What is the appropriate dose of 200 mg/40 mg Bactrim (trimethoprim‑sulfamethoxazole) for a child weighing 21.6 kg?

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Last updated: September 28, 2026 • View editorial policy

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Dosing Bactrim 200/40mg for a 21.6 kg Child

For a child weighing 21.6 kg, administer 2 tablets (200/40 mg each) of Bactrim twice daily, providing a total daily dose of 8 mg/kg trimethoprim and 40 mg/kg sulfamethoxazole.

Calculation Breakdown

The standard pediatric dosing for trimethoprim-sulfamethoxazole is 8-12 mg/kg/day of trimethoprim (based on the trimethoprim component) divided into 2 doses for most infections 1, 2, 3.

For a 21.6 kg child:

  • Target dose: 8-12 mg/kg/day of trimethoprim
  • Calculation: 21.6 kg × 8 mg/kg = 172.8 mg trimethoprim per day
  • Per dose (twice daily): 172.8 mg ÷ 2 = 86.4 mg trimethoprim per dose

Each 200/40 mg tablet contains:

  • 40 mg trimethoprim
  • 200 mg sulfamethoxazole

Therefore: 2 tablets per dose = 80 mg trimethoprim (close to the 86.4 mg target)

This provides:

  • Trimethoprim: 160 mg/day total = 7.4 mg/kg/day
  • Sulfamethoxazole: 800 mg/day total = 37 mg/kg/day

Clinical Context

This dosing applies to common indications including:

  • Urinary tract infections
  • Skin and soft tissue infections (MRSA coverage)
  • Acute otitis media
  • Shigellosis

The FDA-approved dosing for children with UTI or acute otitis media is 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, given in two divided doses every 12 hours 3. The calculated dose of 2 tablets twice daily falls appropriately within this range.

Important Caveats

  • Age restriction: Bactrim is contraindicated in children less than 2 months of age 3
  • Higher doses (up to 12 mg/kg/day trimethoprim) may be needed for more severe infections or MRSA skin infections 1, 2
  • Duration: Typically 10-14 days for UTI, 5 days for shigellosis, 10 days for otitis media 3
  • Renal function: Dose adjustment required if creatinine clearance <30 mL/min 3

Alternative Formulation Consideration

If using the suspension formulation instead of tablets, the equivalent dose would be 10 mL (one teaspoonful) twice daily, as the suspension contains 40 mg trimethoprim and 200 mg sulfamethoxazole per 5 mL.

References

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.