What it is
Metronidazole is an antimicrobial specialised in anaerobic bacteria (those that live without oxygen) and protozoa. It is a reference treatment for giardiasis, amoebiasis, trichomoniasis, bacterial vaginosis and anaerobic abdominal or gynaecological infections.
Its mechanism differs from other antibiotics: inside the micro-organism it is activated and generates toxic radicals that damage its DNA. In Clostridioides difficile colitis it was the first option for years, but current guidelines prefer oral vancomycin or fidaxomicin; metronidazole remains an alternative in mild cases if those are not available.
Mechanism of action
It works by:
- Reduction of its nitro group by anaerobic enzymes, generating cytotoxic free radicals
- Irreversible damage to the micro-organism's DNA
- Activity against protozoa too (Giardia, amoebae, Trichomonas)
At the counter
When to recommend it
- Giardiasis and intestinal amoebiasis
- Trichomoniasis and bacterial vaginosis
- Amoebic liver abscesses or anaerobic abdominal infections, on prescription
- Mild C. difficile colitis, only as an alternative when oral vancomycin or fidaxomicin are not available
When not to
- Allergy to nitroimidazoles
- Pregnancy: for the doctor to assess (traditionally avoided in the first trimester)
- Drinking alcohol during treatment
- Blood disorders or pre-existing peripheral neuropathy
Warnings
- Important Peripheral neuropathy: rare but potentially irreversible. Warn about tingling and weakness in the hands or feet.
- Important Disulfiram-like effect: a reaction similar to the one with disulfiram if alcohol is drunk during or shortly after treatment (flushing, palpitations).
- Caution Nausea and a metallic taste: very common but tolerable.
- Caution Seizures: very rare, but reported with prolonged use.
Pharmacokinetics
- Absorption
- Rapid and almost complete oral absorption (80–95%). Excellent bioavailability. Peak at 1–2 hours.
- Distribution
- Very wide distribution throughout the body, including the CNS (it crosses the blood–brain barrier). High concentrations in liver, kidneys and lungs. Minimal protein binding.
- Metabolism
- Oxidative hepatic metabolism. It produces active metabolites that contribute to the effect but also to toxicity.
- Elimination
- Mainly renal. Half-life of 6–8 hours.
Metronidazole vs oral vancomycin in C. difficile colitis
| Aspect | Metronidazole | Oral vancomycin |
|---|---|---|
| Place in current guidelines | Alternative in mild cases | First line (together with fidaxomicin) |
| Route for this indication | Oral | Oral (given IV it does not reach the colon) |
| Absorption | Almost complete: systemic effects | Almost none: it acts in the gut |
| Systemic effects | Yes (neuropathy, nervous system) | Minimal |
| Cost | Low | Higher |
For C. difficile colitis, oral vancomycin or fidaxomicin now lead. Metronidazole remains a reference treatment for giardiasis, amoebiasis, trichomoniasis, bacterial vaginosis and anaerobic infections.
Self-assessment
Three questions. When you check your answers you will see the explanation for each one.
-
Metronidazole has to be activated by anaerobic enzymes to generate toxic radicals. That is why it only works against anaerobes and certain protozoa.
-
Metronidazole causes a disulfiram-like effect with alcohol. Avoid it during treatment and for 48 hours afterwards.
-
For years it was the first option, but studies showed worse results than oral vancomycin. Current guidelines (IDSA, ESCMID) prefer oral vancomycin or fidaxomicin.
Training content. It does not replace the summary of product characteristics or clinical judgement.