Bactrim Dosing for Soft Tissue Infections
For soft tissue infections, the recommended dose of Bactrim (trimethoprim-sulfamethoxazole) is 1-2 double-strength tablets (160/800 mg) twice daily for 7 days, with the specific dose depending on infection severity and whether MRSA is suspected. 1, 2
Dosing by Clinical Scenario
For Purulent Soft Tissue Infections (MRSA suspected)
- Adults: 1-2 double-strength tablets (160/800 mg or 320/1600 mg) orally twice daily 1, 2
- Pediatric: 8-12 mg/kg/day (based on trimethoprim component) divided into 2 oral doses 1
- Duration: Typically 7 days based on clinical response 1
For Severe Infections Requiring IV Therapy
- Adults: 8-12 mg/kg/day (based on trimethoprim component) divided into 4 doses IV 1, 2
- Pediatric: 8-12 mg/kg/day (based on trimethoprim component) divided into 4 doses IV 1
Critical Clinical Considerations
When to Use Bactrim
- Use for purulent cellulitis or abscesses where MRSA is suspected 1, 2
- Research demonstrates bactericidal activity against both MRSA and beta-hemolytic streptococci with good tissue penetration in soft tissue infections 3
- Clinical studies show similar efficacy between standard dose (160/800 mg) and high dose (320/1600 mg) twice daily, with 73-75% clinical resolution rates 4
When NOT to Use Bactrim Alone
- Do NOT use as monotherapy for non-purulent cellulitis where streptococci are the likely pathogens, as Bactrim has poor activity against beta-hemolytic streptococci 2
- Do NOT use for mixed aerobic-anaerobic wound infections without adding anaerobic coverage, as it lacks activity against anaerobes 2
- Contraindicated in third trimester pregnancy due to kernicterus risk and in nursing mothers 2
- Avoid in patients with sulfa allergies 2
Practical Dosing Algorithm
Step 1: Determine if infection is purulent or non-purulent
- Purulent (abscess, furuncle, carbuncle with MRSA risk) → Bactrim is appropriate 1
- Non-purulent (cellulitis without purulence) → Bactrim should NOT be used alone due to poor streptococcal coverage 2
Step 2: Choose dose based on severity
- Mild-moderate infection: Start with 1 double-strength tablet (160/800 mg) twice daily 1
- More severe infection or treatment failure: Consider 2 double-strength tablets (320/1600 mg) twice daily, though evidence shows similar efficacy to standard dosing 4, 2
Step 3: Duration
- Standard duration is 7 days, adjusted based on clinical response 1
Important Caveats
The 2014 IDSA guidelines note that while Bactrim is bactericidal, there is limited published efficacy data for soft tissue infections compared to other agents 1. However, more recent pharmacokinetic studies demonstrate excellent tissue penetration with trimethoprim achieving tissue/serum ratios of 1.2 and bactericidal activity against relevant pathogens 3. The guidelines consistently classify Bactrim as having "poorly documented efficacy" in earlier versions 1, but it remains a recommended option particularly for MRSA coverage in purulent infections 1, 2.