What is the recommended cephalexin dosing for adult cellulitis, including dosage for uncomplicated versus severe infection, duration of therapy, and adjustments for renal impairment or β‑lactam allergy?

Cephalexin Dosing for Adult Cellulitis

For uncomplicated nonpurulent cellulitis in adults, cephalexin 500 mg orally four times daily for 5-10 days is the recommended first-line treatment, targeting β-hemolytic streptococci as the primary pathogen. 1, 2

Standard Dosing Regimen

Uncomplicated Cellulitis (Nonpurulent)

• Dose: 500 mg orally four times daily (every 6 hours)
• Duration: 5-10 days, individualized based on clinical response 3, 4, 1
• Rationale: Nonpurulent cellulitis (no purulent drainage, exudate, or abscess) is primarily caused by β-hemolytic streptococci, making β-lactam antibiotics like cephalexin appropriate empiric therapy 3, 4

The 2014 IDSA guidelines specifically recommend that typical cases of cellulitis without systemic signs should receive an antimicrobial agent active against methicillin-susceptible S. aureus (MSSA), with cephalexin being a preferred oral option 1, 5. The FDA-approved dosing for skin and soft tissue infections is 250 mg every 6 hours, though 500 mg every 12 hours may be used for uncomplicated cases 2.

Severe or Complicated Cellulitis

• Consider higher doses: Emerging evidence suggests cephalexin 1000 mg four times daily may reduce treatment failure rates (12.9% vs 3.2% in one pilot study) 6
• Alternative: If oral therapy fails or patient has systemic toxicity (fever, tachycardia, hypotension), switch to IV therapy or add MRSA coverage 3, 4

When to Modify Therapy

Add MRSA Coverage If:

• Purulent drainage present (even without drainable abscess) 3, 4
• Failure to respond to β-lactam therapy after 48-72 hours 3, 4, 1
• Systemic toxicity or SIRS present 1
• Penetrating trauma, injection drug use, or known MRSA colonization 1

If MRSA coverage needed: Add trimethoprim-sulfamethoxazole (1-2 double-strength tablets twice daily) OR switch to clindamycin (300-450 mg four times daily) 3, 4, 1

Note: A large RCT found that adding trimethoprim-sulfamethoxazole to cephalexin did NOT improve cure rates for nonpurulent cellulitis (83.5% vs 85.5%), suggesting routine MRSA coverage is unnecessary for typical cases 7. However, in MRSA-prevalent areas, trimethoprim-sulfamethoxazole or clindamycin monotherapy showed superior outcomes to cephalexin alone (91% vs 74% success) 8.

Renal Impairment Adjustments

Cephalexin is renally excreted and requires dose adjustment:

• Normal renal function: No adjustment needed
• Impaired renal function: Reduce frequency or dose based on creatinine clearance 9
• Anephric patients: Single 250-500 mg doses produce prolonged high serum levels; extend dosing interval significantly 9
• Hemodialysis: 58% removed during 6-hour dialysis session; dose after dialysis 9

Specific dosing recommendations for various degrees of renal impairment are not provided in the FDA label, but clinical judgment should guide dose reduction in moderate-to-severe renal dysfunction.

β-Lactam Allergy Alternatives

For Penicillin/Cephalosporin Allergy:

• Non-immediate hypersensitivity: May cautiously use cephalexin (cross-reactivity ~1-3%)
• Immediate hypersensitivity (anaphylaxis, urticaria): Avoid all β-lactams 1, 5

Alternative antibiotics:

• Clindamycin 300-450 mg orally four times daily 1, 5
• Doxycycline 100 mg orally twice daily 1, 5, 10
• Trimethoprim-sulfamethoxazole 1-2 double-strength tablets twice daily (limited streptococcal activity; best combined with another agent or used when MRSA suspected) 1, 5

A recent study showed doxycycline had equivalent low failure rates (6%) compared to cephalexin for presumed streptococcal skin infections, suggesting it's a reasonable alternative in regions with low tetracycline resistance 10.

Key Clinical Pitfalls

1. Don't use first-generation cephalosporins for Lyme disease: Cephalexin is ineffective for erythema migrans despite resembling cellulitis 11

2. Duration matters with β-lactams: While 5 days may suffice, the shorter half-life of cephalexin compared to other antibiotics means 7-10 days is safer for complete eradication 3, 4

3. Obesity considerations: Patients ≥120 kg may benefit from higher doses (cefazolin 3g IV showed better outcomes in one study), though oral cephalexin dosing data in obesity is limited 12

4. Hospitalization criteria: Admit if SIRS present, altered mental status, hemodynamic instability, concern for necrotizing infection, or severe immunocompromise 1

5. Treatment failure definition: Lack of improvement by 48-72 hours (persistent fever, expanding erythema >25%, increased swelling/tenderness) warrants therapy modification 7