What is the recommended duration of treatment with intravenous fosfomycin or oral trimethoprim‑sulfamethoxazole for a skin abscess?

Duration of Antibiotic Therapy for Skin Abscesses

For simple skin abscesses treated with incision and drainage, prescribe trimethoprim-sulfamethoxazole (Bactrim) for 7-10 days when antibiotics are indicated, with 10 days preferred for MRSA infections to reduce treatment failure and recurrence. 1, 2, 3

When Antibiotics Are Actually Needed

• Simple abscesses after incision and drainage do not require antibiotics if the infection is well-localized without surrounding cellulitis, systemic signs, or immunocompromise 4
• Antibiotics are indicated when:
  • Significant surrounding cellulitis extends beyond the abscess borders 4
  • Systemic signs present (fever, elevated white blood cell count) 4
  • Multiple abscess sites exist 2
  • Patient is immunocompromised 4
  • Source control is incomplete 4

Optimal Duration for MRSA Abscesses

The evidence strongly supports 10 days over shorter courses for MRSA:

• A pediatric randomized trial demonstrated that 3-day courses of trimethoprim-sulfamethoxazole resulted in significantly higher treatment failure rates (10.1% absolute increase, p=0.03) and more recurrent infections within 1 month (10.3% absolute increase, p=0.046) compared to 10-day courses specifically in MRSA abscesses 3
• The landmark multicenter trial by Daum et al. used 10-day courses of trimethoprim-sulfamethoxazole, which achieved 81.7% cure rates compared to 68.9% with placebo (p<0.001) in abscesses ≤5 cm 2
• Standard dosing is 1-2 double-strength tablets (160mg/800mg trimethoprim-sulfamethoxazole) twice daily for adults 1

Treatment Duration Recommendations by Clinical Scenario

For abscesses with confirmed or suspected MRSA:

• 10 days of trimethoprim-sulfamethoxazole after incision and drainage 2, 3
• This reduces both immediate treatment failure and recurrence at 1 month 3

For uncomplicated cellulitis (no abscess):

• 5 days is as effective as 10 days if clinical improvement occurs by day 5 4
• This shorter duration does NOT apply to abscesses 4

For mixed abscess plus cellulitis:

• 7-14 days individualized by clinical response 1
• Consider adding a beta-lactam (cephalexin or dicloxacillin) to cover streptococci when cellulitis is prominent 1

Critical Pitfalls to Avoid

• Do not use trimethoprim-sulfamethoxazole alone for pure cellulitis without abscess - it has poor activity against Group A Streptococcus, which commonly causes cellulitis 4, 1
• Do not stop antibiotics at 3-5 days for MRSA abscesses - this increases failure and recurrence rates 3
• Do not prescribe antibiotics for simple abscesses after adequate drainage unless the specific indications above are present 4
• Trimethoprim-sulfamethoxazole has poor anaerobic coverage, so it may not be ideal for perianal or other polymicrobial abscesses 1

Alternative Agents with Same Duration

If trimethoprim-sulfamethoxazole is contraindicated:

• Clindamycin 10 days - equally effective (83.1% cure rate) and may have lower recurrence rates (6.8% vs 13.5% with trimethoprim-sulfamethoxazole, p=0.03), but higher adverse event rates (21.9% vs 11.1%) 2, 5
• Doxycycline 10 days - comparable efficacy but must be combined with a beta-lactam if cellulitis is present 4, 1

Regarding Fosfomycin ("Foxy")

Fosfomycin is NOT indicated for skin abscesses - it is FDA-approved only for uncomplicated urinary tract infections and lacks evidence for soft tissue infections. The question likely refers to oral antibiotics for abscesses, where trimethoprim-sulfamethoxazole remains the evidence-based choice.