Dosing of Bactrim DS (Trimethoprim/Sulfamethoxazole) for Urinary Tract Infections
For uncomplicated urinary tract infections in adults, the recommended dosage of Bactrim DS is one double-strength tablet (160mg trimethoprim/800mg sulfamethoxazole) taken twice daily for 3 days if local resistance is <20%. 1, 2
Standard Dosing Guidelines
Adults
- Uncomplicated UTI: 1 DS tablet (160/800mg) every 12 hours for 3 days 1
- Complicated UTI: 1 DS tablet every 12 hours for 10-14 days 2
Pediatric Patients (≥2 months)
- Contraindicated in children <2 months of age 2
- Recommended dose: 40mg/kg sulfamethoxazole and 8mg/kg trimethoprim per 24 hours, divided into two doses every 12 hours for 10 days 2
- Weight-based dosing:
- 10kg: 1 tablet every 12 hours
- 20kg: 1½ tablets every 12 hours
- 30kg: 1½ tablets every 12 hours
- 40kg: 2 tablets or 1 DS tablet every 12 hours 2
Renal Dosing Adjustments
Dosage adjustment is crucial for patients with impaired renal function:
- CrCl >30 mL/min: Standard regimen
- CrCl 15-30 mL/min: Half the usual regimen
- CrCl <15 mL/min: Use not recommended 2
Treatment Duration Considerations
- Uncomplicated UTI in women: 3 days is sufficient and recommended by guidelines 1
- Complicated UTI: 10-14 days 2
- Shigellosis: 5 days 2
Clinical Pearls and Pitfalls
Important Considerations:
- Check local resistance patterns: Only use TMP-SMX if local resistance is <20% 1
- First-line alternatives when TMP-SMX is not appropriate:
- Nitrofurantoin 100mg twice daily for 5 days
- Fosfomycin 3g single dose 1
Common Pitfalls:
- Prescribing longer courses than necessary: 3-day regimens for uncomplicated UTIs in women are as effective as longer courses and have fewer adverse effects 1, 3
- Not adjusting for renal function: Failure to adjust dosing in renal impairment can lead to toxicity 2
- Using in areas with high resistance: TMP-SMX should be avoided as empiric therapy in regions where resistance exceeds 20% 1
- Adverse reactions: TMP-SMX has a higher rate of adverse effects (24%) compared to some alternatives (e.g., ciprofloxacin 17%) 4
Special Populations
Men with UTI
- Recent evidence supports 7-day treatment courses with TMP-SMX for afebrile men with UTI, which is noninferior to 14-day courses 5
Pregnant Women
- TMP-SMX is generally avoided in pregnancy, especially during the first trimester (risk of neural tube defects) and near term (risk of kernicterus)
Elderly Patients
- Higher risk of adverse effects, especially with renal impairment
- Monitor renal function and adjust dosing accordingly
Monitoring
- No routine post-treatment urinalysis or urine cultures are indicated for asymptomatic patients after treatment 1
- No routine laboratory monitoring is required for short-course therapy 1
By following these evidence-based guidelines for Bactrim DS dosing in UTIs, you can optimize treatment outcomes while minimizing adverse effects and antimicrobial resistance.