India · Paediatric mg/kg & Adult · Amoeba · Giardia · Anaerobic · H. pylori · Dental · Flagyl · Metrogyl · Metronid
✓ Entamoeba histolytica (amoeba)
✓ Giardia lamblia
✓ Trichomonas vaginalis
✓ Anaerobes (Bacteroides, Clostridium, Fusobacterium)
✓ H. pylori (in triple therapy)
✓ C. difficile (mild-moderate)
✗ Aerobic bacteria (no activity)
✗ Microaerophilic streptococci (limited)
| Indication | Paediatric dose | Adult dose | Duration |
|---|---|---|---|
| Amoebic dysentery | 35–50 mg/kg/day in 3 doses (max 800mg/dose) | 800mg TDS (3×/day) | 5–10 days; then luminal agent |
| Amoebic liver abscess | 35–50 mg/kg/day in 3 doses | 800mg TDS or 400mg TDS | 10 days minimum |
| Giardiasis | 15 mg/kg/day in 3 doses (or 2g single dose) | 2g single dose OR 400mg TDS × 5 days | 5 days or single dose |
| Anaerobic / intra-abdominal | 7.5 mg/kg/dose TDS | 400–500mg TDS | 7–10 days |
| Dental abscess | — | 400mg TDS × 3–7 days | 3–7 days (+ amoxicillin) |
| H. pylori (triple therapy) | — | 400mg BD (with PPI + clarithromycin) | 7–14 days |
| Bacterial vaginosis | — | 400mg BD × 5–7 days or 2g single dose | 5–7 days or single dose |
| C. difficile (mild) | — | 400mg TDS × 10–14 days (oral vancomycin preferred) | 10–14 days |
| IV — anaerobic / pelvic | 7.5 mg/kg IV (max 500mg) every 8h | 500mg IV every 8h | 7–10 days or until oral tolerated |
| Surgical prophylaxis (IV) | 15 mg/kg single IV dose | 500mg IV single dose | 30–60 min pre-op (single dose) |
Metronidazole (Flagyl, Metrogyl) is one of the most important and versatile antimicrobials in Indian clinical practice. It has activity against anaerobic bacteria, protozoa (Entamoeba histolytica, Giardia lamblia, Trichomonas vaginalis), and H. pylori. Amoebiasis — both intestinal and hepatic — is endemic in India and metronidazole is the cornerstone of treatment. Oral bioavailability approaches 100%, meaning IV therapy offers little advantage over oral in patients who can swallow — reserve IV metronidazole for patients who cannot take oral medications.
Metronidazole kills the invasive trophozoite form of Entamoeba histolytica and resolves acute dysentery and liver abscess, but it has poor activity against the intestinal cyst form. Without a subsequent luminal agent, intestinal cysts persist and cause relapse or continued transmission. After completing metronidazole for intestinal amoebiasis, always prescribe a luminal agent: diloxanide furoate 500mg TDS × 10 days (adults) or 20 mg/kg/day × 10 days (children), or paromomycin 25–35 mg/kg/day × 7 days. Skipping the luminal stage is the most common reason for recurrent amoebiasis in Indian patients.
Metronidazole inhibits aldehyde dehydrogenase, the enzyme responsible for the second step of alcohol metabolism (acetaldehyde → acetate). When alcohol is consumed during metronidazole treatment, acetaldehyde accumulates, causing flushing, nausea, vomiting, severe headache, sweating, and tachycardia. This reaction can be severe and frightening. Counsel every patient at the time of prescription — avoid all alcohol during treatment and for 48 hours after the last dose. This interaction applies to oral, IV, and topical/vaginal formulations.
Most amoebic liver abscesses respond to metronidazole alone without drainage. Indications for aspiration in addition to metronidazole: large abscess (>10cm), left lobe abscess (risk of rupture into pericardium), failure to respond to 72 hours of medical treatment, imminent rupture, or diagnostic uncertainty. Aspiration fluid in amoebic abscess characteristically appears "anchovy sauce" coloured — reddish-brown — and lacks bacteria on Gram stain.