What is the recommended dose of Bactrim (trimethoprim/sulfamethoxazole) for a 40-pound pediatric patient?

Bactrim Dosing for a 40-Pound Pediatric Patient

For a 40-pound (18 kg) child, the recommended dose of Bactrim oral suspension (40mg trimethoprim/200mg sulfamethoxazole per 5mL) is 10 mL (2 teaspoonfuls) every 12 hours for treatment of most infections, or 5 mL (1 teaspoonful) every 12 hours for prophylaxis. 1

Treatment Dosing (Acute Infections)

The standard treatment dose is based on 8-12 mg/kg/day of trimethoprim (40-60 mg/kg/day of sulfamethoxazole), divided into 2 doses every 12 hours. 2, 1

For your 40-pound (18 kg) patient:

• 10 mL every 12 hours provides 80 mg trimethoprim and 400 mg sulfamethoxazole per dose (160 mg/800 mg daily total) 1
• This achieves approximately 8.9 mg/kg/day of trimethoprim, which falls within the recommended range 2

Duration by Indication:

• Urinary tract infections/acute otitis media: 10-14 days 1
• Shigellosis: 5 days 1
• Skin and soft tissue infections: 7-10 days 2
• Pertussis prophylaxis: 14 days 2

Prophylaxis Dosing

For prophylactic use (such as PCP prophylaxis in immunocompromised children), the CDC recommends 150 mg/m²/day of trimethoprim with 750 mg/m²/day of sulfamethoxazole, divided into two doses and given three consecutive days per week. 3

For practical dosing in your 18 kg patient:

• 5 mL (1 teaspoonful) every 12 hours on 3 consecutive days per week provides adequate prophylaxis 1
• Alternative: 8 mg/kg/day of trimethoprim (approximately 144 mg daily for this child) can be used for daily prophylaxis 3

Critical Safety Considerations

Age restriction: Bactrim is contraindicated in children under 2 months of age due to kernicterus risk. 2, 1

Monitoring requirements:

• Obtain complete blood count with differential and platelet count at treatment initiation 2
• Repeat monthly during prolonged therapy to assess for hematologic toxicity (neutropenia, thrombocytopenia, anemia) 2

Common adverse effects occur in approximately 15% of children and include:

• Rash (most common) 2
• Gastrointestinal disturbances 3
• Hematologic abnormalities 3

Management of rash:

• Mild rash: temporarily discontinue and restart when resolved 2
• Urticarial rash or Stevens-Johnson syndrome: permanently discontinue 2

Dosing Adjustments

Renal impairment: When creatinine clearance is 15-30 mL/min, use half the usual dose; avoid use when creatinine clearance is below 15 mL/min. 1

Higher-dose therapy (12-15 mg/kg/day of trimethoprim) may be considered for severe infections or organisms with higher MICs, particularly in older children, but this exceeds standard dosing and requires careful monitoring. 4

Common Pitfalls to Avoid

• Do not use for non-purulent cellulitis alone, as Bactrim has poor activity against beta-hemolytic streptococci. 5
• Avoid in mixed aerobic-anaerobic infections without additional anaerobic coverage. 5
• Ensure adequate hydration to prevent crystalluria, especially with higher doses 1
• Check for sulfa allergies before prescribing 5