Metronidazole Dosing for a 9-Year-Old Patient
For a 9-year-old child with average weight (approximately 28-32 kg), metronidazole should be dosed at 20-30 mg/kg/day divided into 2-3 doses, with a typical regimen of 500 mg every 8 hours (or 7.5 mg/kg every 8 hours) for most infections. 1
Standard Pediatric Dosing Algorithm
For children under 12 years of age:
- Weight-based dosing: 20-30 mg/kg/day is the recommended range 1
- Practical calculation: For a 30 kg child, this translates to 600-900 mg total daily dose
- Divided dosing: Typically administered every 8 hours (three times daily) or every 12 hours (twice daily)
For children 12 years and older:
- Adult dosing applies: 200 mg to a maximum of 2000 mg per day 1
Specific Dosing Recommendations by Clinical Context
Mild to Moderate Infections
- Standard dose: 7.5 mg/kg every 8 hours (approximately 500 mg three times daily for a 30 kg child)
- This achieves therapeutic concentrations well above minimum inhibitory concentrations for susceptible anaerobic organisms 2
Severe Infections or Anaerobic Coverage
- Higher dose: Up to 30 mg/kg/day may be used, divided into 2-3 doses 1
- Maximum daily dose should not exceed 2000 mg in children under 12 years 1
Pharmacokinetic Considerations in Children
Age-related metabolism:
- Children older than 4 years have pharmacokinetic parameters similar to adults, with normal clearance and elimination 3
- A 9-year-old would be expected to have adult-like drug handling 3
- The elimination half-life in children over 4 years averages 8.7 hours, supporting twice or three times daily dosing 3, 2
Bioavailability and distribution:
- Oral bioavailability approaches 100%, making oral and intravenous dosing equivalent 3
- Metronidazole has excellent tissue penetration, including into the central nervous system 3
- Minimal protein binding allows for effective tissue distribution 2
Renal Impairment Adjustments
Important caveat: While metronidazole itself requires minimal renal dose adjustment (only 10% renal clearance), the metabolites accumulate in renal failure 3, 2
- Mild to moderate renal impairment: No dose adjustment typically needed for the parent drug 3
- Severe renal impairment or dialysis: Consider reducing dose by 50% or extending dosing interval to every 12 hours instead of every 8 hours 3
- Hemodialysis: Metronidazole is substantially removed; supplemental dosing after dialysis may be needed 3
Practical Dosing Example
For a 9-year-old weighing 30 kg:
- Standard regimen: 500 mg (approximately 7.5 mg/kg) every 8 hours
- Alternative regimen: 250 mg every 6 hours for more consistent levels
- Once-daily option: Recent evidence supports 30 mg/kg once daily (900 mg) for certain infections like appendicitis, achieving adequate AUC/MIC ratios 4
Critical Monitoring Points
Therapeutic drug monitoring is generally not required, but clinical response should be evident within 48-72 hours of initiating therapy. Metronidazole achieves peak serum levels of 11.5 mcg/ml after a 500 mg dose, well above the MIC for most anaerobes including Bacteroides fragilis (typically <6 mcg/ml) 2
Common pitfalls to avoid:
- Underdosing in children by using adult fixed doses rather than weight-based calculations
- Failing to account for malnutrition, which significantly reduces clearance and may require 60% dose reduction (12 mg/kg/day instead of 30 mg/kg/day) 5
- Overlooking the disulfiram-like reaction with alcohol exposure
- Not adjusting for severe hepatic dysfunction, which impairs metabolism 3