Cephalexin Dosing for Cellulitis
For adults with cellulitis, prescribe cephalexin 500 mg four times daily (every 6 hours) for 7-10 days. This is the standard dose recommended by both IDSA guidelines and FDA labeling for skin and soft tissue infections 1, 2.
Dosing Algorithm
Standard Adult Dosing
- Cephalexin 500 mg orally four times daily (QID) is the established dose for cellulitis 1, 2
- Treatment duration: 7-10 days minimum for adequate coverage of streptococcal infections 2
- The FDA label specifies the usual adult dose is 250 mg every 6 hours, but for skin and soft tissue infections, 500 mg doses are standard 2
Alternative Dosing Considerations
While the standard remains 500 mg QID, emerging evidence suggests potential alternatives:
High-dose option (1000 mg QID): A 2023 pilot RCT showed lower treatment failure rates (3.2% vs 12.9%) with 1000 mg four times daily compared to standard 500 mg dosing, though with more minor adverse effects 3. This is not yet standard practice but may be considered for severe infections or treatment failures.
Twice-daily dosing (500 mg BID): While FDA labeling mentions 500 mg every 12 hours as an option for "skin and skin structure infections" 2, and older studies from 1981 showed efficacy 4, the IDSA guidelines specifically recommend QID dosing for cellulitis 1. The four-times-daily regimen remains preferred for cellulitis specifically.
Key Clinical Points
MRSA Coverage Not Required
Multiple high-quality RCTs demonstrate that adding MRSA coverage (trimethoprim-sulfamethoxazole) to cephalexin provides no additional benefit for non-purulent cellulitis:
- The 2017 JAMA trial showed no significant difference in cure rates: 83.5% with combination therapy vs 85.5% with cephalexin alone 5
- A 2013 trial confirmed these findings with 85% vs 82% cure rates 6
- Do not routinely add MRSA coverage for typical cellulitis without purulence 1, 5, 6
Treatment Failure Rates
Expect clinical cure rates of 82-85% with standard cephalexin monotherapy 7, 5, 6. Treatment failure (defined as worsening symptoms, need for hospitalization, or antibiotic change) occurs in approximately 15-20% of cases 3, 5.
Common Pitfalls to Avoid
Don't underdose: The 250 mg QID dose is insufficient for cellulitis; use 500 mg QID 1, 2
Don't add unnecessary MRSA coverage: Unless there is purulent drainage, abscess, or systemic signs of severe infection, cephalexin alone is appropriate 1, 5, 6
Don't use twice-daily dosing as first-line: Despite convenience, QID dosing is the guideline-recommended regimen for cellulitis 1
Monitor for treatment failure: Reassess at 48-72 hours; lack of improvement warrants consideration of alternative diagnoses or resistant organisms 1
Consider drug interactions: Cephalexin can increase metformin levels by 24-34%; monitor patients on metformin closely 2
Pediatric Dosing
For children: 25-50 mg/kg/day divided into 4 doses (can be given every 12 hours for convenience in skin infections) 1, 2