Antibiotic Treatment for Skin Infections
First-Line Oral Antibiotics for Uncomplicated Infections
For mild, non-purulent skin infections (cellulitis), cephalexin 500 mg every 6 hours or dicloxacillin 250-500 mg every 6 hours are the recommended first-line oral antibiotics, targeting streptococci and methicillin-susceptible Staphylococcus aureus (MSSA). 1, 2, 3
Standard Oral Regimens for Non-Purulent Infections
- Cephalexin: 500 mg four times daily (every 6 hours) for adults; 25-50 mg/kg/day in divided doses for children 1, 4
- Dicloxacillin: 250-500 mg four times daily for adults; 12 mg/kg/day in 4 divided doses for children 1, 3
- Amoxicillin-clavulanate: 875/125 mg twice daily as an alternative first-line option 1, 2
- Clindamycin: 300-450 mg three times daily for adults; 10-20 mg/kg/day in 3 divided doses for children (useful in penicillin allergy) 1, 3
Treatment duration is 5 days if clinical improvement occurs, with extension if no improvement is seen. 1, 2
MRSA Coverage: When and What to Use
Indications for MRSA-Active Antibiotics
Add MRSA coverage when: 2
- Purulent drainage is present
- Penetrating trauma or injection drug use history
- Evidence of MRSA elsewhere on the body
- Known nasal MRSA colonization
- Failed initial therapy with beta-lactams
MRSA-Active Oral Options
- Trimethoprim-sulfamethoxazole: 1-2 double-strength tablets twice daily 1, 2, 3
- Doxycycline: 100 mg twice daily (avoid in children <8 years) 1, 3
- Clindamycin: 300-450 mg three times daily 1, 3
Note: Clindamycin has potential for cross-resistance with erythromycin-resistant strains and inducible resistance in MRSA, so use cautiously. 1
Purulent Infections (Abscesses, Furuncles)
Incision and drainage is the primary treatment for purulent infections, with antibiotics serving as adjunctive therapy only. 2
When to Add Antibiotics to Drainage
Antibiotics are indicated when: 2
- Fever or systemic signs present
- Multiple lesions found
- Patient is immunocompromised
- Drainage alone has failed
Use MRSA-active antibiotics (trimethoprim-sulfamethoxazole, doxycycline, or clindamycin) for purulent infections requiring antibiotic therapy. 2
Severe Infections Requiring IV Therapy
Hospitalization Criteria
- Systemic inflammatory response syndrome (SIRS)
- Hemodynamic instability
- Altered mental status
- Concern for necrotizing infection
- Severe immunocompromise
- Failed outpatient therapy
IV Antibiotic Regimens
For severe MSSA infections:
- Nafcillin or oxacillin: 1-2 g every 4 hours IV (parenteral drug of choice) 1, 3
- Cefazolin: 1 g every 8 hours IV (for penicillin-allergic patients without immediate hypersensitivity) 1, 2
For severe MRSA infections:
- Vancomycin: 15-20 mg/kg every 8-12 hours IV (parenteral drug of choice for MRSA) 1, 2
- Linezolid: 600 mg every 12 hours IV or PO (bacteriostatic, expensive, no cross-resistance) 1, 3
- Daptomycin: 4 mg/kg every 24 hours IV (bactericidal, monitor for myopathy) 1
For severe polymicrobial or necrotizing infections:
- Vancomycin plus piperacillin-tazobactam (3.375 g every 6 hours or 4.5 g every 8 hours IV) 1, 5
- Vancomycin plus imipenem/meropenem (imipenem 500 mg every 6 hours IV or meropenem 1 g every 8 hours IV) 1, 5
- Ceftriaxone plus metronidazole (with or without vancomycin) for mixed aerobic-anaerobic infections 1
Special Situations
Impetigo (Superficial Infection)
For limited lesions: 1
- Mupirocin ointment: Apply to lesions 3 times daily (topical therapy sufficient)
For widespread impetigo requiring systemic therapy: 1
- Dicloxacillin 250 mg four times daily
- Cephalexin 250 mg four times daily
- Erythromycin 250 mg four times daily (if susceptible)
Necrotizing Fasciitis
Prompt surgical consultation is mandatory for suspected necrotizing fasciitis. 1
- For documented Group A Streptococcal infection: Penicillin plus clindamycin 1, 5
- For polymicrobial necrotizing infection: Clindamycin plus piperacillin-tazobactam (with or without vancomycin) 1
Clindamycin is critical in toxin-mediated infections because it inhibits bacterial protein synthesis, reducing toxin production. 1
Diabetic Foot Infections
Mild infections: 1
- Dicloxacillin, clindamycin, cephalexin, levofloxacin, or amoxicillin-clavulanate
- Add trimethoprim-sulfamethoxazole if MRSA suspected
Moderate to severe infections: 1
- Levofloxacin, ceftriaxone, ampicillin-sulbactam, ertapenem, or imipenem-cilastatin
- Add linezolid, daptomycin, or vancomycin if MRSA confirmed
Animal or Human Bites
- Amoxicillin-clavulanate: 875/125 mg twice daily PO 3
- Ampicillin-sulbactam: 1.5-3.0 g every 6-8 hours IV for severe infections 3
Recurrent Infections
For patients with 3-4 episodes of cellulitis per year despite treating predisposing factors, prophylactic antibiotics should be considered. 1
Prophylactic Regimens
- Oral penicillin or erythromycin twice daily for 4-52 weeks 1
- Intramuscular benzathine penicillin every 2-4 weeks 1
Decolonization for Recurrent MRSA
- Intranasal mupirocin twice daily for 5 days 2
- Daily chlorhexidine washes 2
- Daily decontamination of personal items 2
Critical Pitfalls to Avoid
- Do not use oral anti-staphylococcal penicillins (nafcillin, oxacillin) due to poor oral bioavailability—use dicloxacillin or flucloxacillin instead for oral therapy. 1
- Avoid cephalosporins in patients with immediate penicillin hypersensitivity (urticaria, angioedema, bronchospasm, anaphylaxis). 6
- Do not use rifampicin or fusidic acid as single agents for MRSA—resistance develops rapidly; always use combination therapy. 6
- Examine interdigital toe spaces in lower-extremity cellulitis—treating toe web abnormalities (fissuring, scaling, maceration) reduces recurrence risk. 1
- For streptococcal infections, treat for minimum 10 days to prevent rheumatic fever. 3