Cephalexin (Keflex) for Cellulitis
For uncomplicated cellulitis in adults without penicillin allergy or renal impairment, prescribe cephalexin 500 mg orally four times daily for 7 days. This represents the standard evidence-based approach supported by both FDA labeling and IDSA guidelines 1, 2.
Dosing Regimen
Standard dose: 500 mg orally four times daily (QID) for 7 days 1, 2
- The FDA label explicitly states that for skin and skin structure infections, 500 mg every 12 hours may be administered 2
- However, the IDSA guidelines specifically recommend 500 mg QID for MSSA skin and soft tissue infections 1
- The QID dosing is preferred as it maintains more consistent therapeutic levels and aligns with guideline recommendations for moderate infections
Alternative Dosing Considerations
A 2023 pilot RCT suggests high-dose cephalexin (1000 mg QID) may reduce treatment failure rates (3.2% vs 12.9% with standard dosing), though this came with increased minor adverse effects 3. This remains investigational and is not yet guideline-recommended.
Twice-daily dosing (500 mg BID) has shown equivalent efficacy in older studies 4, but QID dosing remains the standard of care per current guidelines 1.
Duration
7 days is the standard duration for uncomplicated cellulitis 2. The FDA label allows flexibility from 7-14 days for cystitis, but for cellulitis specifically, 7 days is appropriate unless clinical response is inadequate.
Key Precautions and Monitoring
When Cephalexin May NOT Be Appropriate
- Immediate hypersensitivity to penicillins: Cephalexin should be avoided in patients with history of anaphylaxis, angioedema, or urticaria to penicillins 1
- Severe infection with SIRS criteria: Temperature >38°C or <36°C, tachycardia >90 bpm, tachypnea >24 breaths/min, or WBC >12,000 or <4,000 cells/µL requires MRSA coverage 1
- Purulent drainage or abscess: These require incision/drainage and consideration of MRSA-active antibiotics 1
Clinical Pitfalls to Avoid
Do not add trimethoprim-sulfamethoxazole routinely. A large 2017 RCT (n=496) demonstrated that adding TMP-SMX to cephalexin for uncomplicated cellulitis provided no benefit—clinical cure was 83.5% with combination vs 85.5% with cephalexin alone (difference -2.0%, 95% CI -9.7% to 5.7%) 5. Reserve MRSA coverage for patients meeting specific criteria above.
Perform ultrasound if abscess is suspected. The 2017 trial excluded abscesses via ultrasound screening 5, emphasizing that cephalexin monotherapy is only appropriate for true non-purulent cellulitis.
Expected Treatment Failure Rate
With appropriate patient selection, expect treatment failure rates of 3-13% 3, 5. Failure is defined as:
- Fever persisting beyond 3-4 days
- Increase in erythema (>25%), swelling, or tenderness at days 3-4
- No decrease in symptoms by days 8-10
- More than minimal symptoms at days 14-21 5
Comparative Effectiveness
Cephalexin performs equivalently to:
- IV cefazolin plus oral probenecid (failure rate 4.2% vs 6.1%) 6
- Doxycycline in regions with low tetracycline resistance (6% failure rate each) 7
Cephalexin remains the oral agent of choice for methicillin-susceptible strains per IDSA guidelines 1, with >95% clinical response rates in well-selected patients 8.
Renal Dosing
While not explicitly requested, note that the FDA label does not provide specific renal dosing adjustments in the excerpts provided 2, but standard practice requires dose reduction in severe renal impairment (CrCl <30 mL/min).