[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Nitroimidazoles](https://durgical.com/en/pharmacology/classes/nitroimidazoles/) › Metronidazole

[Pharmacology](https://durgical.com/en/pharmacology/) · [Nitroimidazoles](https://durgical.com/en/pharmacology/classes/nitroimidazoles/)

# Metronidazole

The reference drug for anaerobes and for trichomoniasis, giardiasis and amoebiasis. Alcohol is avoided during and after the course.

Updated 2026-11-01

- Class: [Nitroimidazoles](https://durgical.com/en/pharmacology/classes/nitroimidazoles/)
- Target: DNA of anaerobic bacteria and protozoa (after reduction of the nitro group)
- ATC code: J01XD01
- WHO Model List of Essential Medicines

## Mechanism of action

Metronidazole enters cells by passive diffusion. In anaerobic bacteria and protozoa, electron carriers such as ferredoxin reduce its nitro group to a short-lived nitroso radical. The reduced drug and the radicals bind DNA, break it and stop its synthesis, and the cell dies. Human cells and aerobic bacteria hardly activate it.

- Diffuses into the anaerobic microbe
- Ferredoxin reduces its nitro group
- Reactive radicals damage and break DNA
- The microbe dies

## Pharmacokinetics

**Absorption**
Well absorbed by mouth, peak in one to two hours; also given IV and as vaginal gel.

**Distribution**
Wide, including cerebrospinal fluid, saliva, breast milk and liver abscesses.

**Metabolism**
Liver (oxidation and glucuronidation); levels rise in severe liver disease.

**Elimination**
Mainly urine, as metabolites.

**Half-life**
About 8 hours.

## Indications

- Trichomoniasis in women and men (WHO first choice) [Trichomoniasis →](https://durgical.com/en/conditions/trichomoniasis/)
- Bacterial vaginosis [Bacterial vaginosis →](https://durgical.com/en/conditions/bacterial-vaginosis/)
- Amoebiasis, including amoebic liver abscess, and giardiasis [Amoebic liver abscess →](https://durgical.com/en/conditions/amoebic-liver-abscess/)
- Anaerobic intra-abdominal infections and peritonitis, with other antibiotics; prevention in colorectal surgery [Peritonitis →](https://durgical.com/en/conditions/peritonitis/)
- Helicobacter pylori eradication in combination regimens [Helicobacter pylori gastritis →](https://durgical.com/en/conditions/helicobacter-pylori-gastritis/)
- Clostridioides difficile infection (WHO list)

## Adverse effects

### Common

- Metallic taste, nausea and loss of appetite
- Headache and dizziness
- Darker urine
- Vaginal or oral yeast infection

### Serious (rare, important)

- Encephalopathy with cerebellar signs (ataxia, slurred speech) and seizures
- Peripheral and optic neuropathy, mainly with long courses
- Severe liver failure in Cockayne syndrome
- Severe skin reactions; aseptic meningitis

## Contraindications

- Allergy to metronidazole or other nitroimidazoles
- Disulfiram in the previous two weeks
- Alcohol or propylene glycol during treatment and for at least three days after
- Cockayne syndrome; first trimester of pregnancy when treating trichomoniasis

## Key interactions

**Alcohol and propylene glycol**
Disulfiram-like reaction: cramps, vomiting, headache and flushing.

**Warfarin**
Prolongs the prothrombin time; INR is checked.

**Lithium**
Lithium levels can rise to toxic levels.

**Disulfiram**
Psychotic reactions.

## What is monitored

- Neurological symptoms (numbness, unsteadiness, confusion)
- INR on warfarin; lithium levels if taken
- Liver tests in liver disease; blood count in long courses

## Administration and safety

- The IV form is given by slow infusion, not as a push
- Oral forms are best taken with food to reduce stomach upset
- Products with alcohol or propylene glycol are avoided

## Nursing care

- Teach to avoid alcohol, including in mouthwashes and syrups, during and for three days after
- Explain that a metallic taste and dark urine are expected
- Teach to report numbness, tingling, unsteady walking or confusion
- In trichomoniasis, explain that sexual partners are treated too

## Pregnancy and breastfeeding

Contraindicated in the first trimester for trichomoniasis; later in pregnancy it is used when the benefit is clear.

## Memory aid

«GET on the Metro»: Giardia, Entamoeba, Trichomonas, plus Gardnerella and the anaerobes, and no alcohol on the train.

## Test yourself

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

## Why does metronidazole act on anaerobes but not on human cells?

Only anaerobes have the low-redox electron carriers (such as ferredoxin) that reduce its nitro group to the toxic radicals.

## Which reaction occurs with alcohol?

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

## Which neurological adverse effects are possible with long courses?

Peripheral and optic neuropathy, and encephalopathy with cerebellar signs.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/metronidazole/)

## Related drugs

- [Tinidazole](https://durgical.com/en/pharmacology/tinidazole/)
- [Secnidazole](https://durgical.com/en/pharmacology/secnidazole/)
- [Clindamycin](https://durgical.com/en/pharmacology/clindamycin/)

## Related conditions

- [Trichomoniasis](https://durgical.com/en/conditions/trichomoniasis/)
- [Bacterial vaginosis](https://durgical.com/en/conditions/bacterial-vaginosis/)
- [Amoebic liver abscess](https://durgical.com/en/conditions/amoebic-liver-abscess/)
- [Peritonitis](https://durgical.com/en/conditions/peritonitis/)
- [Helicobacter pylori gastritis](https://durgical.com/en/conditions/helicobacter-pylori-gastritis/)

[← Back to the class: Nitroimidazoles](https://durgical.com/en/pharmacology/classes/nitroimidazoles/)

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 - METRONIDAZOLE injection, solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=d46870f5-e2b1-46bd-8413-4a0a8bcf3175) (DailyMed (U.S. National Library of Medicine), 2026); [Metronidazole: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a689011.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/?query=metronidazole) (World Health Organization, 2025).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/pharmacology/metronidazole/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/metronidazole/