Dose Conversion from Keflex to Duricef
When transitioning from Keflex (cephalexin) 500 mg to Duricef (cefadroxil), use cefadroxil 500 mg twice daily 1, 2.
Rationale for Equivalent Dosing
Cephalexin and cefadroxil are both first-generation oral cephalosporins with comparable antimicrobial activity and clinical efficacy, but differ primarily in their pharmacokinetic profiles 3, 2, 4:
MIC distributions are equivalent: Both drugs have identical MIC50 (2 μg/mL) and MIC90 (4 μg/mL) values against methicillin-susceptible Staphylococcus aureus (MSSA), indicating similar in vitro potency 4
Cefadroxil has a longer half-life (approximately 2 hours vs. 1 hour for cephalexin), allowing for less frequent dosing 3, 2
Sustained serum and urine levels: Cefadroxil maintains therapeutic concentrations for longer periods despite lower peak concentrations compared to cephalexin 2
Specific Dosing Recommendation
For the patient currently on cephalexin 500 mg (typically given 4 times daily), switch to cefadroxil 500 mg twice daily 5, 6, 2.
Key Dosing Principles:
Standard adult dose: Cefadroxil 500 mg BID is clinically equivalent to cephalexin 500 mg QID for most infections 5, 6
Bioavailability is similar: Both drugs achieve >80-90% urinary excretion, with comparable systemic availability 3
Dose-proportional pharmacokinetics: Both drugs demonstrate linear kinetics, so higher doses (up to 1000 mg) can be used if clinically indicated 1, 3
Clinical Context Matters:
For mild-to-moderate skin and soft tissue infections (as suggested by IDSA guidelines using cephalexin 500 mg QID) 7, cefadroxil 500 mg BID is appropriate 5, 6.
For more severe infections or MSSA musculoskeletal infections, consider higher dosing: cefadroxil 1000 mg BID may be needed to optimize pharmacodynamic targets 8, 9.
Important Caveats:
Renal function: Both drugs are renally excreted; dose adjustment may be needed in renal impairment 10, 1
Adherence advantage: The twice-daily dosing of cefadroxil improves compliance compared to four-times-daily cephalexin 5, 11
Clinical equivalence proven: Multiple studies demonstrate equal efficacy for UTIs, pneumonia, and SSTIs when comparing cefadroxil BID to cephalexin QID 5, 6