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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Nitroimidazoles

Prodrugs activated only inside anaerobes and protozoa, where they shred DNA. For anaerobes, trichomoniasis, giardiasis and amoebiasis.

Updated 2026-11-01

## What they are

Metronidazole, tinidazole and secnidazole. They act on anaerobic bacteria and on protozoa such as Trichomonas, Giardia and Entamoeba, which have the low-redox electron carriers needed to switch them on.

## Mechanism of action

The drug enters by diffusion. In anaerobic cells, proteins such as ferredoxin pass an electron to its nitro group and form short-lived reactive radicals. These damage DNA, break its strands and kill the cell. Aerobic human cells barely reduce the drug, which explains the selectivity.

Target: DNA, after reductive activation of the nitro group inside the microbe

## Key points

- Disulfiram-like reaction with alcohol or propylene glycol (flushing, vomiting, headache) during treatment and for days afterwards.
- Metallic taste and dark urine are typical and harmless.
- Long courses can cause peripheral neuropathy and, rarely, encephalopathy with cerebellar signs.
- Contraindicated in Cockayne syndrome because of severe liver toxicity.

## Drugs in this class

- [**Metronidazole** The reference drug for anaerobes and for trichomoniasis, giardiasis and amoebiasis. Alcohol is avoided during and… DNA of anaerobic bacteria and protozoa (after reduction of the nitro group)](https://durgical.com/en/pharmacology/metronidazole/)
- [**Tinidazole** A longer-acting relative of metronidazole, often given as a single dose for trichomoniasis and giardiasis. Alcohol is… DNA of anaerobic bacteria and protozoa (after reduction of the nitro group)](https://durgical.com/en/pharmacology/tinidazole/)
- [**Secnidazole** A long-acting nitroimidazole given once, as granules on soft food, for bacterial vaginosis and trichomoniasis. DNA of anaerobic bacteria and protozoa (after reduction of the nitro group)](https://durgical.com/en/pharmacology/secnidazole/)

## Others in the class

- ornidazole
- benznidazole (Chagas disease)
- fexinidazole (sleeping sickness)

## Test yourself

Think of the answer, then tap the question to see it.

## Why do nitroimidazoles act only on anaerobes and protozoa?

Only their low-redox electron carriers can reduce the nitro group to the reactive radicals that damage DNA.

## What happens if a patient drinks alcohol while taking metronidazole?

A disulfiram-like reaction: flushing, nausea, vomiting, cramps and headache.

## Which nitroimidazole effects are expected and harmless?

A metallic taste and darker urine.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [DailyMed - METRONIDAZOLE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=8f1de4ca-a005-4ca9-a43a-380429c80d6f) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - TINIDAZOLE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=95ab57b2-5d7e-4986-abcd-65edf1728383) (DailyMed (U.S. National Library of Medicine), 2026); [Secnidazole: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a617052.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/?query=metronidazole) (World Health Organization, 2025).

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Representative model, for educational purposes only.

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