during or after the course.
- Fever, sore throat or mouth ulcers, or unusual bruising or bleeding.
- 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 at any point, or if a treated infection returns — recurrence may mean a partner needs treating, or that the diagnosis needs revisiting.
Never use antibiotics left over from a previous illness or prescribed for someone else.
Related medicine guides
- Browse Antibiotics, Antifungals & Antimicrobials guides
- Read: Azrotrin used for
- Read: Metronidazole used for
Frequently asked questions
What is Nidasall 400 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.
What are Nidasall tablets used for generally? The same range of infections. It is an antibiotic, prescription-only.
Can I drink alcohol on it? No. The combination can cause flushing, severe headache, vomiting and a racing heart. Avoid alcohol during the course and for at least 48 hours after, including in syrups and mouthwashes.
Is it the same as Flagyl? 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 has my urine gone dark? Metronidazole can darken urine to a reddish-brown colour. This is harmless.
Will it treat a chest or bladder 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.