Bactrim Dosing for 106 lb 12-Year-Old
For a 12-year-old weighing 106 lb (48 kg), the appropriate Bactrim dose depends on the indication, but for most common infections (UTI, otitis media, shigellosis), give 1.5 double-strength tablets (or 3 regular-strength tablets) every 12 hours.
Calculation Breakdown
The child weighs 48 kg. Standard pediatric dosing for most infections is based on 8 mg/kg/day of trimethoprim component divided into two doses 1.
- Daily trimethoprim requirement: 48 kg × 8 mg/kg = 384 mg/day
- Per-dose trimethoprim: 384 mg ÷ 2 = 192 mg every 12 hours
Since each double-strength tablet contains 160 mg trimethoprim (and 800 mg sulfamethoxazole), this child needs approximately 1.2 double-strength tablets per dose. Rounding to practical dosing: 1.5 double-strength tablets every 12 hours provides 240 mg trimethoprim per dose (480 mg/day total), which falls within the acceptable range 1.
Indication-Specific Dosing
For UTI, Otitis Media, or Shigellosis
- 1.5 DS tablets (or 3 regular tablets) every 12 hours for 10-14 days (UTI/otitis) or 5 days (shigellosis) 1
- This delivers 40 mg/kg/day sulfamethoxazole and 8 mg/kg/day trimethoprim 1
For Pneumocystis jirovecii Pneumonia (Treatment)
If treating documented PCP, the dose is substantially higher:
- Treatment dose: 75-100 mg/kg/day sulfamethoxazole (15-20 mg/kg/day trimethoprim) divided every 6 hours 1
- For 48 kg: This equals 3 DS tablets every 6 hours (upper limit) or 2.25 DS tablets every 6 hours (lower limit) for 14-21 days 1
For PCP Prophylaxis
- 750 mg/m²/day sulfamethoxazole with 150 mg/m²/day trimethoprim divided twice daily 2, 1
- For a 12-year-old with typical body surface area (~1.4 m²): approximately 1 DS tablet daily 1
- Alternative: 1 DS tablet 3 times weekly on consecutive days 2, 3
Important Clinical Considerations
Age-appropriate formulation: At 12 years old and 48 kg, this child can take adult-strength tablets. The FDA label specifically states Bactrim is contraindicated in children under 2 months but provides clear dosing for older children 1.
Renal function matters: If creatinine clearance is 15-30 mL/min, reduce dose by 50%; if <15 mL/min, avoid use 1. Standard dosing assumes normal renal function.
Common pitfall: Do not confuse treatment dosing with prophylaxis dosing—PCP treatment requires 10-fold higher doses than prophylaxis 2, 1. The most common error is underdosing serious infections like PCP.
Monitoring: For prolonged therapy (>14 days) or high-dose regimens, monitor CBC monthly to assess for hematologic toxicity (neutropenia, thrombocytopenia) 4, 3.
Recent safety concern: Be aware of FDA warnings regarding acute respiratory failure risk in adolescents and young adults, though absolute risk remains very low (0.03%) 5. This should not preclude appropriate use but warrants informed discussion.
Practical administration: The 1.5 DS tablet dose can be given as three single-strength tablets if double-strength tablets cannot be split accurately. Both formulations contain the same 1:5 trimethoprim:sulfamethoxazole ratio 1.