Cephalexin Dosing for Impetigo
For an otherwise healthy adult with impetigo, cephalexin should be dosed at 250 mg four times daily (every 6 hours) orally, as recommended by IDSA guidelines 1.
Standard Adult Dosing
The IDSA 2014 guidelines specifically list cephalexin 250 mg four times daily (qid) as the recommended dose for impetigo caused by Staphylococcus and Streptococcus 1. The FDA label confirms that the adult dosage range is 1-4 grams daily in divided doses, with the usual dose being 250 mg every 6 hours 2.
For more severe infections or less susceptible organisms, the dose can be increased to 500 mg four times daily 1, 2. The FDA label notes that 500 mg every 12 hours may be used for skin and skin structure infections, though the guideline-recommended qid dosing is more standard for impetigo 2.
Dosing in Renal Impairment
In patients with significant renal impairment (creatinine clearance <30 mL/min), cephalexin dosing must be reduced proportionally to the degree of renal dysfunction 2, 3.
Key Considerations for Renal Adjustment:
- Cephalexin is 70-100% renally excreted unchanged within 6-8 hours 3
- The drug depends heavily on both glomerular filtration AND active tubular secretion 4, 5
- Standard creatinine clearance-based adjustments may underestimate the need for dose reduction because tubular secretion is often disproportionately impaired compared to glomerular filtration 4, 5
Practical Approach:
- Monitor serum creatinine or calculate creatinine clearance 2, 3
- For CrCl <30 mL/min: Reduce dose or extend dosing interval proportionally 3
- Research suggests that in severe renal failure, the dosing interval may need to be prolonged up to 20 times normal (not just 5 times as simple CrCl calculations would suggest) 5
- In anephric patients, single doses of 250-500 mg produce high, prolonged serum levels with peaks sometimes delayed 6-12 hours due to altered absorption 6
- Hemodialysis removes approximately 58% of cephalexin over 6 hours 6
Common Pitfall:
Do not rely solely on creatinine clearance for dose adjustment with cephalexin. Because the drug requires active tubular secretion, patients with tubular dysfunction (common in chronic kidney disease) will have disproportionately reduced clearance 4, 5. Close clinical monitoring is essential in renal impairment 2.
Pediatric Dosing Context
For children, the dose is 25-50 mg/kg/day divided into 3-4 doses 1, 2. This can be given every 12 hours for impetigo specifically 1, 2.
Clinical Efficacy Note
Cephalexin demonstrates superior efficacy for impetigo compared to penicillin V (0% vs 24% treatment failure) and comparable efficacy to erythromycin (0% vs 4% failure) 7, making it an excellent first-line choice given that Staphylococcus aureus is the predominant pathogen in impetigo (62-81% of cases) 7.