What is the appropriate metronidazole dose for an 8‑year‑old boy weighing 19 kg with normal renal and hepatic function?

Metronidazole Dosing for an 8-Year-Old, 19 kg Boy

For this 8-year-old boy weighing 19 kg with normal renal and hepatic function, administer metronidazole 30–40 mg/kg/day divided every 8 hours, with each individual dose not exceeding 500 mg. 1

Practical Dose Calculation

Using the standard pediatric dosing range:

• Lower end (30 mg/kg/day): 30 mg/kg × 19 kg = 570 mg/day ÷ 3 doses = 190 mg every 8 hours
• Higher end (40 mg/kg/day): 40 mg/kg × 19 kg = 760 mg/day ÷ 3 doses = 253 mg every 8 hours

For most infections, start with 200–250 mg every 8 hours (approximately 32–40 mg/kg/day). 1 This dose is well below the 500 mg per-dose maximum and falls within the guideline-recommended range. 1

Condition-Specific Adjustments

The dose should be tailored to the specific infection:

Standard Anaerobic Infections (Intra-Abdominal, Soft Tissue)

• Use 30–40 mg/kg/day divided every 8 hours (200–250 mg per dose for this patient). 1
• Always combine with gram-negative/aerobic coverage (aminoglycoside, carbapenem, or advanced cephalosporin) for intra-abdominal infections—never use metronidazole as monotherapy. 1
• For severe infections with undrained abscesses, maximize the dose toward 40 mg/kg/day. 1

Clostridioides difficile Infection (CDI)

• Non-severe CDI: 7.5 mg/kg/dose three to four times daily = 142 mg every 8 hours or 107 mg every 6 hours (maximum 500 mg per dose). 1
• Important: Metronidazole should only be used for CDI when oral vancomycin or fidaxomicin are unavailable, as these are first-line agents. 1
• For fulminant CDI with ileus, use IV metronidazole combined with oral or rectal vancomycin. 1

Perianal Crohn's Disease

• Use 10–20 mg/kg/day in divided doses = 190–380 mg/day total (approximately 63–127 mg every 8 hours). 1

Route of Administration

• Oral administration is preferred when the patient can tolerate it. 1
• IV administration is indicated for:
  • Fulminant CDI with ileus
  • Severe necrotizing infections requiring rapid tissue levels
  • Inability to tolerate oral medication 1

Duration and Safety Monitoring

• Standard treatment duration is 10 days for most infections. 1
• Do not exceed 10 days without compelling indication due to markedly increased risk of cumulative, potentially irreversible neurotoxicity (peripheral neuropathy, seizures, encephalopathy). 1
• Monitor closely for neurologic symptoms if treatment extends beyond 10 days. 1

Critical Pitfalls to Avoid

1. Never exceed 500 mg per individual dose regardless of weight-based calculations. 1
2. Never use metronidazole as monotherapy for intra-abdominal infections—always pair with gram-negative coverage. 1
3. Never select metronidazole as first-line for CDI—prioritize vancomycin or fidaxomicin. 1
4. Never continue beyond 10 days without reassessing necessity due to neurotoxicity risk. 1
5. Recognize that CDI dosing (7.5 mg/kg/dose) is LOWER than standard anaerobic infection dosing (10–13 mg/kg/dose). 1

Supporting Pharmacokinetic Data

Research in pediatric appendicitis patients demonstrates that 30 mg/kg once-daily dosing achieves adequate AUC/MIC ratios for Bacteroides fragilis with MIC ≤2 mcg/mL in 96–100% of patients, though traditional every-8-hour dosing remains the guideline standard. 2 Historical studies confirm that 30–40 mg/kg/day divided dosing achieves therapeutic levels (peak 15–30 mcg/mL, trough 4–12 mcg/mL) with excellent safety profiles in children. 3