Keflex (Cephalexin) Dosing and Treatment Duration
Standard Adult Dosing
For most skin and soft tissue infections in adults, cephalexin should be dosed at 500 mg four times daily (every 6 hours) for 7-10 days. 1, 2
Dosing by Infection Type
Skin and Soft Tissue Infections (MSSA):
- 500 mg four times daily (every 6 hours) is the recommended regimen for methicillin-susceptible Staphylococcus aureus infections 1, 2
- For mild to moderate infections, 250-500 mg every 6 hours may be sufficient 2
- Treatment duration is typically 7-10 days depending on clinical response 2, 3
Uncomplicated Cystitis:
- 500 mg every 12 hours (twice daily) for patients over 15 years of age 4
- Continue therapy for 7-14 days 4
Streptococcal Pharyngitis:
- 500 mg every 12 hours (twice daily) 4
- Must treat for at least 10 days for β-hemolytic streptococcal infections 4
Mastitis:
- 500 mg orally every 6 hours (four times daily) for lactating women 2
Pediatric Dosing
Standard Dosing:
- 25-50 mg/kg/day divided into 4 doses for mild to moderate infections 2, 4
- 75-100 mg/kg/day divided into 3-4 doses for MSSA infections 1, 2
Otitis Media:
- Requires 75-100 mg/kg/day in 4 divided doses 4
Alternative Dosing for Compliance:
- For streptococcal pharyngitis and skin infections in children over 1 year, the total daily dose may be divided and given every 12 hours 4
Critical Limitations and When NOT to Use Cephalexin
Cephalexin is completely ineffective against:
- MRSA - use trimethoprim-sulfamethoxazole, doxycycline, or clindamycin instead 1, 3
- Pasteurella multocida (animal bites) 1, 2
- Eikenella corrodens (human bites) 2
- Pseudomonas aeruginosa 2
- Most anaerobes 1, 2
Poor Activity Against:
- Haemophilus influenzae - do not use for sinusitis or many respiratory infections 2
- Beta-lactamase-producing Moraxella catarrhalis 2
Severe Infections
If daily doses exceeding 4 grams are required, switch to parenteral cephalosporins (e.g., cefazolin 1 g IV every 8 hours) 2, 4
Allergy Considerations
- Patients with immediate hypersensitivity reactions to penicillin should avoid cephalexin due to cross-reactivity 1
- For non-immediate penicillin reactions, cephalexin remains appropriate 1
- Contraindicated in patients with history of anaphylaxis, angioedema, or urticaria to penicillins 2
Treatment Duration Algorithm
Standard infections: 7-10 days 2, 3
If no improvement within 5 days: Extend treatment and consider broadening coverage 3
β-hemolytic streptococcal infections: Minimum 10 days 4
Uncomplicated cystitis: 7-14 days 4
Renal Dosing
Patients with creatinine clearance less than 30 mL/min require dose reduction proportional to reduced renal function 5