Cephalexin Dosing and Treatment Duration
For adults with bacterial infections, cephalexin should be dosed at 500 mg four times daily (every 6 hours) for 7-10 days, with the specific duration determined by clinical response and infection type. 1, 2
Adult Dosing Regimens
Standard Dosing
- 500 mg every 6 hours (four times daily) is the recommended dose for most infections, including skin and soft tissue infections caused by methicillin-susceptible Staphylococcus aureus (MSSA) 1, 2
- For mild to moderate infections, 250 mg every 6 hours may be adequate 2
- The total daily dose ranges from 1-4 grams divided throughout the day 2
Alternative Dosing for Specific Infections
- For streptococcal pharyngitis, uncomplicated skin infections, and uncomplicated cystitis in patients over 15 years: 500 mg every 12 hours (twice daily) is acceptable 2
- Cystitis requires 7-14 days of treatment 2
- If doses exceeding 4 grams daily are needed, parenteral cephalosporins should be considered instead 2
Pediatric Dosing
Standard Pediatric Regimens
- For most infections: 25-50 mg/kg/day divided into 4 doses 2
- For MSSA infections: 75-100 mg/kg/day divided into 3-4 doses 1, 3
- For streptococcal pharyngitis and skin infections in children over 1 year: the total daily dose may be divided every 12 hours 2
Severe Infections
- For otitis media: 75-100 mg/kg/day in 4 divided doses is required 2
- In severe infections, the standard pediatric dosage may be doubled 2
Treatment Duration
- 7-10 days is typical for most infections, with duration adjusted based on clinical response 1
- β-hemolytic streptococcal infections require at least 10 days of treatment to prevent rheumatic fever 2
- Cystitis requires 7-14 days 2
Critical Microbiological Limitations
When Cephalexin Should NOT Be Used
- Cephalexin is completely ineffective against MRSA and alternative agents (trimethoprim-sulfamethoxazole, doxycycline, or clindamycin) must be used if MRSA is suspected 1, 3
- Poor activity against Haemophilus influenzae and Moraxella catarrhalis makes it inappropriate for sinusitis and many respiratory infections 1
- No activity against Pasteurella multocida, making it unsuitable for animal bite infections 3
- Inadequate anaerobic coverage limits its use in mixed infections 3
- No activity against Pseudomonas aeruginosa 1
Appropriate Spectrum
- Excellent activity against MSSA and streptococcal species 3, 4
- Good coverage of Escherichia coli, Klebsiella species, and Proteus mirabilis 4
Pharmacokinetic Considerations
- Cephalexin is rapidly and completely absorbed in the upper intestine, with peak serum levels of 7.7 mcg/mL after 250 mg and 12.3 mcg/mL after 500 mg 4
- 70-100% is excreted unchanged in urine within 6-8 hours, achieving urinary concentrations of 500-1000 mcg/mL 5
- Food delays but does not prevent absorption 4
- Renal dose adjustment is required when creatinine clearance is <30 mL/min 5