What is the recommended dose of metronidazole (Flagyl) per kilogram of body weight?

Metronidazole Dosing by Body Weight

For pediatric patients, the recommended dose of metronidazole is 7.5 mg/kg/dose administered three to four times daily (tid or qid), with a maximum dose of 500 mg per dose. 1

Dosing Guidelines by Indication

Clostridium difficile Infection (CDI)

• Initial episode, non-severe: 7.5 mg/kg/dose tid or qid for 10 days (maximum 500 mg per dose) 1
• Initial episode, severe/fulminant: For severe CDI, oral vancomycin is preferred over metronidazole
• IV dosing for severe/fulminant cases: 10 mg/kg/dose tid (maximum 500 mg per dose) when used in combination with oral vancomycin 1

Alternative Dosing Regimens

Recent pharmacokinetic studies suggest once-daily dosing may be effective in certain situations:

• For appendicitis: 30 mg/kg once daily has shown adequate coverage against Bacteroides fragilis with MIC ≤2 mcg/mL 2
• This dosing achieved similar area under the curve (AUC) to adults receiving 1000-1500 mg once daily or 500 mg every 8 hours 2

Special Populations

Malnourished Children

• Reduced dosing recommended: 12 mg/kg/day (60% reduction from standard dose)
• Metronidazole clearance is significantly decreased in severe malnutrition, leading to potential drug accumulation 3

Administration Considerations

• Oral or intravenous administration is acceptable depending on clinical situation
• Duration of therapy typically 10 days for most infections 1
• For recurrent C. difficile infections, vancomycin is preferred over a second course of metronidazole 1

Monitoring and Precautions

• Total and differential leukocyte counts should be monitored before and after therapy, especially if a second course is necessary 4
• Safety and effectiveness in pediatric patients have not been established by the FDA except for the treatment of amebiasis 4
• Drug interactions: Metronidazole potentiates warfarin, interacts with alcohol, and has altered clearance when co-administered with drugs affecting liver enzymes 4

Common Pitfalls to Avoid

1. Underdosing in normal weight children: Ensure proper weight-based calculation at 7.5 mg/kg/dose
2. Overdosing in malnourished children: Consider reduced dosing (12 mg/kg/day) in severely malnourished patients 3
3. Continuing metronidazole for recurrent C. difficile: Guidelines recommend switching to vancomycin for recurrent episodes 1
4. Ignoring drug interactions: Be aware of interactions with warfarin, alcohol, disulfiram, and other medications 4

While some studies suggest the possibility of less frequent dosing regimens, the most established evidence-based recommendation remains 7.5 mg/kg/dose three to four times daily for most pediatric indications, with appropriate adjustments for specific clinical scenarios.