Bactrim (Trimethoprim-Sulfamethoxazole) Dosing and Duration
For acute uncomplicated cystitis in women, use Bactrim 160/800 mg (one double-strength tablet) twice daily for 3 days, but only if local resistance rates are below 20% and the pathogen is known to be susceptible. 1
Acute Uncomplicated Cystitis
Standard Dosing
- 160/800 mg (one double-strength tablet) twice daily for 3 days is the evidence-based regimen 1
- Clinical cure rates of 85-100% and bacterial cure rates of 85-100% have been demonstrated with 3-day therapy 1
Critical Resistance Considerations
- Bactrim is no longer recommended as first-line empiric therapy due to rising resistance rates globally 1
- Use only when local E. coli resistance is documented to be <20% 1
- If susceptibility is unknown, choose an alternative agent (nitrofurantoin or fosfomycin) 1
- When resistance is present, clinical cure drops dramatically from 84% to 41% 1
Alternative Duration
- 7-day regimens (160/800 mg twice daily) show similar efficacy to 3-day courses but with significantly higher adverse event rates (38% vs 31%) 1
Acute Uncomplicated Pyelonephritis
Standard Dosing
- 160/800 mg (one double-strength tablet) twice daily for 14 days when the pathogen is known to be susceptible 1
- Requires an initial IV dose of ceftriaxone 1g or consolidated aminoglycoside if susceptibility is unknown at treatment initiation 1
Key Restrictions
- Never use empirically without knowing susceptibility 1
- Fluoroquinolones are preferred for empiric therapy when local resistance is <10% 1
Pneumocystis jirovecii Pneumonia (PCP)
Treatment Dosing
- 75-100 mg/kg/day sulfamethoxazole with 15-20 mg/kg/day trimethoprim divided every 6 hours for 14-21 days 2
- For a 70 kg adult: 4 teaspoonfuls (20 mL) every 6 hours 2
Prophylaxis Dosing
- Adults: 160/800 mg (one double-strength tablet) once daily 1, 2
- Children: 150 mg/m²/day trimethoprim with 750 mg/m²/day sulfamethoxazole divided twice daily, given 3 consecutive days per week 3, 2
- Alternative pediatric dosing: 8 mg/kg/day of trimethoprim component divided twice daily 3
Other Indications
Shigellosis
- Adults: 160/800 mg twice daily for 5 days 2
- Children: 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours divided every 12 hours for 5 days 2
Acute Otitis Media (Children)
- 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours divided every 12 hours for 10 days 2
Chronic Bronchitis Exacerbations
- 160/800 mg twice daily for 14 days 2
Traveler's Diarrhea
- 160/800 mg twice daily for 5 days 2
Renal Dosing Adjustments
- CrCl >30 mL/min: Standard dosing 2
- CrCl 15-30 mL/min: Reduce dose by 50% 2
- CrCl <15 mL/min: Not recommended 2
Critical Monitoring Requirements
For Prophylactic Use
- Complete blood counts with differential and platelet count at initiation and monthly thereafter to detect hematologic toxicity 3
- Approximately 2% of patients develop severe anemia by 18 months when used long-term 1
Common Pitfalls
- Do not use for empiric UTI treatment without knowing local resistance patterns - resistance correlates directly with treatment failure 1
- Do not use amoxicillin or ampicillin instead - these have very high resistance rates worldwide and poor efficacy 1
- Avoid 10-day courses for simple cystitis - 3-day therapy is equally effective with fewer side effects 1
- Never use for pyelonephritis without susceptibility data or initial IV therapy - this is a critical safety issue 1