Bactrim Dosing for Skin Infections
For uncomplicated skin and soft tissue infections in adults, prescribe Bactrim 1-2 double-strength tablets (800mg/160mg sulfamethoxazole/trimethoprim) twice daily for 7-10 days. 1
Adult Dosing Recommendations
Oral Therapy (Outpatient)
- Standard dose: 1-2 double-strength tablets (DS: 800mg/160mg) twice daily by mouth 1
- Treatment duration: 7-10 days, with 10 days preferred for MRSA infections to reduce treatment failure and recurrence 1, 2, 3
- The higher dose (2 DS tablets twice daily = 320mg/1600mg) does not improve clinical outcomes compared to standard dosing (1 DS tablet twice daily = 160mg/800mg) for most skin infections 4
Intravenous Therapy (Severe Infections)
- Dose: 8-12 mg/kg/day (based on trimethoprim component) divided into 4 doses 1
- Consider IV therapy for complicated or severe infections requiring inpatient management 1
Pediatric Dosing
- Oral or IV: 8-12 mg/kg/day (based on trimethoprim component) divided into 2 doses (oral) or 4 doses (IV) 1
- Contraindication: Do not use in children under 2 months of age 1
- Duration: 10 days for MRSA abscesses is superior to 3 days in reducing treatment failure (10.1% rate difference) and recurrence (10.3% rate difference) 3
Clinical Context and Appropriate Use
When Bactrim is Optimal
- Purulent cellulitis or abscesses where MRSA is suspected or confirmed 1
- Post-drainage therapy for skin abscesses, particularly in MRSA-prevalent settings where it increases cure rates by 6.9% compared to drainage alone 2
- Empiric coverage when beta-lactam therapy has failed 1
Critical Limitations Requiring Combination Therapy
- Non-purulent cellulitis: Bactrim has poor activity against beta-hemolytic streptococci and should be combined with a beta-lactam (e.g., amoxicillin) when streptococcal coverage is needed 1, 5
- Animal/human bites: Bactrim (SMX-TMP 160-800mg bid) has good aerobic activity but poor anaerobic coverage; combine with metronidazole or use amoxicillin-clavulanate instead 5
Important Safety Considerations
Contraindications and Precautions
- Pregnancy: Category C/D; avoid in third trimester 1
- Sulfa allergies: Consider alternative agents (clindamycin, doxycycline, linezolid) 1
- Drug interactions: Monitor carefully when combining with methotrexate, as trimethoprim/sulfamethoxazole at treatment doses (not prophylactic doses) can cause severe interactions 5
Adverse Effects
- Gastrointestinal side effects are common but typically mild 2
- Monitor for severe hypersensitivity reactions, particularly when combined with other immunosuppressive agents 5
Pharmacokinetic Advantages
- Excellent tissue penetration with trimethoprim tissue/serum ratio of 1.2 (range 0.3-4.5) 6
- Demonstrates bactericidal activity (>3 log kill) against both S. aureus and beta-hemolytic streptococci in diabetic foot infections 6
- Effective against MRSA with good clinical outcomes in outpatient settings 2