What is the recommended dose of Bactrim (trimethoprim/sulfamethoxazole) for an uncomplicated urinary tract infection (UTI)?

Bactrim Dosing for UTI

For uncomplicated UTI in women, prescribe Bactrim DS (160 mg trimethoprim/800 mg sulfamethoxazole) twice daily for 3 days. 1, 2

Adult Dosing by Clinical Scenario

Women with Uncomplicated Cystitis

• Bactrim DS (160/800 mg) twice daily for 3 days is the standard first-line regimen 1, 2
• This 3-day course achieves clinical cure rates of 90-100% when the pathogen is susceptible 1
• The shorter 3-day regimen has comparable efficacy to 10-day courses but with significantly fewer adverse effects (1.4-31% vs 15-38%) 2

Men with UTI

• Bactrim DS (160/800 mg) twice daily for 7 days is required 1
• Men need longer treatment duration than women due to higher risk of prostatic involvement

Complicated UTI or Pyelonephritis

• Bactrim DS (160/800 mg) twice daily for 14 days based on antibiotic susceptibility 1
• The FDA label indicates 10-14 days for complicated UTI 3, 4

Critical Prescribing Caveats

Resistance Threshold

• Only use Bactrim empirically when local E. coli resistance is <20% 1, 2
• If local resistance exceeds 20%, choose alternative first-line agents (nitrofurantoin, fosfomycin, or pivmecillinam) 2
• When the organism is resistant, clinical cure rates plummet to 41-54% 1

Pregnancy Considerations

• Avoid Bactrim in the last trimester of pregnancy due to potential contraindications 1

Renal Impairment Dosing

• Creatinine clearance >30 mL/min: standard dosing 3, 4
• Creatinine clearance 15-30 mL/min: reduce to half the usual regimen 3, 4
• Creatinine clearance <15 mL/min: do not use 3, 4

Why 3 Days Instead of Longer

The Infectious Diseases Society of America specifically recommends the 3-day regimen over longer courses because each additional day beyond the recommended duration carries a 5% increased risk for antibiotic-associated adverse events without additional clinical benefit 1. The 3-day course achieves bacterial cure rates of 85-100% 2, making longer courses unnecessary for uncomplicated cases.

Common Pitfalls to Avoid

• Do not prescribe amoxicillin or ampicillin empirically for UTI due to poor efficacy and high resistance rates 2
• Do not extend treatment beyond recommended durations without clear indication, as this only increases adverse events 1
• Do not use fluoroquinolones as first-line therapy for uncomplicated cystitis; reserve these for more invasive infections 5