Nitroimidazoles · Anti-infectives
Metronidazole
Flagyl · Flagyl®(brand names vary by market)
Only works where there is no oxygen, and never with alcohol.
Also: oral liquid, infusion, topical, suppository
Mechanism
Reduced by anaerobic organisms into nitro radicals that fragment DNA. Aerobic cells cannot perform that reduction, which is the whole basis of its selectivity and its spectrum.
Indications
- Anaerobic infection
- Intra-abdominal infection in combination
- Bacterial vaginosis and trichomoniasis
- Giardiasis and amoebiasis
- Clostridioides difficile, no longer first line
Formulations
Dosing concepts
Oral equals intravenous
Oral bioavailability approaches 100 percent, so there is rarely a pharmacological reason to keep a patient on the intravenous form once they can swallow.
Duration
Peripheral neuropathy is cumulative-dose related, so long courses are reviewed rather than renewed automatically.
Concepts, not prescribing instructions. Practice numbers live in current references and local protocols.
Pharmacokinetics
- Bioavailability
- ~100%
- Half-life
- ~8 h
- Elimination
- hepatic: Hepatic metabolism; reduce in severe hepatic impairment
- Protein binding
- <20%
Adverse effects
- Metallic tastecommon
- Nauseacommon
- Disulfiram-like reaction with alcoholseriousFlushing, vomiting, tachycardia; unpleasant enough to require explicit warning.
- Peripheral neuropathyseriousWith prolonged courses; may not fully reverse.
- Dark urinecommonHarmless.
Contraindications & precautions
Contraindications
- Alcohol during and for 48 hours after
- First trimester of pregnancy, relative
Precautions
- Prolonged courses
- Hepatic impairment
- Warfarin, whose effect it substantially increases
Interactions
Monitoring
Neuropathy symptoms on courses beyond a few weeks
Cumulative and sometimes irreversible
INR in warfarin patients
A strong potentiating interaction
Whether it is duplicating anaerobic cover
Adding it to piperacillin-tazobactam is a very common redundancy
Counselling points
- No alcohol at all during the course and for 48 hours afterwards, including in cough medicines and mouthwash.
- A metallic taste is common and goes away.
- Your urine may darken; that is harmless.
- Report numbness or tingling in the hands or feet.
Clinical pearls
- It has no aerobic Gram-negative activity, so it is always a partner drug in intra-abdominal sepsis rather than a solo one.
- Adding it to a carbapenem or to piperacillin-tazobactam duplicates anaerobic cover that is already there, one of the easiest stewardship wins on any ward round.
References & review
- Metronidazole product monograph (consult the current version for your market) (monograph)
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. (reference-work)
Last reviewed 2026-08-16 · jurisdiction: global · drug information changes; verify against current references before practice use.