Trimethoprim-Sulfamethoxazole Dosing for Urinary Tract Infections
For uncomplicated cystitis in women, prescribe TMP-SMX 160/800 mg (one double-strength tablet) twice daily for 3 days. 1, 2, 3
Uncomplicated Cystitis (Women)
- Standard regimen: TMP-SMX DS (160/800 mg) twice daily for 3 days 1, 2
- Clinical cure rates reach 90-100% when organisms are susceptible 1, 2
- Bacterial eradication rates are 91-100% for susceptible pathogens 1, 2
- This 3-day course is equally effective as 7-day regimens but with significantly fewer adverse effects (8.7% vs 31-38%) 1
Uncomplicated Cystitis (Men)
- Use TMP-SMX DS (160/800 mg) twice daily for 7 days 2
- Men require longer duration than women due to anatomical differences and higher risk of prostatic involvement 2
Uncomplicated Pyelonephritis
- TMP-SMX DS (160/800 mg) twice daily for 14 days 1, 2
- Critical caveat: Only use if susceptibility is confirmed by culture 1
- Do not use empirically without culture results due to high resistance rates 1
- Clinical cure rates are 85-92% when organisms are susceptible 1
Critical Resistance Threshold
Do not use TMP-SMX empirically if local E. coli resistance exceeds 20% 1, 2
- When resistance is <20%, cure rates are 84% for susceptible organisms vs only 41-54% for resistant organisms 1, 2
- Avoid in patients who used TMP-SMX in the preceding 3-6 months 2
- Avoid in patients who traveled outside the United States in the preceding 3-6 months 2
Alternative First-Line Agents When TMP-SMX is Inappropriate
- Nitrofurantoin monohydrate/macrocrystals 100 mg twice daily for 5 days 1, 2
- Fosfomycin 3 grams as a single dose 1, 2
- These alternatives maintain excellent activity with resistance rates generally below 10% 2
Renal Impairment Dosing
- Creatinine clearance 15-30 mL/min: Use half the usual dose 3
- Creatinine clearance <15 mL/min: Do not use 3
Common Pitfalls to Avoid
- Never treat asymptomatic bacteriuria with TMP-SMX 1
- Do not extend treatment beyond 7 days for uncomplicated cystitis—each additional day increases adverse events by 5% without added benefit 2
- Hospital antibiograms overestimate community resistance; use local outpatient surveillance data instead 2
- Avoid in last trimester of pregnancy 1