What is the most up-to-date dosing of cephalexin (Cephalexin) for an adult patient with a normal renal function and an uncomplicated urinary tract infection (UTI)?

Cephalexin Dosing for Uncomplicated UTI in Adults

For uncomplicated urinary tract infections in adults with normal renal function, cephalexin should be dosed at 500 mg orally every 12 hours for 7 to 14 days. 1

Standard Dosing Regimen

• The FDA-approved dosing for uncomplicated cystitis is 500 mg every 12 hours, with therapy continued for 7 to 14 days 1
• The Infectious Diseases Society of America recommends cephalexin 500 mg orally twice daily for 7 days as the standard duration for uncomplicated UTIs 2
• Recent high-quality evidence demonstrates that twice-daily dosing (500 mg BID) is equally effective as four-times-daily dosing (500 mg QID) for uncomplicated UTIs, with no difference in treatment failure rates (12.7% vs 17%, P = 0.343) 3

Key Clinical Considerations

Cephalexin achieves urinary concentrations of 500-1000 mcg/mL following 250-500 mg oral doses, which far exceeds the minimum inhibitory concentration for common uropathogens 4. This makes it highly effective for UTI treatment despite being considered an alternative rather than first-line agent by current guidelines 3.

Advantages of Twice-Daily Dosing

• Twice-daily dosing improves patient adherence compared to four-times-daily regimens without compromising efficacy 3
• No difference in adverse events between BID (4.6%) and QID (5.6%) dosing schedules 3
• Cephalexin is completely absorbed in the upper intestine and does not disturb lower bowel flora 4

Important Caveats and Pitfalls

When NOT to Use Cephalexin

• Do not use cephalexin for febrile UTIs or suspected pyelonephritis, as agents that achieve therapeutic blood concentrations (not just urinary concentrations) are required for parenchymal infections 5
• Avoid in patients with known local resistance patterns showing high cephalexin resistance among E. coli isolates 5

Renal Dosing Adjustments

• Patients with creatinine clearance 30-50 mL/min can receive standard doses with close monitoring 2
• Patients with creatinine clearance <30 mL/min require proportional dose reduction based on degree of renal impairment 4
• In anephric patients, single doses produce high, prolonged serum concentrations that may persist for 6-12 hours 6

Pre-Treatment Requirements

• Obtain urine culture with antibiotic sensitivity testing before starting treatment, especially in patients with altered renal function or recurrent infections 2
• Evaluate clinical response at 48-72 hours; if no improvement occurs, obtain repeat culture and consider alternative antibiotics 2

Alternative Considerations

While cephalexin is effective, first-line agents remain preferred when local resistance is acceptable:

• Trimethoprim-sulfamethoxazole is first-line when local resistance is <20% 3
• Fluoroquinolones (ciprofloxacin 500 mg twice daily for 7 days) are first-line when local resistance is <10% 2
• For men with UTI, treatment duration should be extended to 14 days due to the complicated nature of male UTIs and potential prostatic involvement 7