Bactrim Dosing for MSSA Step-Down Therapy
For step-down therapy of MSSA infections, use trimethoprim-sulfamethoxazole 2 double-strength tablets (320/1600 mg) twice daily for 7-14 days, though beta-lactams remain the preferred oral agents when MSSA susceptibility is confirmed. 1
Recommended Dosing Regimen
Adult Dosing:
- Standard dose: 1 double-strength tablet (160/800 mg) twice daily 1
- High dose (preferred for step-down from IV therapy): 2 double-strength tablets (320/1600 mg) twice daily 1
- Duration: 7-14 days depending on infection severity and clinical response 1
Pediatric Dosing:
- 8-12 mg/kg/day (based on trimethoprim component) divided into 2 doses 1
Clinical Decision Algorithm
When to use the higher dose (320/1600 mg twice daily):
- Serious infections requiring step-down from IV therapy 1
- Infections with higher bacterial burden
- When transitioning from parenteral antibiotics
When standard dose may suffice (160/800 mg twice daily):
Note that observational data showed no difference in clinical resolution between high-dose (320/1600 mg) and standard-dose (160/800 mg) for MRSA skin infections (73% vs 75% cure rates), though this was for MRSA, not MSSA 2
Critical Caveats for MSSA
Beta-lactams are preferred over TMP-SMX for MSSA: When susceptibility is confirmed, dicloxacillin or cephalexin remain the first-line oral agents 1. TMP-SMX should be reserved for situations where beta-lactams cannot be used (allergy, intolerance, or resistance).
Lack streptococcal coverage: TMP-SMX has no reliable activity against beta-hemolytic streptococci 1. If streptococcal coverage is needed, combine with a beta-lactam or use clindamycin instead 1.
Limited evidence base: The efficacy of TMP-SMX against staphylococcal infections is poorly documented, with recommendations primarily extrapolated from MRSA guidelines rather than MSSA-specific data 1