Metronidazole, a nitroimidazole antimicrobial, acts as a prodrug activated by anaerobic organisms and certain protozoa. Once absorbed, it penetrates microbial cells and undergoes reduction by bacterial nitroreductases, forming cytotoxic free radicals. These radicals disrupt DNA and RNA synthesis, causing strand breaks and cell death. It is bactericidal against most anaerobic bacteria (e.g., Bacteroides, Clostridia) and protozoa (e.g., Giardia, Entamoeba histolytica, Trichomonas vaginalis).

Metronidazole is effective for:

1. Anaerobic Infections:
   • Intra-abdominal sepsis, pelvic inflammatory disease, brain abscesses.
   • Clostridioides difficile colitis (oral/IV formulation).
2. Protozoal Infections:
   • Trichomoniasis, giardiasis, amoebic dysentery, and hepatic abscesses.
3. Bacterial Vaginosis: First-line therapy.
4. Dental Infections: Periodontitis, acute necrotizing gingivitis.
5. Surgical Prophylaxis: For colorectal/gynaecological surgeries.
6. Helicobacter pylori Eradication: Combined with other antibiotics.

Adults:

• Oral/IV: 400–500 mg every 8 hours (adjust based on infection severity).
  • C. difficile: 500 mg TID for 10–14 days.
  • Trichomoniasis: 2 g single dose or 400 mg BD for 7 days.
• Topical: Gel/cream for rosacea or bacterial vaginosis.

Children:

• Oral/IV: 7.5 mg/kg every 8 hours (max 12 g/day).

Special Populations:

• Hepatic Impairment: Reduce dose by 50% in severe liver disease.
• Renal Impairment: No adjustment for oral use; avoid IV in severe renal failure.

• Common: Nausea, metallic taste, headache, dry mouth.
• Serious:
  • Peripheral neuropathy (risk increases with prolonged use).
  • Seizures, encephalopathy (rare, high doses).
  • Pancreatitis, hepatitis, or leukopenia.
  • Disulfiram-like reaction with alcohol (flushing, vomiting, tachycardia).

1. Alcohol Avoidance: During treatment and ≥48 hours post-therapy.
2. Pregnancy & Lactation:
   • Avoid in first trimester; use in second/third trimester only if essential (BNF Category: Use with caution).
   • Excreted in breast milk; suspend breastfeeding during treatment.
3. Neurologic Disorders: Monitor for neuropathy or seizures.
4. Carcinogenicity: Rodent studies show tumor risk; human relevance unclear.

• Alcohol: Disulfiram-like reaction (violent nausea/vomiting).
• Warfarin: Potentiates anticoagulant effect (monitor INR).
• Lithium: Increased risk of toxicity (monitor levels).
• CYP450 Inhibitors (e.g., cimetidine): May elevate metronidazole levels.
• Phenytoin: Reduced phenytoin metabolism (risk of toxicity).

Generic & Brand Names

• Generic: Metronidazole.
• Brand Names: Flagyl (oral/IV), Rozex (topical), Zidoval (vaginal gel).