Cephalexin Dosing Frequency
For most skin and soft tissue infections in adults, cephalexin should be dosed at 500 mg every 6 hours (four times daily), though 500 mg every 12 hours (twice daily) is acceptable for uncomplicated cases like simple cellulitis, streptococcal pharyngitis, and uncomplicated cystitis. 1
Adult Dosing by Indication
The FDA-approved dosing provides clear guidance 1:
- Standard dosing: 250 mg every 6 hours (four times daily)
- Moderate infections: 500 mg every 12 hours (twice daily) is specifically approved for:
- Streptococcal pharyngitis
- Skin and skin structure infections
- Uncomplicated cystitis (patients >15 years)
- Severe infections: Up to 4 g daily in divided doses; consider parenteral therapy if higher doses needed
Pediatric Dosing
Children require 25-50 mg/kg/day in divided doses 1:
- Standard infections: Can be divided every 6 hours (QID) or every 12 hours (BID)
- Otitis media: Requires 75-100 mg/kg/day in 4 divided doses
- Streptococcal infections: Continue for at least 10 days
Recent evidence supports less frequent dosing in children with bone/joint infections—45 mg/kg (maximum 1.5 g) three times daily achieved 99% cure rates 2.
Evidence for Reduced Frequency Dosing
Multiple recent studies challenge traditional four-times-daily dosing:
- Urinary tract infections: Two studies found no difference in treatment failure between twice-daily versus four-times-daily dosing (12.7% vs 17%, p=0.343 3; 18.7% vs 15.0%, p=0.465 4)
- Pediatric osteoarticular infections: TID versus QID dosing showed equivalent clinical cure rates with no failures in either group 5
The key advantage of less frequent dosing is improved adherence without compromising efficacy, particularly important for outpatient therapy.
Clinical Decision Algorithm
For mild-moderate infections (simple cellulitis, pharyngitis, uncomplicated UTI):
- Use 500 mg twice daily
- Improves adherence, reduces cost
- Supported by FDA labeling and recent clinical data
For moderate-severe infections (complicated SSTIs, suspected MSSA with systemic signs):
- Use 500 mg four times daily
- Optimizes PK/PD parameters 6
- Particularly important for S. aureus infections
For severe/life-threatening infections:
- Consider parenteral cephalosporins per IDSA guidelines 7
- Cefazolin 1 g every 8 hours IV for adults, 33 mg/kg/dose every 8 hours IV for children
Important Caveats
- Duration: Typically 5-7 days for most SSTIs; extend if not improving 7
- Streptococcal infections: Treat for minimum 10 days 1
- Cystitis: Continue 7-14 days 1
- Refrigeration: Oral suspension stable 14 days refrigerated 1
- MRSA coverage: Cephalexin is ineffective; use vancomycin, linezolid, or other MRSA-active agents if suspected 7
The shift toward twice-daily dosing for uncomplicated infections represents practical, evidence-based prescribing that balances pharmacology with real-world adherence challenges.