Metronidazole
FLAGYL
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.
FDA-approved for
- anaerobic/protozoal infections (~1963)
FDA label
Read the full label on DailyMed ↗
| FDA dosing | Population | Start | Target | Max |
|---|---|---|---|---|
| Trichomoniasis | Adult | 2 g single dose (or two 1 g doses same day), or 250 mg TID ×7 days | ||
| Amebiasis (acute intestinal) | Adult | 750 mg TID ×5–10 days | ||
| Amebic liver abscess | Adult | 500–750 mg TID ×5–10 days | ||
| Amebiasis | Pediatric | 35–50 mg/kg/day divided TID ×10 days | ||
| Anaerobic bacterial infection | Adult | 7.5 mg/kg q6h (~500 mg for 70 kg) | 4 g/day |
Renal Hemodialysis removes significant drug; consider supplemental dose after the session
Hepatic Severe hepatic impairment (Child-Pugh C): reduce dose by 50%
Repeat trichomoniasis courses spaced 4–6 weeks apart, with trichomonad reconfirmed
Instructions
- Avoid alcohol and propylene glycol during and for at least 3 days after therapy
- Do not give within 2 weeks of disulfiram
- Do not use during first trimester for trichomoniasis
Contraindications
- First trimester of pregnancy (in patients with trichomoniasis)
- Disulfiram use within the last 2 weeks (psychotic reactions)
- Alcohol use during and for ≥3 days after therapy (disulfiram-like reaction)
- Cockayne syndrome (severe/fatal hepatotoxicity)
Cautions
- Peripheral and optic neuropathy (may be persistent)
- CNS: convulsive seizures, encephalopathy, aseptic meningitis
- QT prolongation
- Severe cutaneous reactions (SJS, TEN, DRESS, AGEP)
- Hepatotoxicity/acute liver failure in Cockayne syndrome
Adverse reactions
- Nausea, sometimes with headache
- Metallic taste
- Anorexia, vomiting
- Diarrhea, epigastric distress, cramping
- Peripheral neuropathy/paresthesia
- Dizziness
- Darkened urine
Pregnancy Crosses placenta; contraindicated first trimester for trichomoniasis; most studies show no increased anomaly risk
Lactation Undefined
NO BOXED WARNINGS
Principal riskDisulfiram-like reaction with alcohol; peripheral neuropathy with prolonged use
Cost, est. cash$5–$150/month
Generic entry1984
Classes and tags
Clinical profile
Legacy pregnancy categoryB
Defining liabilityAllergy, gastrointestinal effects and agent-specific renal, hepatic, hematologic or interaction risks.
Mechanism of action: Metronidazole ballicule
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