What is the appropriate dose, duration, and precautions for using Bactrim (trimethoprim‑sulfamethoxazole) to treat an uncomplicated urinary tract infection in a child?

Bactrim Dosing for Pediatric Uncomplicated UTI

For uncomplicated urinary tract infections in children, use trimethoprim-sulfamethoxazole at 8 mg/kg/day trimethoprim with 40 mg/kg/day sulfamethoxazole, divided every 12 hours, for 7–14 days total duration. 1

Dosing Recommendations

Standard Dosing

• Primary regimen: 8 mg/kg/day trimethoprim combined with 40 mg/kg/day sulfamethoxazole, divided into two doses given every 12 hours 1
• Acceptable alternative range: 6–12 mg/kg/day trimethoprim with 30–60 mg/kg/day sulfamethoxazole, divided twice daily, when the standard dose is not feasible 1, 2
• FDA-approved dosing: 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, given in two divided doses every 12 hours for 10 days 3

Weight-Based Tablet Dosing (every 12 hours)

• 20 kg (≈44 lb): 1 single-strength tablet (80 mg TMP/400 mg SMX) 1
• 30 kg (≈66 lb): 1½ single-strength tablets 1
• 40 kg (≈88 lb): 2 single-strength tablets OR 1 double-strength tablet 1

Treatment Duration

Treat for 7–14 days regardless of oral versus parenteral route. 1, 2

• Avoid short courses: Regimens shorter than 7 days are proven inferior for febrile UTIs and should not be used 1, 2, 4
• No single optimal duration: The American Academy of Pediatrics found insufficient evidence to recommend specifically 7,10, or 14 days, but all fall within the acceptable range 1
• Single-dose regimens clear bacteria initially but carry a 23% risk of early recurrence (often asymptomatic) compared to 2% with standard courses 4

Route of Administration

Oral therapy is equally effective as parenteral for most children with uncomplicated UTI. 1

Reserve IV therapy for:

• Toxic-appearing children 1, 2
• Children unable to retain oral medications 1, 2
• Compliance concerns with oral administration 2

Critical Resistance Considerations

Only use trimethoprim-sulfamethoxazole empirically when local E. coli resistance rates are ≤10–20%. 1, 2

• Resistance prevalence varies substantially by geographic region—always consult your local antibiogram 1
• Modify therapy immediately based on culture and susceptibility results once available 1
• For febrile UTIs/pyelonephritis specifically, use only if local resistance is <10% 1

Age Restrictions & Contraindications

• Not recommended for infants <2 months of age 3
• Do not use nitrofurantoin for febrile UTIs, as it achieves only urinary (not tissue/serum) concentrations 1, 2

Common Pitfalls to Avoid

• Inadequate duration: Using <7 days for febrile UTIs leads to treatment failure 1, 2
• Ignoring local resistance: Empiric use without checking antibiogram data risks treatment failure 1, 2
• Wrong agent for pyelonephritis: Nitrofurantoin is inappropriate for any febrile/upper tract infection 1, 2
• Delayed treatment: Initiate antibiotics promptly (ideally within 48 hours of fever onset) to reduce renal scarring risk 1

Monitoring & Safety

• Perform complete blood counts at treatment initiation and monthly for long-term therapy 2
• Permanently discontinue if serious adverse reactions occur (anaphylaxis, Stevens-Johnson syndrome) 2
• Adjust dosing in renal impairment: use half the usual regimen if creatinine clearance is 15–30 mL/min; avoid use if <15 mL/min 3