Cephalexin Dosing Frequency: Impetigo vs SSTI
For impetigo, cephalexin should be dosed at 250 mg four times daily (QID) in adults and 25-50 mg/kg/day divided into 3-4 doses in children, while for skin and soft tissue infections (SSTI), cephalexin is dosed at 500 mg QID in adults and 25-50 mg/kg/day in 4 divided doses in children.
Impetigo Dosing
The IDSA guidelines 1 and FDA labeling 2 are aligned on impetigo treatment:
- Adults: 250 mg QID orally
- Children: 25-50 mg/kg/day divided into 3-4 doses orally
The lower dose for impetigo reflects that this is a superficial skin infection primarily caused by Staphylococcus aureus and Streptococcus pyogenes 3. The guideline specifically lists this as the standard regimen for impetigo 1.
SSTI (Non-Impetigo) Dosing
For methicillin-susceptible S. aureus (MSSA) skin and soft tissue infections, the dosing is higher:
The FDA label 2 explicitly states that 500 mg every 12 hours (BID) may be used for "skin and skin structure infections" in adults, but the IDSA guideline 1 specifically recommends QID dosing for MSSA SSTI, which takes precedence as the authoritative clinical guidance.
Key Algorithmic Approach
Step 1: Identify the infection type
- Impetigo (superficial, honey-crusted lesions) → 250 mg QID adults
- Deeper SSTI (cellulitis, abscess, wound infection) → 500 mg QID adults
Step 2: Adjust for pediatric patients
- Both conditions: 25-50 mg/kg/day divided into doses
- Impetigo: Can divide into 3-4 doses
- SSTI: Divide into 4 doses
Step 3: Consider BID alternative dosing
- The FDA label 2 permits 500 mg BID for skin infections
- Research supports BID dosing effectiveness 4, 5
- However, this is not the guideline-recommended approach for MSSA SSTI 1
Important Caveats
MRSA Considerations: While cephalexin lacks in vitro activity against MRSA, clinical studies show surprisingly good outcomes 6, 7. However, this likely reflects the benefit of incision and drainage rather than antibiotic efficacy. For purulent infections with suspected MRSA, consider alternative agents like clindamycin or trimethoprim-sulfamethoxazole 1.
Dosing Frequency Trade-offs: While QID dosing is guideline-recommended 1, BID dosing improves compliance and has demonstrated comparable efficacy in research studies 4, 5. For impetigo specifically, twice-daily dosing showed equal effectiveness to four-times-daily dosing 5. However, the IDSA guideline takes precedence and recommends QID for SSTI 1.
Duration: Both impetigo and SSTI typically require 7-10 days of therapy 2. For streptococcal infections, ensure at least 10 days of treatment 2.
Severe Infections: If daily doses exceed 4 grams or the patient has severe infection with systemic signs, parenteral cephalosporins should be considered 2.