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
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 →
Bacterial vaginosis Bacterial vaginosis →
Amoebiasis, including amoebic liver abscess, and giardiasis Amoebic liver abscess →
Anaerobic intra-abdominal infections and peritonitis, with other antibiotics; prevention in colorectal surgery Peritonitis →
Helicobacter pylori eradication in combination regimens 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

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.

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.

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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 (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - METRONIDAZOLE injection, solution (DailyMed (U.S. National Library of Medicine), 2026); Metronidazole: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025).

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Representative model, for educational purposes only.