Cotrimoxazole 40/200 Syrup Dosing
For pediatric treatment of infections, administer 8 mg/kg/day of trimethoprim (40 mg/kg/day sulfamethoxazole) divided into two doses every 12 hours, which translates to 1 mL per kg of body weight per dose using the 40/200 suspension. 1
Standard Treatment Dosing
The FDA-approved dosing for children over 2 months of age with urinary tract infections, acute otitis media, or shigellosis is:
- 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, divided every 12 hours 1
- Using the 40/200 mg/5mL suspension (8/40 mg per mL), this equals 1 mL/kg/dose given twice daily 1
Weight-Based Dosing Table
For practical administration using the 40/200 suspension 1:
- 10 kg child: 10 mL twice daily
- 20 kg child: 20 mL twice daily
- 30 kg child: 30 mL twice daily (or 1.5 double-strength tablets)
- 40 kg child: 40 mL twice daily (or 2 double-strength tablets)
Treatment Duration by Indication
- Urinary tract infections: 10-14 days 1
- Acute otitis media: 10 days 1
- Shigellosis/dysentery: 5 days 1, 2
- Pneumonia (severe): 15-20 mg/kg/day TMP divided every 6-8 hours 2
Prophylaxis Dosing
For Pneumocystis jiroveci pneumonia (PCP) prophylaxis in immunocompromised children:
- 150 mg/m²/day trimethoprim with 750 mg/m²/day sulfamethoxazole, divided twice daily 3, 4
- Alternative schedule: Same total daily dose given 3 consecutive days per week 4
- The CDC recommends this three-day weekly schedule as preferred for prophylaxis 4
For simple prophylaxis of recurrent UTIs:
- 2 mg/kg trimethoprim with 10 mg/kg sulfamethoxazole once daily 5
Critical Safety Considerations
Contraindications:
- Do not use in infants under 2 months of age 1
- Avoid in patients with G6PD deficiency due to hemolytic anemia risk 3
- Avoid in pregnancy at term 3
Monitoring requirements:
- Obtain baseline complete blood count and monitor monthly for hematologic toxicity, particularly thrombocytopenia 3, 4
- If life-threatening toxicity occurs, permanently discontinue 4
Renal Dose Adjustment
For children with impaired renal function 2, 1:
- CrCl 15-30 mL/min: Reduce dose by 50%
- CrCl <15 mL/min: Reduce dose by 50% or use alternative agent
- Hemodialysis: Give 50% dose after each dialysis session
Common Pitfalls to Avoid
The most critical error is underdosing in serious infections—the standard 8 mg/kg/day TMP is appropriate for uncomplicated infections, but severe infections like PCP require 15-20 mg/kg/day TMP divided every 6 hours 1. Research demonstrates that even double-dose cotrimoxazole (16 mg/kg/day TMP) shows equivalent efficacy to standard dosing for pneumonia without increased toxicity 6, though this is not FDA-approved.
Ensure accurate weight-based dosing rather than age-based approximations, as plasma concentrations vary significantly with improper dosing 7. Treatment failure is more likely when medication is not administered correctly 6.
Do not use concurrent methotrexate at treatment doses due to severe bone marrow suppression risk, though lower prophylactic methotrexate doses are generally tolerated 3.