Sulfatrim (Trimethoprim/Sulfamethoxazole) Dosing Based on Weight for Pediatric Patients
The recommended dose of trimethoprim-sulfamethoxazole (TMP-SMX) for pediatric patients is 8 mg/kg/day of trimethoprim and 40 mg/kg/day of sulfamethoxazole, divided into two doses given every 12 hours. 1, 2
Standard Dosing Guidelines
For children 2 months of age or older, the FDA-approved dosing is:
| Weight | Dose (every 12 hours) |
|---|---|
| 10 kg | 1 regular-strength tablet (80 mg TMP/400 mg SMX) |
| 20 kg | 1 regular-strength tablet |
| 30 kg | 1½ regular-strength tablets |
| 40 kg | 2 regular-strength tablets or 1 double-strength tablet |
Weight-Based Calculation Method
When precise dosing is needed, calculate using:
- Trimethoprim: 8 mg/kg/day divided into 2 doses (4 mg/kg/dose q12h)
- Sulfamethoxazole: 40 mg/kg/day divided into 2 doses (20 mg/kg/dose q12h)
For example:
- A 15 kg child would receive 60 mg TMP and 300 mg SMX per dose, twice daily
- A 25 kg child would receive 100 mg TMP and 500 mg SMX per dose, twice daily
Special Situations and Adjustments
For Pneumocystis jirovecii Pneumonia (PJP)
- Treatment: 15-20 mg/kg/day of TMP and 75-100 mg/kg/day of SMX divided into doses given every 6 hours for 14-21 days 1, 2
- Prophylaxis: 150 mg/m²/day of TMP and 750 mg/m²/day of SMX given in equally divided doses twice daily, 3 consecutive days per week 1, 2
For Impaired Renal Function
- Creatinine clearance >30 mL/min: Standard regimen
- Creatinine clearance 15-30 mL/min: Half the usual regimen
- Creatinine clearance <15 mL/min: Not recommended 1, 2
Important Considerations
TMP-SMX is contraindicated in infants younger than 2 months due to risk of kernicterus 1, 2
Duration of therapy varies by indication:
Monitor for adverse effects, particularly in prolonged treatment:
- Complete blood counts should be monitored at initiation and monthly intervals 3
- Watch for skin reactions, especially in the first 8 weeks of therapy
Clinical Pearls
- The oral suspension formulation (40 mg TMP/200 mg SMX per 5 mL) allows for more precise dosing in smaller children
- Population pharmacokinetic studies suggest that the standard 8/40 mg/kg/day dosing achieves adequate exposure for organisms with MICs ≤0.5 mg/L 4
- For resistant organisms with higher MICs (up to 1 mg/L), higher doses of 12/60 mg/kg/day or 15/75 mg/kg/day may be needed 4
- When treating MRSA skin infections, ensure adequate dosing as TMP-SMX is a primary oral option 5, 3
By following these weight-based dosing guidelines, you can provide appropriate TMP-SMX therapy while minimizing the risk of adverse effects and treatment failure.