Metronidazole
FLAGYL
Pronunciation: me-troe-NI-da-zoel
“Metro needs a Flagellum”
What it isNitroimidazole reduced to DNA-damaging radicals; anaerobic bacterial and protozoal infections.
Why this oneIts spectrum spans anaerobes plus trichomoniasis and giardiasis, with strong tissue and CNS penetration.
What limits itAlcohol can provoke a disulfiram-like reaction; metallic taste, neuropathy with prolonged use, and increased warfarin effect matter.
Metronidazole prescription history
Animated chart of estimated U.S. prescriptions per year, Antibacterials (systemic) (1938–2026). Metronidazole enters in 1963 (estimated).
FDA-approved for
- anaerobic/protozoal infections (~1963)
Principal riskDisulfiram-like reaction with alcohol; peripheral neuropathy with prolonged use
Classes and tags
Clinical profile
Legacy pregnancy categoryB
Defining liabilityAllergy, gastrointestinal effects and agent-specific renal, hepatic, hematologic or interaction risks.
FDA label
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| FDA dosing | Population | Start | Target | Max |
|---|---|---|---|---|
| Trichomoniasis (symptomatic, asymptomatic, sexual partners) | Adult (female & male) | 2 g once (or 1 g ×2 same day), or 250 mg TID ×7 days | Same | Undefined |
| Acute intestinal amebiasis (amebic dysentery) | Adult | 750 mg TID ×5-10 days | Same | Undefined |
| Amebic liver abscess | Adult | 500 mg or 750 mg TID ×5-10 days | Same | Undefined |
| Serious anaerobic bacterial infections (intra-abdominal, skin, gyn, septicemia, bone/joint, CNS, lower respiratory, endocarditis) | Adult | 7.5 mg/kg q6h (approx. 500 mg for 70 kg) ×7-10 days | Same | 4 g/24 h |
| Amebiasis | Pediatric | 35-50 mg/kg/24 h divided TID ×10 days | Same | Undefined |
Renal ESRD: metabolites accumulate, monitor for AEs; hemodialysis removes significant drug — consider supplemental dose after session
Hepatic Severe (Child-Pugh C): reduce dose by 50%; mild-moderate: no adjustment, monitor for AEs
Trichomoniasis repeat courses 4-6 weeks apart with trichomonad reconfirmed and leukocyte counts before/after; no one-day course in pregnancy. Anaerobic: bone/joint, lower respiratory, endocardial infections may need longer
Instructions
- Avoid alcohol and propylene glycol during and for at least 3 days after therapy
- Take full course exactly as directed; do not skip doses
Available strengths
- tab 250, 500 mg
Contraindications
- First trimester of pregnancy (in patients with trichomoniasis)
- Disulfiram use within the last 2 weeks (psychotic reactions)
- Alcohol or propylene glycol use during and for ≥3 days after therapy (disulfiram-like reaction)
- Cockayne syndrome (severe/fatal hepatotoxicity)
Cautions
- Hypersensitivity reactions
- Severe cutaneous adverse reactions (TEN, SJS, DRESS, AGEP)
- CNS/peripheral nervous system effects: encephalopathy, peripheral/optic neuropathy, seizures, aseptic meningitis
- Hepatic impairment (accumulation)
- Renal impairment/ESRD (metabolite accumulation)
- Fungal superinfection (candidiasis)
- Blood dyscrasias (leukopenia)
- Drug-resistant bacteria and parasites
Adverse reactions
- Nausea, sometimes with headache
- Anorexia, vomiting
- Diarrhea
- Epigastric distress, abdominal cramping
- Constipation
- Metallic taste
Pregnancy Crosses placenta; contraindicated first trimester for trichomoniasis; most studies show no increased anomaly risk
Lactation Present in milk at levels similar to maternal serum; weigh benefits, or pump and discard during therapy and for 48 hours after last dose
Cost
Generic entry1984
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Memory aids: interaction avatars
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Self-check
- How can metronidazole affect warfarin?
Increases its effect. - What neurologic toxicity can follow prolonged metronidazole use?
Neuropathy.