What is the recommended dosing 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) one double-strength tablet twice daily for 3 days. 1

Standard Dosing by Patient Population

Women with Uncomplicated Cystitis

• Bactrim DS 160/800 mg twice daily for 3 days is the recommended regimen 1
• This short-course achieves 90-100% clinical cure rates when organisms are susceptible 1
• Bacterial eradication rates reach 91-100% for susceptible pathogens 1
• Each additional day beyond 3 days increases adverse event risk by 5% without added benefit 1

Men with UTI

• Bactrim DS 160/800 mg twice daily for 7 days is required 1, 2
• Male UTIs are considered complicated and require longer treatment duration than the 3-day regimen used in women 2
• Some guidelines support extending to 7-14 days depending on clinical response 2

Pyelonephritis

• Bactrim DS 160/800 mg twice daily for 14 days when susceptibility is confirmed 1
• Only use if culture demonstrates susceptibility, as empiric use is not recommended for upper tract infections 1

Critical Resistance Threshold

Do not use Bactrim empirically when local E. coli resistance exceeds 20%. 1

• When resistance is <20%, cure rates are 84% for susceptible organisms versus only 41% for resistant organisms 1
• Treatment efficacy drops dramatically to 41-54% when the pathogen is resistant 1

Additional Risk Factors for Resistance

• Recent Bactrim use within the preceding 3-6 months 1
• Travel outside the United States within the preceding 3-6 months 1
• In these scenarios, choose alternative first-line agents instead 1

Renal Dose Adjustments

For CrCl >30 mL/min: Standard dose of 1 double-strength tablet twice daily 2, 3

For CrCl 15-30 mL/min: Reduce to half-dose (1 single-strength tablet or half of double-strength) 2, 3

For CrCl <15 mL/min: Use not recommended; select alternative agent 2, 3

Monitoring in Renal Impairment

• Calculate baseline creatinine clearance before initiating therapy 2
• Monitor electrolytes regularly as trimethoprim causes hyperkalemia 2
• Check serum creatinine and BUN 2-3 times weekly during therapy in patients with renal impairment 2
• Ensure adequate hydration (at least 1.5 liters daily) to prevent crystalluria 2

Alternative First-Line Agents

When Bactrim cannot be used due to resistance or contraindications:

• Nitrofurantoin monohydrate/macrocrystals 100 mg twice daily for 5 days (90% clinical cure rate) 1, 2
• Fosfomycin trometamol 3g single dose 2
• Pivmecillinam 400 mg three times daily for 3-5 days 2

These alternatives maintain resistance rates generally below 10% across all regions 1

Special Populations

Pregnancy

• Avoid Bactrim in the last trimester due to potential contraindications 1, 2
• Avoid trimethoprim alone in the first trimester 2

Complicated UTIs

• Fluoroquinolones may be appropriate if local resistance is low 2
• Longer treatment durations (7-14 days) are required 2

Common Pitfalls to Avoid

• Never use the 3-day regimen for male patients—this is inadequate treatment and requires 7 days minimum 2
• Do not fail to adjust dose when CrCl <30 mL/min—this significantly increases toxicity risk 2
• Do not rely on hospital antibiograms for outpatient UTIs—they overestimate community resistance rates; use local outpatient surveillance data instead 1
• Avoid rapid IV bolus administration—use proper hydration to prevent crystalluria 2

Side Effects

Common adverse effects include rash, urticaria, nausea, vomiting, and hematologic abnormalities 1. Side effects occur in approximately 5% of patients on 3-day regimens versus 24-28% on 10-day regimens 4, 5.