Is Keflex (cephalexin) used to treat urinary tract infections?

Yes, Keflex (cephalexin) is used to treat UTIs, but it is not a first-line agent for uncomplicated cystitis in women.

For uncomplicated cystitis in women, cephalexin is listed as an alternative therapy rather than first-line treatment 1. The 2024 European Association of Urology guidelines recommend fosfomycin, nitrofurantoin, or pivmecillinam as first-line agents, with cephalosporins like cefadroxil (and by extension, cephalexin) reserved as alternatives when local E. coli resistance is <20% 1.

FDA-Approved Indication and Microbiology

Keflex is FDA-approved and active against the most common UTI pathogens 2:

• E. coli (the primary UTI pathogen)
• Klebsiella pneumoniae
• Proteus mirabilis

The drug achieves excellent urinary concentrations—following a 500 mg dose, peak urine levels reach approximately 2200 μg/mL, with over 90% excreted unchanged in urine within 8 hours 2. This pharmacokinetic profile makes it effective for UTI treatment.

Clinical Context and Positioning

For Women with Uncomplicated Cystitis:

• Use cephalexin 500 mg twice daily for 3 days when first-line agents are contraindicated or unavailable 1
• Only appropriate when local E. coli resistance to cephalosporins is <20% 1
• Recent comparative data shows cephalexin achieves 85.3% susceptibility coverage for lower UTI isolates 3

For Men with UTI:

• Men require 7-day treatment courses 1
• Trimethoprim-sulfamethoxazole is preferred, but cephalexin represents a reasonable alternative based on susceptibility testing

For Recurrent UTI Prevention:

• Postcoital prophylaxis with cephalexin 250 mg is highly effective, reducing UTI episodes from 127 infections over 6 months to just 1 infection over 12 months in sexually active women 4
• This approach uses only one-third the tablets required for daily prophylaxis while achieving identical results 4

Important Caveats

Resistance considerations: While cephalexin has good activity, it is not active against:

• Methicillin-resistant staphylococci
• Most enterococci
• Enterobacter species
• Pseudomonas
• Penicillin-resistant S. pneumoniae 2

Duration matters: Pediatric data reveals providers often overtreated lower UTIs with 8.3-day courses when guidelines recommend shorter durations 3. For uncomplicated cystitis, 3 days is sufficient 1.

Drug interactions: Cephalexin increases metformin levels by 24-34%, requiring dose adjustment and careful monitoring when co-administered 2.

Practical Algorithm

1. First, determine UTI type: uncomplicated cystitis vs. pyelonephritis vs. complicated UTI
2. For uncomplicated cystitis in women: Start with fosfomycin, nitrofurantoin, or pivmecillinam 1
3. Use cephalexin when:
   • First-line agents contraindicated
   • Local resistance data supports use (<20% resistance)
   • For postcoital prophylaxis in recurrent UTI 1, 4
4. Dose: 500 mg twice daily for 3 days (women) or 7 days (men) 1
5. For recurrent UTI prevention: 250 mg single dose postcoitally 4

The evidence shows cephalexin works for UTIs with comparable efficacy to other cephalosporins 5, but antimicrobial stewardship principles favor reserving it as an alternative to preserve first-line narrow-spectrum options 6.