Metronidazole Dosing for Bacillary Diarrhea
Metronidazole is NOT recommended for bacillary diarrhea (bacterial dysentery caused by organisms like Shigella, Salmonella, or Campylobacter), as it lacks activity against these aerobic/facultative anaerobic pathogens and is not indicated for this condition. 1, 2, 3
Critical Distinction: What Metronidazole Actually Treats
Metronidazole is specifically active against:
- Anaerobic bacteria (like Bacteroides species and Clostridioides difficile)
- Protozoal infections (like Giardia lamblia and Entamoeba histolytica)
- It has no activity against typical bacillary diarrhea pathogens, which are aerobic or facultative anaerobic gram-negative rods 3, 4
If You Mean Clostridioides difficile Infection (CDI)
For non-severe CDI, use vancomycin 125 mg orally four times daily for 10 days or fidaxomicin 200 mg twice daily for 10 days as first-line therapy. 1, 2
- Metronidazole 500 mg orally three times daily for 10 days should only be used when vancomycin or fidaxomicin are unavailable due to access or cost barriers 1, 2
- Metronidazole is inferior to vancomycin, with cure rates of 84% versus 97% overall, and 76% versus 97% in severe disease 1, 2
- Never use metronidazole for severe CDI (defined as WBC ≥15,000 cells/mL or serum creatinine >1.5 mg/dL) 1, 5
Pediatric CDI Dosing
- If metronidazole must be used: 35-50 mg/kg/24 hours divided into three doses orally for 10 days 3
If You Mean Amebiasis (Entamoeba histolytica)
For acute intestinal amebiasis (amebic dysentery):
- Adults: 750 mg orally three times daily for 5-10 days 3
- Pediatric: 35-50 mg/kg/24 hours divided into three doses orally for 10 days 3
If You Mean Giardiasis (Giardia lamblia)
Tinidazole is the preferred first-line agent, NOT metronidazole. 1, 2
If metronidazole must be used:
- Adults: 250 mg orally three times daily for 5-7 days 1, 2
- Pediatric: 15 mg/kg/day divided into three doses for 5 days 2
Critical Safety Warnings
- Avoid repeated or prolonged courses beyond 14 days due to cumulative and potentially irreversible neurotoxicity 1, 2
- Never use for recurrent CDI—switch to vancomycin tapered/pulsed regimens instead 1
- Patients must avoid alcohol during treatment and for 24 hours after completion due to disulfiram-like reaction 2
Common Pitfall to Avoid
The term "bacillary diarrhea" traditionally refers to dysentery caused by aerobic bacteria (Shigella, Salmonella, Campylobacter), for which fluoroquinolones or azithromycin are appropriate, not metronidazole. 3 If you are treating diarrhea with metronidazole, you must first confirm the pathogen is either anaerobic (C. difficile) or protozoal (Giardia, Entamoeba). 1, 2, 3