Cephalexin Dosing for Bacterial Infections
For adults with skin and soft tissue infections, cephalexin 500 mg four times daily is the standard dose, while pediatric patients require 25-50 mg/kg/day divided into four doses. 1
Adult Dosing
The FDA-approved adult dosage ranges from 1 to 4 grams daily in divided doses. 1
Standard Regimens by Indication:
- Skin and soft tissue infections (MSSA): 500 mg four times daily (QID) 2, 1
- Streptococcal pharyngitis: 500 mg every 12 hours 1
- Uncomplicated cystitis (patients >15 years): 500 mg every 12 hours for 7-14 days 1
- Routine infections: 250 mg every 6 hours 1
For more severe infections or less susceptible organisms, larger doses may be needed, but if daily doses exceed 4 grams, parenteral cephalosporins should be considered. 1
Recent Evidence on Dosing Frequency:
A 2023 study demonstrated that cephalexin 500 mg twice daily is as effective as four-times-daily dosing for uncomplicated urinary tract infections, with no difference in treatment failure rates (12.7% vs 17%, P=0.343) and may improve patient adherence. 3 However, this applies specifically to UTIs, not other infections.
Pediatric Dosing
The usual recommended daily dosage for pediatric patients is 25-50 mg/kg/day in divided doses. 1
Age-Specific Considerations:
- Infants 29-60 days old with UTI: 50-100 mg/kg/day divided into 4 doses 2
- Streptococcal pharyngitis and skin infections (>1 year): Total daily dose may be divided every 12 hours 1
- Otitis media: 75-100 mg/kg/day in 4 divided doses is required 1
- Severe infections: Dosage may be doubled 1
For β-hemolytic streptococcal infections, therapeutic dosage must be administered for at least 10 days. 1
Duration of Therapy
The typical treatment duration for uncomplicated skin infections is 7 days, depending on clinical response. 4 If the infection has not improved within 5 days, treatment should be extended. 4
Critical Limitations and Pitfalls
MRSA Coverage:
Cephalexin is NOT effective against MRSA; if MRSA is suspected or confirmed, alternative agents (vancomycin, linezolid, clindamycin, or daptomycin) must be used. 2, 4 This is a common prescribing error that can lead to treatment failure.
Limited Pathogen Coverage:
- Cephalexin has limited activity against Pasteurella multocida and anaerobes, making it inappropriate for animal bites without additional coverage. 4
- For Haemophilus influenzae infections, cephalexin fails in approximately 50% of cases, particularly in otitis media. 5
Penicillin Allergy:
Patients with immediate hypersensitivity reactions to penicillin should use cephalexin cautiously due to potential cross-reactivity. 2, 4 However, for non-immediate reactions, cephalexin is an acceptable alternative. 2
Renal Dosing Adjustments
Patients with creatinine clearance <30 mL/min require dosage reduction proportional to their reduced renal function. 6 Cephalexin is 70-100% renally excreted within 6-8 hours. 6
Clinical Pharmacology
Cephalexin achieves urinary concentrations of 500-1000 mcg/mL following 250-500 mg oral doses, far exceeding the minimum inhibitory concentration for typical urinary pathogens. 6 It is rapidly and completely absorbed in the upper intestine, with minimal gastrointestinal disturbance since it does not affect lower bowel flora. 6