Bactrim for Uncomplicated UTI
Bactrim (trimethoprim-sulfamethoxazole) 160/800 mg twice daily for 3 days is an excellent first-line treatment for uncomplicated UTI in adult women, but only when local E. coli resistance rates are below 20%. 1
Critical Resistance Threshold
- Do not use Bactrim empirically if local E. coli resistance exceeds 20%, as clinical cure rates plummet from 84-90% with susceptible organisms to only 41-54% with resistant organisms 1, 2
- Verify your local antibiogram before prescribing—hospital data often overestimates community resistance, so outpatient surveillance data is more accurate 1
- Avoid Bactrim in patients who used it within the preceding 3-6 months or traveled outside the United States in that timeframe, as these factors independently predict resistance 1
Dosing Recommendations
For women with uncomplicated cystitis:
- Bactrim DS (160 mg trimethoprim/800 mg sulfamethoxazole) twice daily for 3 days achieves 90-100% clinical cure rates when organisms are susceptible 1
- This 3-day regimen has equivalent efficacy to longer courses but with fewer adverse effects 1
For men with UTI:
- Extend duration to 7 days at the same dose (160/800 mg twice daily) 1
For uncomplicated pyelonephritis:
- 14 days of Bactrim DS twice daily is required, but only after confirming susceptibility 1
Alternative First-Line Agents When Bactrim Is Not Appropriate
- Nitrofurantoin monohydrate/macrocrystals 100 mg twice daily for 5 days achieves 90% clinical cure and 92% bacterial cure rates with resistance rates generally below 10% 1
- Fosfomycin trometamol 3g single dose offers equivalent efficacy with the convenience of single-dose therapy 1
- These alternatives maintain excellent activity with minimal collateral damage to normal flora 1
Common Pitfalls to Avoid
- Never prescribe Bactrim in the last trimester of pregnancy due to potential contraindications 1
- Each additional day of treatment beyond the recommended 3-day duration carries a 5% increased risk for antibiotic-associated adverse events without additional benefits 1
- Common side effects include rash, urticaria, nausea, vomiting, and hematologic abnormalities 1
- Reserve fluoroquinolones for pyelonephritis rather than simple cystitis to minimize collateral damage, despite their high efficacy 1
When Susceptibility Is Confirmed
- If culture results confirm TMP-SMX susceptibility, bacterial eradication rates reach 91-100% 1
- A study demonstrated 100% clinical cure with the 3-day regimen when organisms were susceptible 1
- Treatment efficacy is excellent when used appropriately, making resistance patterns the key determinant of whether to prescribe Bactrim 1, 2