What is the dosage of Bactrim (trimethoprim-sulfamethoxazole) for a urinary tract infection (UTI)?

Bactrim Dosing for Urinary Tract Infections

For uncomplicated UTI in women, use Bactrim DS (trimethoprim-sulfamethoxazole 160/800 mg) twice daily for 3 days; for men or complicated UTIs, extend treatment to 7-14 days. 1, 2, 3

Dosing by Patient Population

Women with Uncomplicated Cystitis

• Standard dose: 160/800 mg (1 DS tablet) twice daily for 3 days 1, 3
• This 3-day regimen achieves 90-100% cure rates with 79-90% sustained cure at 30 days 2
• The 3-day course is as effective as 7-day treatment but with fewer side effects and less resistance development 2
• Important caveat: Bactrim should only be used as an alternative agent when first-line options (fosfomycin, nitrofurantoin, pivmecillinam) are unavailable or contraindicated 1, 2

Men with UTI

• Dose: 160/800 mg (1 DS tablet) twice daily for 7-14 days 2, 3
• The standard 3-day regimen used in women is inadequate for male patients—this is a critical error to avoid 2
• Male UTIs are considered complicated and require longer treatment duration 2

Complicated UTIs (Both Sexes)

• Dose: 160/800 mg (1 DS tablet) twice daily for 10-14 days 3
• FDA labeling specifies 10-14 days for standard UTI treatment 3

Renal Dose Adjustments

You must adjust dosing based on creatinine clearance to prevent toxicity: 2, 3

• CrCl >30 mL/min: Standard dose (160/800 mg twice daily) 2, 3
• CrCl 15-30 mL/min: Reduce to half-dose (80/400 mg twice daily or 1 single-strength tablet) 2, 3
• CrCl <15 mL/min: Use not recommended; consider alternative agent 2, 3

Critical Resistance Considerations

Bactrim is no longer automatically first-line therapy due to rising resistance rates: 2, 4, 5

• Treatment failure rates increase from 16% to 59% when local E. coli resistance exceeds 20% 2
• Only use Bactrim empirically if local resistance rates are <20% 1, 2
• If symptoms don't resolve within 4 weeks or recur within 2 weeks, obtain urine culture and susceptibility testing before retreating 1

Pregnancy Restrictions

• Avoid in first trimester (trimethoprim component) 1
• Contraindicated in last trimester (sulfamethoxazole component) 1

Common Pitfalls to Avoid

• Never use 3-day regimens in men—this guarantees treatment failure 2
• Never skip dose adjustment in renal impairment (CrCl <30)—this significantly increases toxicity risk including hyperkalemia 2
• Never use amoxicillin or ampicillin as alternatives—these have poor efficacy and very high resistance rates 2
• Don't forget to check local resistance patterns—empiric Bactrim use is inappropriate in high-resistance areas 1, 2, 5

Monitoring Requirements in Renal Impairment

When using Bactrim in patients with reduced kidney function: 2

• Calculate baseline creatinine clearance before starting therapy
• Monitor electrolytes regularly (trimethoprim causes hyperkalemia)
• Check serum creatinine and BUN 2-3 times weekly during treatment
• Ensure adequate hydration (≥1.5 liters daily) to prevent crystalluria