Bactrim DS Dosing for Urinary Tract and Skin Infections
Urinary Tract Infections
For uncomplicated UTIs in women, use Bactrim DS (160 mg trimethoprim/800 mg sulfamethoxazole) one tablet twice daily for 3 days. 1
Women with Uncomplicated Cystitis
- Standard regimen: 1 double-strength tablet (160/800 mg) twice daily for 3 days 1
- Clinical cure rates reach 90-100% when organisms are susceptible 1
- Bacterial eradication rates are 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
- Require longer duration: 1 double-strength tablet (160/800 mg) twice daily for 7-14 days 1, 2
- The 3-day regimen used in women is inadequate for male patients 2
- Male UTIs are considered complicated and demand extended therapy 2
Pyelonephritis
- Use 1 double-strength tablet twice daily for 14 days, only if susceptibility is confirmed 1
Critical Resistance Threshold
- Only use Bactrim empirically when local E. coli resistance is <20% 1
- When resistance exceeds 20%, treatment failures outweigh benefits 1
- Cure rates drop from 84% (susceptible organisms) to 41% (resistant organisms) 1
When to Avoid Bactrim
- Recent trimethoprim-sulfamethoxazole use in preceding 3-6 months 1
- Travel outside the United States in preceding 3-6 months 1
- Last trimester of pregnancy 1, 2
- Local resistance data showing >20% E. coli resistance 1
Renal Dose Adjustments
- CrCl >30 mL/min: Standard dose (1 DS tablet twice daily) 3
- CrCl 15-30 mL/min: Half-dose (1 single-strength tablet or half DS tablet) 3
- CrCl <15 mL/min: Use not recommended 3
- Monitor electrolytes regularly as trimethoprim causes hyperkalemia 2
- Ensure adequate hydration (at least 1.5 liters daily) to prevent crystalluria 2
Skin Infections
The FDA label does not provide specific dosing for skin infections. 3 For skin and soft tissue infections, standard adult dosing would follow the general UTI dosing pattern of 1 double-strength tablet twice daily, though duration would depend on infection severity and clinical response. 3
Alternative First-Line Agents
When Bactrim cannot be used due to resistance or contraindications:
- Nitrofurantoin 100 mg twice daily for 5 days (90% clinical cure rate) 1
- Fosfomycin 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
Common Pitfalls to Avoid
- Do not use 3-day regimens in men—this is inadequate treatment and a common error 2
- Do not fail to adjust dose when CrCl <30 mL/min—this significantly increases toxicity risk 2, 3
- Do not use empirically without checking local resistance patterns—hospital antibiograms often overestimate community resistance 1
- Do not prescribe in last trimester of pregnancy 1, 2