What is the first-line oral antibiotic treatment for impetigo in adults?

First-Line Oral Antibiotic Treatment for Impetigo in Adults

For adults with impetigo, dicloxacillin or cephalexin is the recommended first-line oral antibiotic treatment when methicillin-susceptible Staphylococcus aureus (MSSA) is suspected, which is the most common scenario. 1

Pathogen Considerations

Impetigo is primarily caused by:

• Staphylococcus aureus (most common)
• Streptococcus pyogenes (Group A Streptococcus)
• Sometimes both organisms simultaneously

First-Line Oral Antibiotic Options

For suspected MSSA (most common scenario):

• Dicloxacillin (recommended dosing: 250-500 mg four times daily)
• Cephalexin (recommended dosing: 500 mg three to four times daily)

Both options are equally effective for treating staphylococcal skin infections 2, though cephalexin may offer the advantage of twice-daily dosing in some cases, potentially improving medication adherence.

For suspected or confirmed MRSA:

If methicillin-resistant S. aureus (MRSA) is suspected or confirmed, alternative first-line options include:

• Doxycycline
• Clindamycin
• Trimethoprim-sulfamethoxazole (SMX-TMP)

Treatment Duration

The standard treatment duration is 7 days for impetigo 1. This is sufficient in most cases to eliminate the infection and prevent complications.

Special Considerations

• Streptococcal-only infections: If cultures yield streptococci alone (uncommon), oral penicillin is the recommended agent 1
• Penicillin ineffectiveness: Standard penicillin is generally not effective for impetigo due to the high prevalence of penicillin-resistant S. aureus 3, 4
• Poststreptococcal glomerulonephritis risk: Systemic antimicrobials should be used during outbreaks of poststreptococcal glomerulonephritis to eliminate nephritogenic strains of S. pyogenes 1

Treatment Algorithm

1. Limited disease (few lesions):

   • Consider topical antibiotics (mupirocin, retapamulin, or fusidic acid) 3

2. Extensive disease (multiple lesions or widespread):

   • Use oral antibiotics:
     • First-line: Dicloxacillin or cephalexin
     • If MRSA suspected: Doxycycline, clindamycin, or SMX-TMP

3. Treatment failure after 72 hours:

   • Reassess diagnosis
   • Consider culture and sensitivity testing
   • Switch to alternative antibiotic based on suspected resistance pattern

Clinical Pearls

• Impetigo typically resolves within 2-3 weeks without scarring 3
• Complications are rare, with poststreptococcal glomerulonephritis being the most serious 3
• Rising rates of antibiotic resistance should be considered when selecting treatment 5
• Oral penicillin V is seldom effective and should not be used as first-line therapy 4
• Topical disinfectants are inferior to antibiotics and should not be used 3, 4

Monitoring Response

Assess for clinical improvement within 72 hours of starting treatment, including:

• Decreasing erythema, warmth, and swelling
• Resolution of fever and systemic symptoms
• Improvement in pain 6

If no improvement after 4-7 days, investigate with new cultures, including both aerobic and anaerobic cultures for persistent infections 6.