Bactrim Dosing for Cystitis (Bladder Infection)
For uncomplicated bacterial cystitis in women, prescribe Bactrim DS (160 mg trimethoprim/800 mg sulfamethoxazole) one tablet twice daily for 3 days. 1, 2
Standard Dosing Regimen
- Women with uncomplicated cystitis: Bactrim DS (160/800 mg) twice daily for 3 days is the guideline-recommended duration 1, 2
- Men with UTI: A longer duration of 7 days is required at the same dose (160/800 mg twice daily) 2
- Complicated UTI or pyelonephritis: 14 days of Bactrim DS twice daily, but only if susceptibility is confirmed 1, 2
Critical Prescribing Considerations
Only use Bactrim empirically when local E. coli resistance rates are below 20%. 1, 2 This is the most important caveat that determines whether you should prescribe Bactrim at all.
- Clinical cure rates are 90-100% when the pathogen is susceptible to Bactrim 1, 2
- However, cure rates plummet to 41-54% when the organism is resistant 1, 2
- Patients who used Bactrim in the preceding 3-6 months or traveled internationally recently should receive alternative antibiotics due to higher resistance risk 2
Alternative First-Line Options When Bactrim Cannot Be Used
- Nitrofurantoin: 100 mg twice daily for 5 days (equivalent efficacy to 3-day Bactrim) 1, 2, 3
- Fosfomycin: Single 3-gram dose (slightly lower efficacy but excellent convenience) 1, 2
- Fluoroquinolones: Reserve for pyelonephritis or resistant organisms, not for simple cystitis 1, 2
Important Clinical Pearls
- Hospital antibiograms overestimate community resistance rates—use local outpatient surveillance data when available 2
- Each additional day of antibiotics beyond the recommended 3-day course increases adverse event risk by 5% without added benefit 2
- Do not extend treatment beyond 7 days unless symptoms persist 1, 2
- Common side effects include rash, nausea, vomiting, and hematologic abnormalities 2
Special Populations
- Pregnancy: Contraindicated in the third trimester 4, 2
- Renal impairment: Reduce dose by 50% if creatinine clearance is 15-30 mL/min; avoid if <15 mL/min 5
- Pediatric patients ≥2 months: 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours divided twice daily for 10 days 5
Note on "Cyst" Terminology: If you are asking about polycystic kidney disease cyst infections (not bladder cystitis), Bactrim achieves excellent cyst fluid penetration with mean concentrations of 15.2 mcg/mL trimethoprim and 42.5 mcg/mL sulfamethoxazole, demonstrating bactericidal activity against common pathogens 6. However, standard dosing and duration for infected renal cysts are not well-established in guidelines and typically require longer courses than simple cystitis.