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# Metrogyl (metronidazole)
## Overview
Metronidazole is a nitroimidazole antibiotic with bactericidal activity against anaerobic bacteria and antiprotozoal activity against certain parasites. It is available in oral, intravenous, topical, and vaginal formulations.
## Primary Indications
- Anaerobic bacterial infections (intra-abdominal, gynecologic, bone/joint, skin, CNS abscess)
- Bacterial vaginosis
- *Clostridioides difficile* infection (CDI)
- Protozoal infections: trichomoniasis, amebiasis, giardiasis
- Perioperative prophylaxis (colorectal, gynecologic surgery)
- Rosacea (topical)
## Adult Dosing
*Exact dosing depends on local protocol, infection severity, and renal/hepatic function.*
**Anaerobic infections (IV/oral)**
- Loading dose: 15 mg/kg IV; then 7.5 mg/kg IV/oral every 6 hours (max 4 g/day for severe infections)
- Alternative: 500 mg IV/oral every 8 hours
**C. difficile infection**
- 500 mg oral every 8 hours for 10–14 days (mild-moderate) — now less preferred; vancomycin/fidaxomicin often recommended first line per guidelines
**Bacterial vaginosis**
- 500 mg oral twice daily for 7 days; or 2 g single dose (oral or vaginal gel available)
**Trichomoniasis**
- 2 g oral single dose; or 500 mg twice daily for 7 days
**Amebiasis**
- 750 mg oral three times daily for 5–10 days (followed by luminal amebicide)
## Pediatric Dosing
*Dose depends on indication, weight, and formulation. Verify with local protocols.*
**Anaerobic infections (IV/oral)**
- Children: 7.5 mg/kg/dose every 6 hours (max 4 g/day)
- Neonates (postmenstrual age dependent): often 7.5–15 mg/kg/dose every 12–48 hours
**C. difficile**
- 7.5 mg/kg/dose oral every 6–8 hours (max 2 g/day) for 10–14 days — not first line in infants
**Amebiasis**
- 35–50 mg/kg/day divided every 8 hours × 7–10 days (max 2 g/day)
**Giardiasis**
- 5 mg/kg/dose oral three times daily × 5–7 days; or 15 mg/kg/day divided every 8 hours × 5–7 days
## Dose Adjustments
- **Hepatic impairment:** Severe (Child-Pugh C): reduce dose by 50% (e.g., to 7.5 mg/kg every 8 hours) or extend interval; avoid in acute liver disease
- **Renal impairment:** No adjustment needed for intermittent dosing; for continuous therapy, may consider interval extension (every 12 hours) if GFR <10 mL/min; metabolites accumulate but dose reduction usually not required
- **Hemodialysis:** Give after dialysis; monitor for accumulation with prolonged use
## Contraindications
- Hypersensitivity to metronidazole or any nitroimidazole
- First trimester of pregnancy (controversial; avoid unless essential; use caution throughout pregnancy)
- Active CNS disease (e.g., epilepsy, active brain lesion) due to risk of seizures/neuropathy
## Adverse Effects
- **Common:** Metallic taste, nausea, headache, dry mouth, darkening of urine (harmless)
- **Serious:** Peripheral neuropathy (long-term/high doses), seizures, encephalopathy, aseptic meningitis, pancreatitis, leukopenia, QT prolongation (IV formulation with excipient)
- **Other:** Disulfiram-like reaction with alcohol (nausea, flushing, vomiting) — avoid alcohol during therapy and for 48 hours after last dose
## Key Drug Interactions
- **Warfarin:** Potentiates anticoagulation (INR increase) — monitor closely
- **Disulfiram:** Psychotic reactions (avoid concurrent use)
- **Lithium:** May increase lithium levels (toxicity risk)
- **Phenytoin/phenobarbital:** May decrease metronidazole levels
- **Cimetidine:** May increase metronidazole levels
- **Cholestyramine:** Reduces absorption of oral metronidazole (space doses)
- Alcohol (including in preparations): disulfiram-like reaction
## Monitoring
- Baseline and periodic CBC (leukopenia risk)
- Liver function tests with prolonged therapy
- Neurologic exam for signs of peripheral neuropathy or CNS toxicity (discontinue if abnormal)
- INR if on warfarin
- Therapeutic drug monitoring not routinely needed; levels rarely used for severe hepatic impairment or toxicity
## Clinical Pearls
- Metronidazole penetrates well into all tissues including CNS and abscess cavities
- IV formulation contains sodium; use with caution in heart failure/hypertension
- For C. difficile in children, oral vancomycin is now first line over metronidazole per IDSA guidelines
- Topical metronidazole (gel/cream) is first line for rosacea, but not for infections
- Avoid prolonged courses (>4 weeks) due to neuropathy risk; if >2 weeks needed, monitor neurologic function weekly
**Disclaimer:** This information is for educational purposes and does not substitute for professional judgment. Always verify dosing with current local protocols, official prescribing information, and patient-specific factors.
