What is the recommended dosing of Bactrim (sulfamethoxazole and trimethoprim) for an adult patient with uncomplicated urinary tract infection and normal renal function?

Bactrim Dosing for Uncomplicated UTI in Adults with Normal Renal Function

For an adult patient with uncomplicated urinary tract infection and normal renal function, the recommended dose is 160 mg trimethoprim/800 mg sulfamethoxazole (one double-strength tablet) twice daily for 3 days in women, or 7 days in men. 1, 2, 3

Standard Dosing by Gender

• Women with uncomplicated cystitis: 160/800 mg (one DS tablet) twice daily for 3 days achieves clinical cure rates of 90-100% when organisms are susceptible 1, 2
• Men with UTI: 160/800 mg twice daily for 7 days is required, as the 3-day regimen used in women is inadequate for male patients and leads to treatment failure 1
• Complicated UTI or pyelonephritis: 160/800 mg twice daily for 14 days, only after confirming susceptibility testing 1, 3

Critical Resistance Threshold—When NOT to Use

Bactrim should only be used empirically when local E. coli resistance is <20%. 1, 2

• Clinical cure rates drop dramatically from 84% (susceptible organisms) to only 41% (resistant organisms), making treatment failure the expected outcome when resistance exceeds this threshold 1, 2
• Avoid empiric use in patients who have used trimethoprim-sulfamethoxazole in the preceding 3-6 months or traveled outside the United States recently, as these factors independently predict resistance 2

Renal Dosing Adjustments (For Completeness)

While your patient has normal renal function, be aware of these adjustments for future reference:

• CrCl >30 mL/min: Standard dose of 160/800 mg twice daily 1, 3
• CrCl 15-30 mL/min: Reduce to half-dose (80/400 mg or one single-strength tablet twice daily) 4, 1, 3
• CrCl <15 mL/min: Use alternative agent or half-dose with extreme caution 4, 3

Alternative First-Line Agents

When Bactrim cannot be used due to resistance or contraindications:

• Nitrofurantoin monohydrate/macrocrystals: 100 mg twice daily for 5 days (clinical cure rate 90%, bacterial cure rate 92%) 1, 2
• Fosfomycin trometamol: 3g single dose 1, 2
• Pivmecillinam: 400 mg three times daily for 3-5 days 1

Key Clinical Pitfalls to Avoid

• Never use the 3-day regimen for male patients—this is inadequate treatment and leads to treatment failure 1
• Never use empirically when local resistance exceeds 20%—treatment failure outweighs benefits 1, 2
• Avoid in the last trimester of pregnancy due to risk of kernicterus; use with caution in second trimester 1, 2
• Monitor for hyperkalemia, as trimethoprim blocks potassium excretion, particularly in patients with renal impairment or those on ACE inhibitors/ARBs 1

Common Adverse Effects to Counsel About

• Rash and urticaria (most common) 1, 2
• Nausea and vomiting 1, 2
• Rare but serious: Stevens-Johnson syndrome, hematologic abnormalities 1, 2, 3