What is the recommended dose of metronidazole (Flagyl) oral suspension for a 20kg child?

Metronidazole Dosing for a 20kg Child

For a 20kg child requiring metronidazole oral suspension, the recommended dose is 7.5 mg/kg per dose given 3-4 times daily (maximum 500 mg per dose), which translates to 150 mg per dose, 3-4 times daily for most indications. 1, 2

Standard Pediatric Dosing by Indication

For Crohn's Disease with Perianal Fistulizing Disease

• The usual daily dose is 10-20 mg/kg/day, which for a 20kg child equals 200-400 mg daily, typically divided into 2-3 doses 1
• This provides a bridge to immunosuppressive medications and has good short-term response in perianal disease 1

For Clostridium difficile Infection (Non-Severe)

• 7.5 mg/kg per dose, 3-4 times daily (oral), with a maximum of 500 mg per dose 1, 2
• For a 20kg child: 150 mg per dose, 3-4 times daily for 10 days 1
• Metronidazole is recommended only when access to vancomycin or fidaxomicin is limited for initial episodes of non-severe CDI 1
• Avoid repeated or prolonged courses due to risk of cumulative and potentially irreversible neurotoxicity 1

For Severe/Fulminant C. difficile Infection

• Vancomycin oral (10 mg/kg/dose, 4 times daily, maximum 500 mg) is strongly preferred over metronidazole 1, 2
• Intravenous metronidazole (10 mg/kg/dose, 3 times daily, maximum 500 mg) may be added as adjunctive therapy with oral vancomycin in critically ill patients 1

For Anaerobic Infections (General)

• 35-50 mg/kg/24 hours, divided into three doses, orally for 10 days for amebiasis 3
• For serious anaerobic infections, the usual pediatric oral dosage is 7.5 mg/kg every 6 hours (approximately 150 mg every 6 hours for a 20kg child) 3
• Maximum daily dose should not exceed 4 grams in 24 hours 3

Practical Dosing Calculation for 20kg Child

Most Common Regimen (Non-Severe Infections):

• 150 mg three times daily (7.5 mg/kg × 20kg = 150 mg per dose) 1, 2
• Alternative: 150 mg four times daily for more severe anaerobic infections 3

For Crohn's Disease/Perianal Fistulas:

• 200-400 mg daily divided into 2-3 doses (10-20 mg/kg/day) 1
• Typically given as 100-200 mg twice daily or 65-135 mg three times daily

Important Clinical Considerations

Treatment Duration

• Standard duration is 10 days for most anaerobic infections and CDI 1, 3
• Bone and joint infections, lower respiratory tract infections, and endocardium infections may require longer treatment 3

Monitoring and Safety

• Avoid repeated courses without a 4-6 week interval between treatments due to neurotoxicity risk 1, 3
• Total and differential leukocyte counts should be obtained before and after retreatment 3
• Monitor for signs of peripheral neuropathy, especially with prolonged use 1

When to Choose Alternative Therapy

• For severe or fulminant CDI, vancomycin is strongly preferred over metronidazole 1, 2
• For initial CDI episodes, vancomycin or fidaxomicin are now preferred over metronidazole when available 1
• Metronidazole should only be used for non-severe CDI when access to preferred agents is limited 1

Common Pitfalls to Avoid

• Do not use metronidazole as first-line for severe CDI—this is a critical error that can worsen outcomes 1
• Do not exceed maximum single dose of 500 mg even if weight-based calculation suggests higher dose 1, 2, 3
• Do not prescribe repeated courses without appropriate interval and laboratory monitoring 3