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

Metronidazole Dosing in Pediatric Patients

For most pediatric infections requiring metronidazole, use 30-40 mg/kg/day divided every 8 hours (maximum 500 mg per dose), with dosing adjustments based on the specific clinical scenario. 1

Standard Dosing by Clinical Indication

General Anaerobic Infections

• Administer 30-40 mg/kg/day divided every 8 hours, with a maximum of 500 mg per dose 1
• The FDA-approved pediatric dosing for amebiasis is 35-50 mg/kg/24 hours divided into three doses for 10 days 2
• For serious anaerobic infections, the adult-equivalent oral dosing is 7.5 mg/kg every 6 hours (approximately 500 mg for a 70 kg adult) 2
• Standard treatment duration is 7-10 days for most infections, though bone/joint, lower respiratory tract, and endocardial infections may require longer courses 2

Intra-Abdominal Infections

• Use 30-40 mg/kg/day divided every 8 hours as part of combination therapy 1
• Combine with aminoglycosides, carbapenems, or advanced-generation cephalosporins for complicated infections 1
• For mixed necrotizing infections requiring anaerobic coverage, use 7.5 mg/kg/dose every 6 hours IV, typically combined with cefotaxime (50 mg/kg/dose every 6 hours) 1

Clostridioides difficile Infection (CDI)

Non-Severe CDI:

• Administer 7.5 mg/kg/dose three or four times daily (maximum 500 mg per dose) for 10 days 3, 4
• This dosing is LOWER than for other anaerobic infections (10-13 mg/kg/dose) 1
• Oral vancomycin (10 mg/kg/dose four times daily, maximum 125 mg per dose) is an alternative 4

Severe or Fulminant CDI:

• Oral vancomycin is strongly recommended over metronidazole (strong recommendation, moderate quality evidence) 3
• If metronidazole must be used in severe infections, administer 10 mg/kg/dose three times daily IV (maximum 500 mg per dose) 3

Recurrent CDI:

• For first recurrence after metronidazole: use oral vancomycin 10 mg/kg/dose four times daily (maximum 125 mg per dose) for 10 days 4
• For second or subsequent recurrences: oral vancomycin is recommended over metronidazole (weak recommendation, low quality evidence) 3
• Avoid repeated or prolonged metronidazole courses due to peripheral neuropathy risk 4

Perianal Fistulizing Crohn's Disease

• Use 10-20 mg/kg/day in divided doses 5, 1
• Ciprofloxacin 20 mg/kg/day is an alternative or adjunctive option 5
• These antibiotics offer good short-term response and may bridge to immunosuppressive medications 5

Age-Specific Dosing Considerations

Neonates

• Dosing varies significantly by postnatal age and weight 5
• Postnatal age ≤7 days and ≤2000 g: 7.5 mg/kg every 12 hours 5
• Postnatal age ≤7 days and >2000 g: 7.5-10 mg/kg every 12 hours 5
• Postnatal age >7 days and <1200 g: 7.5-10 mg/kg every 8-12 hours 5
• Postnatal age >7 days and >2000 g: 10 mg/kg every 8 hours 5

Infants and Children

• Standard dosing: 15-22.5 mg/kg/day divided every 8 hours 5
• Some patients may require up to 30 mg/kg/day divided every 8 hours based on serum levels (e.g., cystic fibrosis or febrile neutropenic patients) 5

Alternative Dosing Strategies

Once-Daily Dosing for Appendicitis

• Recent pharmacokinetic data supports 30 mg/kg per dose once daily for pediatric appendicitis 6
• This achieves AUC/MIC ratio ≥70 for Bacteroides fragilis with MIC ≤2 mcg/mL in 96-100% of patients 6
• Target attainment decreases for higher MICs (61-97% for MIC 4 mcg/mL; 9-71% for MIC 8 mcg/mL) 6
• This dosing strategy is based on metronidazole's long half-life, concentration-dependent killing, and 3-hour postantibiotic effect 6

Special Populations and Considerations

Malnourished Children

• Severe malnutrition requires a 60% dose reduction to 12 mg/kg/day to avoid drug accumulation 7
• Biotransformation is significantly impaired in malnourished children, leading to elevated plasma concentrations 7
• This dosing maintains therapeutic plasma concentrations of 6.0 mcg/mL 7

Hepatic Impairment

• Patients with severe hepatic disease metabolize metronidazole slowly, requiring cautious dose reduction 2
• Close monitoring of plasma metronidazole levels and toxicity is recommended 2

Renal Impairment

• Do not specifically reduce doses in anuric patients since accumulated metabolites are rapidly removed by dialysis 2

Route of Administration

• Intravenous metronidazole is usually administered initially for most serious anaerobic infections 2
• Transition to oral therapy is appropriate once clinical improvement occurs 8
• In one study, patients received initial parenteral therapy for 5-21 days (average 11.6 days) before transitioning to oral therapy 8

Common Pitfalls and Caveats

• Do not use the same dosing for CDI as for other anaerobic infections—CDI requires lower doses (7.5 mg/kg/dose vs. 10-13 mg/kg/dose) 1
• Avoid metronidazole in first-trimester pregnancy (contraindicated) 2
• Do not exceed 4 g in 24 hours regardless of indication 2
• Monitor for peripheral neuropathy with prolonged or repeated courses 4
• Therapeutic drug monitoring may be necessary in elderly patients, those with hepatic impairment, or malnourished children 2, 7
• Peak concentrations on day 3 of therapy typically range from 15.2-30 mcg/mL, with trough levels of 4-11.6 mcg/mL 8