Bactrim Dosing for MSSA Step-Down Therapy
For step-down oral therapy of MSSA infections, use trimethoprim-sulfamethoxazole (TMP-SMX) 1-2 double-strength tablets (160/800 mg) twice daily, with the higher dose (2 double-strength tablets = 320/1600 mg twice daily) preferred for more serious infections requiring step-down from IV therapy. 1
Dosing Recommendations
Adults
- Standard dose: 1 double-strength tablet (160 mg TMP/800 mg SMX) twice daily 1
- Higher dose: 2 double-strength tablets (320 mg TMP/1600 mg SMX) twice daily 1
- Duration: 7-14 days depending on infection severity and clinical response 1
Pediatric Patients
- Dose: 8-12 mg/kg/day (based on trimethoprim component) divided into 2 doses orally 1
- Alternative dosing: Can be given in 4 divided doses IV if transitioning from parenteral therapy 1
Clinical Context and Evidence Strength
The evidence supporting TMP-SMX for MSSA is primarily extrapolated from MRSA guidelines, as IDSA guidelines specifically recommend it as an oral option for CA-MRSA skin and soft tissue infections. 1 While the guidelines note "efficacy poorly documented" and "limited published efficacy data" for TMP-SMX against staphylococcal infections 1, it remains a guideline-recommended option.
When to Use Higher vs. Standard Dosing
Use the higher dose (320/1600 mg twice daily) when:
- Stepping down from IV therapy for complicated skin and soft tissue infections 1
- Treating deeper infections, major abscesses, or cellulitis requiring hospitalization 1
- Patient has significant comorbidities or immunosuppression 1
The standard dose (160/800 mg twice daily) is appropriate for:
- Simple purulent cellulitis in outpatients 1
- Uncomplicated skin abscesses after drainage 2
- Less severe infections 1
Important Caveats
Research evidence shows no significant difference in clinical resolution between high-dose (320/1600 mg) and standard-dose (160/800 mg) TMP-SMX for MRSA skin infections (73% vs 75% cure rates, P=0.79). 3 However, this study was observational and focused on MRSA, not MSSA.
For MSSA specifically, beta-lactams (dicloxacillin 500 mg four times daily or cephalexin 500 mg four times daily) remain the preferred oral agents when susceptibility is confirmed. 1 TMP-SMX should be reserved for situations where beta-lactams cannot be used (allergy, intolerance, or specific clinical scenarios).
TMP-SMX lacks reliable activity against beta-hemolytic streptococci, so if streptococcal coverage is needed, combine with a beta-lactam (such as amoxicillin) or use an alternative agent like clindamycin. 1