What is the recommended dose of metronidazole (Flagyl) for pediatric patients?

Metronidazole Pediatric Dosing

For most pediatric infections requiring metronidazole, use 30-40 mg/kg/day divided every 8 hours (maximum 500 mg per dose), as recommended by the Infectious Diseases Society of America 1.

Standard Dosing by Clinical Indication

General Anaerobic Infections

• Administer 30-40 mg/kg/day divided into three doses (every 8 hours), with a maximum of 500 mg per dose 1
• For severe anaerobic infections, the FDA-approved adult dosing translates to 7.5 mg/kg every 6 hours (approximately 30 mg/kg/day), not to exceed 4 grams in 24 hours 2
• Standard treatment duration is 7-10 days for most infections, though bone, joint, lower respiratory tract, and endocardial infections may require longer courses 1, 2

Clostridium Difficile Infection (CDI)

Critical distinction: CDI requires LOWER dosing than other anaerobic infections 3

• Non-severe CDI: 7.5 mg/kg/dose three or four times daily (maximum 500 mg per dose) for 10 days 1, 3
• Severe CDI requiring IV therapy: 10 mg/kg/dose three times daily IV (maximum 500 mg per dose) 1
• For severe or fulminant CDI, oral vancomycin is preferred over metronidazole 3
• For second or subsequent recurrences, oral vancomycin is recommended over metronidazole 3

Intra-Abdominal Infections

• Use 30-40 mg/kg/day divided every 8 hours as part of combination therapy with aminoglycosides, carbapenems, or advanced-generation cephalosporins 1

Mixed Necrotizing Infections

• Administer 7.5 mg/kg/dose every 6 hours IV, typically combined with cefotaxime (50 mg/kg/dose every 6 hours) or other broad-spectrum agents 1

Amebiasis

• FDA-approved dosing: 35-50 mg/kg/24 hours divided into three doses orally for 10 days 2

Crohn's Disease (Perianal Disease)

• Use 10-20 mg/kg/day in divided doses 1

Route of Administration Considerations

• Oral dosing: 40-50 mg/kg/day for adequate bioavailability 4
• IV dosing: 30 mg/kg/day provides equivalent exposure 4
• Transition from IV to oral therapy is appropriate once clinical improvement occurs, typically after 5-21 days of parenteral therapy 4

Important Clinical Caveats

Age-Specific Considerations

• Children under 12 years: Use weight-based dosing of 20-30 mg/kg/day 5
• Children 12 years and older: Can receive adult dosing of 200 mg to maximum 2000 mg per day 5

Special Populations

• Malnourished children: Reduce dose by 60% (to approximately 12 mg/kg/day) due to impaired drug clearance and risk of accumulation 6
• Severe hepatic disease: Administer doses below usual recommendations with close monitoring of plasma levels and toxicity 2
• Anuric patients: No specific dose reduction needed as metabolites are rapidly removed by dialysis 2

Pharmacokinetic Evidence

Recent PK/PD data demonstrates that once-daily dosing (30 mg/kg) achieves adequate AUC/MIC ratios (≥70) for Bacteroides fragilis with MIC ≤2 mcg/mL in 96-100% of pediatric appendicitis patients 7. However, this approach is not yet widely adopted in guidelines, and the traditional every-8-hour dosing remains standard practice 1, 3.

Common Pitfalls

• Do not confuse CDI dosing (7.5 mg/kg/dose) with general anaerobic infection dosing (10-13 mg/kg/dose) 1
• Maximum single dose is 500 mg regardless of total daily dose calculation 1, 3, 2
• Avoid first-trimester use in pregnancy; if treatment is necessary, avoid single high-dose regimens that result in higher fetal exposure 2