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

Trimethoprim-Sulfamethoxazole (Bactrim) Dosing for Uncomplicated UTI

For uncomplicated urinary tract infections in adults, the recommended dosage of trimethoprim-sulfamethoxazole (TMP-SMX) is one double-strength tablet (160/800 mg) twice daily for 3 days. 1

Dosing Guidelines

Standard Adult Dosing

• One double-strength tablet (160/800 mg) or two regular-strength tablets (80/400 mg) every 12 hours 2
• Duration: 3 days for uncomplicated UTI 1
  • Note: While the FDA label mentions 10-14 days 2, more recent guidelines from the Infectious Diseases Society of America (IDSA) recommend the shorter 3-day course for uncomplicated UTIs

Special Populations

• Renal Impairment:

  • CrCl >30 mL/min: Standard regimen
  • CrCl 15-30 mL/min: Half the usual regimen (one double-strength tablet daily)
  • CrCl <15 mL/min: Not recommended 2

• Severe renal impairment (GFR of 23 mL/min): 160/800 mg every 24 hours 1

Clinical Efficacy

TMP-SMX demonstrates excellent efficacy for uncomplicated UTIs:

• Clinical efficacy: 93% (90-100%)
• Microbiological efficacy: 94% (91-100%) 1

Important Considerations

Resistance Patterns

• Consider local resistance patterns when selecting TMP-SMX
• Resistance now approaches 18-22% in some regions of the US 1
• Check baseline electrolytes, particularly potassium, before starting TMP-SMX as it can cause hyperkalemia 1

Alternative First-Line Options

If TMP-SMX is not appropriate, consider these alternatives:

• Nitrofurantoin monohydrate/macrocrystals: 100 mg twice daily for 5 days (first-line per IDSA)
• Fosfomycin trometamol: 3 g single dose 1

Follow-up

• No routine post-treatment urinalysis or cultures needed if symptoms resolve 1
• If symptoms don't improve within 72 hours, obtain urine culture and consider alternative antibiotics based on susceptibility 1

Common Pitfalls to Avoid

1. Overtreatment: Using the longer 10-14 day course for uncomplicated UTIs increases risk of side effects and resistance
2. Ignoring resistance patterns: Check local resistance before prescribing TMP-SMX
3. Missing renal function assessment: Always check renal function and adjust dosing accordingly
4. Overlooking drug interactions: TMP-SMX can cause hyperkalemia, especially in patients on other potassium-sparing medications
5. Using in contraindicated populations: Avoid in severe renal impairment (CrCl <15 mL/min) 2

Remember that while TMP-SMX is effective and economical, nitrofurantoin is now considered first-line by IDSA due to lower resistance rates and excellent efficacy.