Trimethoprim-Sulfamethoxazole Dosing in Children
For most pediatric infections, use 8-12 mg/kg/day of trimethoprim (40-60 mg/kg/day of sulfamethoxazole) divided into two doses every 12 hours.
Standard Dosing by Indication
Urinary Tract Infections and Acute Otitis Media
- Dose: 8 mg/kg/day TMP with 40 mg/kg/day SMX, divided every 12 hours for 10 days 1
- Maximum daily dose: 320 mg TMP with 1600 mg SMX
- Not recommended for infants <2 months of age 1
Skin and Soft Tissue Infections (MRSA)
- Dose: 8-12 mg/kg/day TMP (based on TMP component) in 2-4 divided doses 2, 3
- This dosing applies to both oral and IV administration
- Duration typically 7 days depending on clinical response
Pneumocystis jirovecii Pneumonia (PCP)
Treatment:
- Dose: 15-20 mg/kg/day TMP with 75-100 mg/kg/day SMX, divided into 3-4 doses every 6 hours for 14-21 days 4
- Infuse IV doses over 1 hour
- Can switch to oral after acute pneumonitis resolves if no malabsorption or diarrhea 4
Prophylaxis:
- Dose: 150 mg/m²/day TMP with 750 mg/m²/day SMX, divided into 2 doses, given 3 days per week on consecutive days 5, 6
- Alternative schedule: Same daily dose given as single dose 3 times weekly, or divided twice daily, or divided twice daily 3 times weekly on alternate days 6
- Maximum daily dose: 320 mg TMP with 1600 mg SMX 5
Age-Specific Considerations
Neonates (<3 days old)
- Loading dose: 10 mg/kg SMX with 3 mg/kg TMP
- Maintenance: 3 mg/kg SMX with 1 mg/kg TMP twice daily 7
- Half-life is significantly prolonged (16.5 hours for SMX, 19 hours for TMP) compared to older children
Infants and Young Children
Recent pharmacokinetic data demonstrates that 8 mg/kg/day TMP with 40 mg/kg/day SMX divided every 12 hours achieves adult-equivalent exposure and meets pharmacodynamic targets for organisms with MIC ≤0.5 mg/L in >90% of children 8. For organisms with MIC up to 1 mg/L, higher doses may be needed: 12 mg/kg/day TMP (60 mg/kg/day SMX) for children 6-21 years, or 15 mg/kg/day TMP (75 mg/kg/day SMX) for children <6 years 8.
Renal Impairment Dosing
Creatinine clearance is the primary determinant of TMP-SMX clearance 9:
- CrCl >30 mL/min: Standard dosing
- CrCl 15-30 mL/min: 50% of standard dose
- CrCl <15 mL/min: Not recommended by FDA label 1, though recent data suggests 50 mg/kg/day in 3 divided doses may be feasible 9
Critical Dosing Pitfalls
Avoid excessive dosing: Historical high-dose regimens (>10 mg/kg/day TMP) for non-PCP infections are associated with increased adverse events without improved efficacy 10, 11. A recent study showed 66.7% of patients on high-dose therapy required dose adjustment or discontinuation compared to only 24-26% on lower doses 10.
Monitor for adverse effects: Approximately 15% of HIV-infected children experience substantial adverse reactions, lower than adults 4. Common reactions include rash, neutropenia, thrombocytopenia, and GI complaints. For mild rash, temporarily discontinue and restart when resolved; permanently discontinue for Stevens-Johnson syndrome or anaphylaxis 5, 4.
Body surface area vs. weight-based dosing: For PCP prophylaxis, BSA-based dosing (150/750 mg/m²/day) is preferred over weight-based dosing to minimize toxicity while maintaining efficacy 5, 6.