What is the recommended dose of Keflex (cephalexin) for an adult patient with an uncomplicated urinary tract infection (UTI)?

Keflex Dosing for Uncomplicated UTI

For uncomplicated cystitis in adults, the FDA-approved dose of cephalexin (Keflex) is 500 mg every 12 hours for 7 to 14 days, though cephalexin should be considered an alternative rather than first-line agent. 1

Treatment Positioning and Selection Criteria

Cephalexin is classified as an alternative agent for uncomplicated UTI, not first-line therapy, due to inferior efficacy and more adverse effects compared to preferred antimicrobials. 2, 3

First-line agents you should use instead include:

• Fosfomycin trometamol 3 g single dose 2
• Nitrofurantoin 100 mg twice daily for 5 days 2
• Pivmecillinam 400 mg three times daily for 3-5 days 2

Only use cephalexin when:

• Local E. coli resistance to β-lactams is <20% 2, 3
• First-line agents cannot be used due to allergy, intolerance, or unavailability 3

Specific Dosing Regimen

The FDA-approved dosing is 500 mg every 12 hours (twice daily) for 7 to 14 days for uncomplicated cystitis in patients over 15 years of age. 1

Recent evidence demonstrates that 500 mg twice daily is equally effective as 500 mg four times daily, with no difference in treatment failure rates (12.7% vs 17%, p=0.343) or adverse events. 4 The twice-daily regimen improves adherence and should be preferred. 4

Alternative dosing of 250 mg every 6 hours is also FDA-approved but offers no clinical advantage and reduces compliance. 1

Treatment Duration Considerations

• Uncomplicated cystitis: 7-14 days 1
• Do not use shorter courses - single-dose cephalexin (3 g) showed only 67% cure rates, inferior to standard therapy 5
• Males with UTI: Consider 14 days when prostatitis cannot be excluded 3
• Catheter-associated UTI: 7-14 days (but this represents complicated, not uncomplicated infection) 3

Critical Monitoring and Follow-Up

Do not perform routine post-treatment cultures in asymptomatic patients. 2, 3

Obtain urine culture and susceptibility testing if:

• Symptoms do not resolve by end of treatment 2, 3
• Symptoms recur within 2-4 weeks 2, 3

If treatment fails, assume resistance to cephalexin and switch to a different antimicrobial class for 7 days. 3 Do not retreat with cephalexin. 3

Common Pitfalls to Avoid

• Do not confuse uncomplicated cystitis with pyelonephritis - upper tract infections require different treatment approaches and longer durations 3
• Do not use cephalexin in high-risk scenarios such as patients who used fluoroquinolones in the last 6 months or those from urology departments with higher resistance rates 3
• Do not prescribe four-times-daily dosing - twice-daily dosing is equally effective and improves adherence 4
• Verify local resistance patterns before empiric use - cephalexin should only be used when local E. coli resistance is <20% 2, 3