## Treatment of Invasive Amebic Colitis **Key Point:** Metronidazole is the first-line drug for invasive amebiasis (amebic colitis and liver abscess) because it achieves high tissue penetration and effectively kills trophozoites in the intestinal wall and extraintestinal sites. ### Mechanism of Action Metronidazole is a nitroimidazole that generates toxic free radicals in anaerobic organisms, causing DNA damage and cell death. It is highly effective against trophozoites in tissue but has poor activity against cysts. ### Two-Drug Regimen for Complete Cure | Stage | Drug | Dose | Duration | Purpose | |-------|------|------|----------|----------| | **Tissue invasion** | Metronidazole | 750 mg TDS | 7–10 days | Kill trophozoites in intestinal wall and liver | | **Luminal cysts** | Paromomycin OR Iodoquinol | 25–35 mg/kg/day OR 650 mg TDS | 7 days | Eliminate cysts from intestinal lumen | **Clinical Pearl:** Metronidazole alone is insufficient for cure because it does not reliably eliminate cysts from the bowel lumen. A second luminal agent (paromomycin or iodoquinol) must always follow to prevent relapse and transmission. **High-Yield:** The classic regimen is **metronidazole (tissue) + paromomycin (lumen)**. This combination ensures both eradication of invasive trophozoites and prevention of chronic carriage. ### Why Metronidazole First? - Rapid tissue penetration (serum and liver abscess levels are therapeutic) - Kills trophozoites in the intestinal wall within 24–48 hours - Prevents progression to toxic megacolon and perforation - Standard of care in all major guidelines (WHO, CDC, Harrison's) [cite:KD Tripathi 8e Ch 51]
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