Are Keflex (cephalexin) and ampicillin the same for treating a urinary tract infection (UTI)?

Keflex (Cephalexin) and Ampicillin Are Not the Same for Treating UTIs

Keflex (cephalexin) and ampicillin are not the same medication, and ampicillin should not be used for empirical treatment of UTIs due to its relatively poor efficacy and high prevalence of antimicrobial resistance worldwide. 1

Differences Between Cephalexin and Ampicillin

• Drug Class:

  • Cephalexin is a first-generation cephalosporin
  • Ampicillin is an aminopenicillin

• Efficacy for UTIs:

  • Cephalexin is recommended as an appropriate treatment option for UTIs 1, 2
  • Ampicillin is specifically not recommended for empirical UTI treatment due to poor efficacy and high resistance rates 1

• Resistance Patterns:

  • Ampicillin has very high worldwide resistance rates among common UTI pathogens 1
  • Cephalexin maintains better activity against many uropathogens, particularly when susceptibility is confirmed 3

Current Guideline Recommendations for UTI Treatment

First-line Options:

• Nitrofurantoin 100 mg twice daily for 5 days
• Trimethoprim-sulfamethoxazole 160/800 mg twice daily for 3 days (if local resistance <20%)
• Fosfomycin 3 g single dose 2

Alternative Options (including Cephalexin):

• Cephalexin 500 mg twice daily or four times daily for 5-7 days 2, 4
• Other β-lactams (including cephalexin) are appropriate when first-line agents cannot be used 1

Not Recommended:

• Ampicillin should not be used for empirical treatment of UTIs 1

Efficacy of Cephalexin for UTIs

Recent research supports cephalexin's effectiveness for UTIs:

• A 2023 study found no significant difference in treatment failure between twice-daily and four-times-daily dosing of cephalexin for UTIs (18.7% vs 15.0%, P = 0.465), suggesting that the more convenient twice-daily regimen is reasonable 4

• Another 2023 study comparing cephalexin with other antibiotics for pediatric UTIs found it to be a reasonable first-line therapy with a favorable side-effect profile 5

• A 2020 review concluded that cephalexin has very good early bacteriological and clinical cure rates in uncomplicated UTIs due to non-ESBL-producing Enterobacteriaceae, comparable to many first-line agents 3

Important Clinical Considerations

• Urine Culture: Always obtain a urine culture before starting antibiotics to confirm the causative pathogen and determine susceptibility 2

• Local Resistance Patterns: Consider local antibiogram data when selecting antibiotics 2

• Treatment Duration:

  • Uncomplicated UTI: 3-5 days
  • Complicated UTI: 7-14 days 2

• Dosing Convenience: Cephalexin can be administered as 500 mg twice daily, which may improve patient adherence compared to more frequent dosing regimens 6, 4

Common Pitfalls to Avoid

• Using Ampicillin Empirically: Due to high resistance rates, ampicillin should not be used for empirical UTI treatment 1

• Treating Asymptomatic Bacteriuria: Antibiotic therapy is only indicated for symptomatic UTIs in most populations 1, 2

• Ignoring Culture Results: Therapy should be adjusted based on culture results to ensure effective treatment and minimize resistance development 2

In conclusion, while both are antibiotics, cephalexin (Keflex) is an appropriate option for UTI treatment when first-line agents cannot be used, whereas ampicillin is specifically not recommended due to poor efficacy and high resistance rates.