What is the appropriate pediatric Bactrim (trimethoprim‑sulfamethoxazole) dose for a child weighing 36 pounds (≈16 kg)?

Bactrim Dosing for a 36-Pound Child

For a child weighing 36 pounds (16.4 kg), administer Bactrim at 8–12 mg/kg/day of trimethoprim (131–197 mg/day total) divided into two doses every 12 hours, which translates to approximately 1 tablet of single-strength Bactrim (80 mg TMP/400 mg SMX) every 12 hours for most common infections. 1, 2

Standard Dosing Calculation

Weight-based dosing:

• The American Academy of Pediatrics recommends 8–12 mg/kg/day of trimethoprim and 40–60 mg/kg/day of sulfamethoxazole, divided every 12 hours for children over 2 months of age 1
• For a 16.4 kg child, this equals:
  • Trimethoprim: 131–197 mg/day (65.5–98.5 mg per dose)
  • Sulfamethoxazole: 656–984 mg/day (328–492 mg per dose)

Practical tablet dosing:

• The FDA label recommends 1 single-strength tablet (80 mg TMP/400 mg SMX) every 12 hours for children weighing 22–44 pounds 2
• This provides 160 mg TMP/day, which falls within the recommended 8–12 mg/kg/day range for this weight 2

Indication-Specific Adjustments

For mild-to-moderate infections (UTI, otitis media, skin infections):

• Use the standard 8–10 mg/kg/day range (lower end) 1, 3
• 1 single-strength tablet every 12 hours is appropriate 2

For serious infections (severe MRSA, complicated infections):

• Use 10–12 mg/kg/day or higher 1
• May require 1.5 single-strength tablets every 12 hours (120 mg TMP per dose) to achieve therapeutic targets 1

Formulation Considerations

Liquid formulation may be preferred for this weight:

• The American Academy of Pediatrics recommends using liquid formulation for accurate dosing in children weighing <16 kg, though at 36 pounds (16.4 kg) tablets are acceptable 1
• Liquid suspension contains 40 mg TMP/200 mg SMX per 5 mL 2
• For precise dosing at 8 mg/kg/day: approximately 8 mL (65 mg TMP) every 12 hours 1

Duration and Monitoring

Treatment duration:

• UTI and otitis media: 10–14 days 2
• Shigellosis: 5 days 2
• Skin/soft tissue infections: 7–10 days 1

Safety monitoring:

• Obtain complete blood count with differential and platelet count at treatment initiation 1
• Repeat monthly during prolonged therapy to assess for hematologic toxicity 1
• Ensure adequate hydration (age-appropriate fluid intake) to prevent crystalluria 1

Important Contraindications and Precautions

Absolute contraindications:

• Age <2 months 2
• G6PD deficiency (risk of hemolytic anemia) 1
• Severe hepatic or renal impairment without dose adjustment 1

Drug interactions to monitor:

• Increases methotrexate toxicity 1
• Enhances warfarin anticoagulant effect 1
• Increases hypoglycemia risk with oral hypoglycemics 1

Common Pitfalls to Avoid

• Do not underdose: Using only 6 mg/kg/day may fail to achieve therapeutic targets for bacteria with MIC >0.5 mg/L 1
• Do not use in infants <2 months: Risk of kernicterus due to bilirubin displacement 2
• Do not forget renal adjustment: If creatinine clearance is impaired, reduce dose by 50% for CrCl 15–30 mL/min 1
• Avoid inadequate hydration: Insufficient fluid intake increases crystalluria risk during therapy 1