What is the weight‑based single dose of trimethoprim‑sulfamethoxazole (Bactrim) for a child?

Weight-Based Single Dose of Trimethoprim-Sulfamethoxazole for Children

For a single dose in children, trimethoprim-sulfamethoxazole is dosed at 8 mg/kg of the trimethoprim component (equivalent to 40 mg/kg of sulfamethoxazole). 1

Standard Pediatric Dosing Framework

The FDA-approved dosing for children over 2 months of age is 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, divided into two doses given every 12 hours—meaning each individual dose is 4 mg/kg trimethoprim and 20 mg/kg sulfamethoxazole. 1

Practical Calculation Example

• For a child weighing 43.6 kg (96 pounds), the single dose would be:
  • Trimethoprim: 4 mg/kg × 43.6 kg = 174.4 mg per dose
  • Sulfamethoxazole: 20 mg/kg × 43.6 kg = 872 mg per dose
  • This translates to approximately 8.75 mL of pediatric suspension (which contains 40 mg trimethoprim per 5 mL) given every 12 hours. 2

Indication-Specific Single-Dose Considerations

Mild-to-Moderate Infections (UTI, Skin Infections)

• Use 4 mg/kg trimethoprim per dose (the lower end of the range) every 12 hours for 7–10 days. 3, 2
• The American Academy of Pediatrics supports 8–10 mg/kg/day total daily dose divided twice daily for uncomplicated infections. 3

Serious Infections (Severe MRSA, Complicated Soft Tissue)

• Increase to 5–6 mg/kg trimethoprim per dose every 12 hours (total 10–12 mg/kg/day). 3
• For life-threatening infections, consider 3.75–5 mg/kg trimethoprim per dose every 6–8 hours (total 15–20 mg/kg/day in four divided doses). 3

Pneumocystis Jirovecii Pneumonia (PCP) Treatment

• The CDC recommends 3.75–5 mg/kg trimethoprim per dose every 6 hours (total 15–20 mg/kg/day) for 14–21 days. 3, 1
• Each single dose for a 40 kg child would be approximately 150–200 mg trimethoprim given four times daily. 1

Age and Safety Restrictions

• Contraindicated in infants under 2 months due to kernicterus risk from sulfonamide displacement of bilirubin. 4, 1
• The liquid formulation is preferred for children weighing less than 16 kg to ensure accurate dosing. 3

Renal Impairment Adjustments

When creatinine clearance is reduced, adjust the single dose as follows:

• CrCl 15–30 mL/min: Reduce each dose by 50% (e.g., 2 mg/kg trimethoprim per dose instead of 4 mg/kg). 3, 1
• CrCl <15 mL/min: Either reduce by 50% or select an alternative agent; the FDA label advises against use in severe renal failure. 3, 1
• For PCP treatment with CrCl 10–50 mL/min, give 3–5 mg/kg trimethoprim every 12 hours instead of every 6–8 hours. 3
• For CrCl <10 mL/min, extend the interval to every 24 hours. 3

Monitoring and Safety

• Obtain a baseline complete blood count with differential and platelet count before starting therapy, then repeat monthly during prolonged treatment to detect neutropenia, thrombocytopenia, or anemia. 3, 4
• Ensure the child drinks at least 1.5 liters of fluid daily during high-dose therapy to prevent crystalluria. 3
• Screen for G6PD deficiency before initiating therapy, as hemolytic anemia can occur in deficient patients. 3, 4

Common Pitfalls

• Do not use in neonates or infants under 2 months: The risk of kernicterus is substantial. 4, 1
• Avoid underdosing in serious infections: Mild infections require 8–10 mg/kg/day total, but severe MRSA or PCP may need 15–20 mg/kg/day divided into more frequent doses. 3
• Adjust for renal function: Failure to reduce the dose when CrCl is <30 mL/min markedly increases toxicity risk. 3
• Ensure adequate hydration: Insufficient fluid intake during high-dose therapy raises the risk of crystal formation in the urinary tract. 3