What is the recommended dose of Bactrim (sulfamethoxazole and trimethoprim) for an uncomplicated urinary tract infection (UTI) in a healthy, non-pregnant adult woman with normal renal function?

Bactrim Dosing for Uncomplicated UTI

For uncomplicated cystitis in healthy, non-pregnant adult women with normal renal function, prescribe Bactrim DS (160 mg trimethoprim/800 mg sulfamethoxazole) one tablet twice daily for 3 days. 1

Standard Dosing Regimen

• The recommended dose is one double-strength tablet (160/800 mg) twice daily for 3 days, as established by the Infectious Diseases Society of America 1
• This 3-day regimen achieves clinical cure rates of 90-100% when the pathogen is susceptible 1
• Bacterial eradication rates are similarly high at 91-100% for susceptible organisms 1

Critical Resistance Threshold - When NOT to Use Bactrim

Do not prescribe Bactrim empirically if local E. coli resistance exceeds 20%, as treatment failures will outweigh benefits 1

• When organisms are resistant, clinical cure rates plummet to only 41-54%, making treatment failure the expected outcome 1
• Avoid Bactrim in patients who have used trimethoprim-sulfamethoxazole in the preceding 3-6 months, as this independently predicts resistance 1
• Avoid Bactrim in patients who have traveled outside the United States in the preceding 3-6 months, as this also predicts resistance 1

Alternative First-Line Agents When Bactrim Cannot Be Used

If local resistance exceeds 20% or the patient has risk factors for resistance, use these alternatives:

• Nitrofurantoin monohydrate/macrocrystals 100 mg twice daily for 5 days - achieves 90% clinical cure and 92% bacterial cure rates 1, 2
• Fosfomycin trometamol 3 g single dose - offers the convenience of single-dose therapy with equivalent efficacy 1
• Pivmecillinam 400 mg twice daily for 5 days - maintains excellent activity with resistance rates generally below 10% (where available) 1

Important Contraindications and Warnings

• Avoid Bactrim in pregnant women, especially in the last trimester, due to potential fetal risks 1
• Avoid in patients with marked hepatic damage 1
• Monitor for hematologic abnormalities including thrombocytopenia and neutropenia 1
• Common side effects include rash, urticaria, nausea, vomiting, and hematologic abnormalities 1

Duration Considerations for Men

• Men with UTI require 7 days of treatment (not 3 days) at the same dose of 160/800 mg twice daily 1

Key Clinical Pitfall to Avoid

Hospital antibiograms often overestimate community resistance rates - use local outpatient surveillance data when available to guide empiric therapy decisions 1. Each additional day of antibiotic treatment beyond the recommended 3-day duration carries a 5% increased risk for antibiotic-associated adverse events without additional benefits 1.