Bactrim DS Dosing for Abscess
For treating a skin abscess, the recommended dose is 1-2 double-strength tablets (160 mg trimethoprim/800 mg sulfamethoxazole per tablet) twice daily for 7-10 days.
Recommended Dosing Regimen
The standard adult dose is 1-2 DS tablets (160-320 mg trimethoprim/800-1600 mg sulfamethoxazole) twice daily 1. This translates to:
- Standard dose: 1 DS tablet (160/800 mg) BID
- High dose: 2 DS tablets (320/1600 mg) BID
Duration: 7-10 days depending on clinical response 1.
Which Dose Should You Choose?
The evidence strongly supports that both doses are equally effective for uncomplicated skin abscesses. A prospective observational study of 291 patients with MRSA skin infections found no difference in clinical resolution between high-dose (320/1600 mg BID) and standard-dose (160/800 mg BID) therapy—73% vs 75% cure rates respectively (P=0.79) 2.
Start with the standard dose (1 DS tablet BID) for most patients, as it provides equivalent efficacy with fewer side effects. Reserve the high dose for:
- Patients with systemic signs of infection
- Those with diabetes or immunocompromise
- History of recurrent MRSA infections
- Larger abscesses (>5 cm)
Critical Context: When Antibiotics Are Actually Needed
Incision and drainage alone is often sufficient for simple abscesses, but antibiotics provide measurable benefit. A landmark randomized trial of 1,247 patients demonstrated that trimethoprim-sulfamethoxazole after drainage achieved 92.9% clinical cure versus 85.7% with placebo (P<0.001) 3. More importantly, antibiotics reduced:
- Subsequent surgical drainage procedures: 3.4% vs 8.6%
- New skin infections: 3.1% vs 10.3%
- Household member infections: 1.7% vs 4.1%
These benefits persisted across all abscess sizes and regardless of whether patients met traditional guideline criteria for antibiotics 4.
Pediatric Dosing
For children, dose based on the trimethoprim component: 8-12 mg/kg/day of trimethoprim in 2 divided doses (oral) or 4 divided doses (IV) 1.
Important Caveats
- MRSA coverage: In settings where MRSA prevalence is high (>45% of cultured abscesses), trimethoprim-sulfamethoxazole is an appropriate first-line choice 3.
- Side effects: The most common adverse effect is mild gastrointestinal upset. High-dose therapy has higher rates of dose adjustment or discontinuation (66.7% vs 24-27% for lower doses in other infections) 5.
- Bactericidal activity: Unlike clindamycin (which is bacteriostatic), TMP-SMX is bactericidal against staphylococci 1.
- Efficacy concerns: The IDSA guidelines note that efficacy data for TMP-SMX in skin infections is "poorly documented" or "limited" 1, but subsequent high-quality RCTs have filled this evidence gap 3, 4.
When NOT to Use Bactrim Alone
Avoid TMP-SMX monotherapy for:
- Non-purulent cellulitis (lacks streptococcal coverage—add beta-lactam)
- Severe infections requiring hospitalization (use vancomycin IV)
- Patients with sulfa allergy (use clindamycin or doxycycline instead)