Bactrim (Trimethoprim/Sulfamethoxazole) Dosing for Uncomplicated UTI
For uncomplicated urinary tract infections (UTIs) in adults, the recommended dose of Bactrim is one double-strength tablet (160mg trimethoprim/800mg sulfamethoxazole) or two regular-strength tablets (80mg trimethoprim/400mg sulfamethoxazole) taken every 12 hours for 3 days. 1, 2, 3
Dosing Guidelines
Adult Dosing
- Standard dose: 160mg trimethoprim/800mg sulfamethoxazole (one double-strength tablet) every 12 hours 2, 3
- Duration: 3 days for uncomplicated UTI 1
- While the FDA label mentions 10-14 days, current guidelines from the Infectious Diseases Society of America (IDSA) recommend the shorter 3-day course for uncomplicated UTIs 1
- Short-course therapy (≤6 days) is as effective as longer treatment with fewer adverse events 1
Pediatric Dosing
- For children ≥2 months: 40mg/kg sulfamethoxazole and 8mg/kg trimethoprim per 24 hours, divided into two doses given every 12 hours 2, 3
- Duration: 10 days 2, 3
- Note: Bactrim is contraindicated in infants less than 2 months of age 3
Renal Impairment Adjustments
- CrCl >30 mL/min: Standard dosing
- CrCl 15-30 mL/min: Half the usual regimen
- CrCl <15 mL/min: Not recommended 2, 3
Clinical Considerations
First-Line Status
Trimethoprim-sulfamethoxazole remains a first-line agent for uncomplicated UTIs, along with:
- Nitrofurantoin (100mg twice daily for 5 days)
- Fosfomycin (3g single dose) 1
Resistance Patterns
- Only use TMP-SMX where local resistance rates are <20% 1
- In some regions of the US, resistance approaches 18-22% 1, 4
- If local resistance exceeds 20%, consider alternative agents like nitrofurantoin or fosfomycin 1, 5
Common Pitfalls
- Treating for too long: The 3-day course is sufficient for uncomplicated UTIs in adults; longer courses increase risk of side effects without improving outcomes 1
- Ignoring local resistance patterns: Treatment failure is more likely when using TMP-SMX for infections caused by resistant organisms (clinical cure <60%) 4
- Not adjusting for renal function: Dose reduction is necessary for patients with impaired renal function 2, 3
- Using in complicated UTIs: The 3-day regimen is only for uncomplicated UTIs; complicated UTIs require longer treatment (7-14 days) 1
Special Populations
Pregnant Women
- Pregnant women should be screened for and treated for bacteriuria, but alternative agents may be preferred 1
Complicated UTIs
- Extended treatment duration (7-14 days) is necessary 1
- Consider alternative agents based on culture and sensitivity results
Remember that urine culture is not routinely needed for uncomplicated cystitis but should be obtained for suspected pyelonephritis, symptoms that don't resolve within 4 weeks after treatment, women with atypical symptoms, or pregnant women 1.