Bactrim Dosage for Children
For treatment of urinary tract infections, acute otitis media, and shigellosis in children ≥2 months old, administer 40 mg/kg/day of sulfamethoxazole and 8 mg/kg/day of trimethoprim, divided into two doses every 12 hours. 1
Age Restrictions
- Bactrim is contraindicated in children under 2 months of age 1
- This restriction applies to all indications and formulations 1
Treatment Dosing by Indication
Urinary Tract Infections and Acute Otitis Media
- Standard dose: 40 mg/kg sulfamethoxazole + 8 mg/kg trimethoprim per 24 hours, divided every 12 hours for 10 days 1
- Weight-based tablet dosing guide 1:
- 22 lb (10 kg): 1 tablet (400/80 mg) every 12 hours
- 44 lb (20 kg): 2 tablets (400/80 mg) or 1 DS tablet every 12 hours
- 66 lb (30 kg): 3 tablets (400/80 mg) or 1½ DS tablets every 12 hours
- 88 lb (40 kg): 4 tablets (400/80 mg) or 2 DS tablets every 12 hours
Shigellosis
- Identical dosing to UTI/otitis media but for only 5 days 1
Pneumocystis Jiroveci Pneumonia (PCP) Treatment
- Treatment dose: 75-100 mg/kg/day sulfamethoxazole + 15-20 mg/kg/day trimethoprim, divided every 6 hours for 14-21 days 1
- This represents significantly higher dosing than for routine bacterial infections 1
- Upper limit dosing guide for PCP 1:
- 35 lb (16 kg): 1 tablet every 6 hours
- 53 lb (24 kg): 1½ tablets every 6 hours
- 70 lb (32 kg): 2 tablets (400/80 mg) or 1 DS tablet every 6 hours
Prophylaxis Dosing
PCP Prophylaxis in Immunocompromised Children
- CDC-recommended regimen: 150 mg/m²/day trimethoprim + 750 mg/m²/day sulfamethoxazole, divided into two doses, given on 3 consecutive days per week 2
- Alternative weight-based approach: 8 mg/kg/day of trimethoprim component divided into two doses 2
- Maximum daily dose should not exceed 1600 mg sulfamethoxazole and 320 mg trimethoprim 1
- Body surface area dosing guide 1:
- 0.26 m²: ½ tablet every 12 hours
- 0.53 m²: 1 tablet every 12 hours
- 1.06 m²: 2 tablets every 12 hours
Indications for PCP Prophylaxis
The CDC recommends prophylaxis for 2:
- All HIV-infected infants 1-12 months of age
- HIV-infected children 1-5 years with CD4+ count <500/μL or CD4+ percentage <15%
- HIV-infected children 6-12 years with CD4+ count <200/μL or CD4+ percentage <15%
Renal Impairment Adjustments
Dose reduction is mandatory when creatinine clearance falls below 30 mL/min 1:
- CrCl >30 mL/min: Standard dosing
- CrCl 15-30 mL/min: 50% of usual dose
- CrCl <15 mL/min: Use not recommended 1
Pharmacokinetic Considerations
- Younger children require higher weight-based doses than adults to achieve equivalent serum concentrations due to faster clearance 3
- Half-lives of both trimethoprim and sulfamethoxazole increase with age and correlate directly with serum creatinine levels 3
- Intravenous administration produces more reliable peak concentrations than oral dosing, with peak increments significantly higher (P <0.001) 3
- For IV dosing in children ≤10 years: loading dose of 250 mg TMP + 1,250 mg SMX/m², followed by 150 mg TMP + 750 mg SMX/m² every 8 hours 3
Formulation Selection
- Liquid formulation is more appropriate for accurate dosing in younger children and those weighing less than 40 kg 2
- Tablet formulations become practical for older children who can reliably swallow pills 1
Monitoring Requirements
For children on prophylactic therapy 2:
- Complete blood count with differential and platelet count at initiation
- Monthly CBC monitoring thereafter to detect hematologic toxicity
- Discontinue permanently if life-threatening toxicity occurs 2
Common Pitfalls
- Do not use test doses when initiating low-dose Bactrim for any indication in children 4
- Concomitant use with methotrexate is NOT contraindicated, contrary to common belief 4
- NSAIDs and salicylates can be given concurrently in children with normal renal function 4
- Thrombocytopenia risk increases with higher serum trimethoprim levels and longer treatment duration 3
- Common adverse effects include rash (most frequent), gastrointestinal disturbances, and hematologic abnormalities 2