Is cephalexin (cephalexin) a suitable alternative to Bactrim (trimethoprim/sulfamethoxazole) for treating a urinary tract infection (UTI) in a patient with a Bactrim allergy?

Cephalexin for UTI in Bactrim-Allergic Patient

Yes, cephalexin is an appropriate alternative to Bactrim for treating uncomplicated UTI in a patient with Bactrim allergy, though you should first confirm she has current symptoms requiring treatment. 1, 2, 3

Critical First Step: Confirm Active Infection

• Do not prescribe antibiotics without current UTI symptoms, as treating asymptomatic bacteriuria (even with a history of UTI) promotes antimicrobial resistance without clinical benefit. 1
• Melody states she has "no recent symptoms"—you need to clarify if she has dysuria, frequency, urgency, or suprapubic pain now before prescribing any antibiotic. 1
• If she is truly asymptomatic, a referral for antibiotics is not indicated regardless of her UTI history. 1

Why Cephalexin Over Trimethoprim (Bactrim)

Allergy Consideration

• Since Melody has a documented Bactrim (trimethoprim-sulfamethoxazole) allergy, this agent is contraindicated and cephalexin becomes a reasonable fluoroquinolone-sparing alternative. 4, 3
• Important caveat: Screen for any history of penicillin or cephalosporin allergy, as cross-reactivity can occur (though it's relatively low at ~1-3%). 4

Efficacy of Cephalexin for Uncomplicated UTI

• Cephalexin 500 mg twice daily for 5-7 days demonstrates 81% clinical success rates for uncomplicated UTI in recent studies, with only 10.6% requiring antibiotic changes and 6.8% returning for persistent symptoms. 2
• Cephalexin achieves high urinary concentrations and maintains full activity against common uropathogens (E. coli, Klebsiella) when they are non-ESBL producers. 3, 5
• Modern cefazolin-cephalexin surrogate testing has reclassified many previously "resistant" organisms as susceptible, improving cephalexin's utility. 3

Guideline Support

• While the 2024 European Association of Urology guidelines do not list cephalexin as a first-line agent for uncomplicated cystitis, they do recommend oral cephalosporins (cefpodoxime, ceftibuten) for pyelonephritis when fluoroquinolones are contraindicated. 6
• The guidelines acknowledge that treatment must be tailored to local resistance patterns and patient-specific factors like allergies. 6

Dosing and Duration

• Prescribe cephalexin 500 mg orally twice daily for 5-7 days for uncomplicated lower UTI. 2, 3
• Twice-daily dosing is preferred over once-daily to reduce gastrointestinal side effects. 7
• For complicated UTI or if pyelonephritis is suspected, consider 7-14 days of therapy. 6

When Cephalexin May Not Be Optimal

Resistance Concerns

• Avoid cephalexin for organisms with known high resistance rates, such as Serratia marcescens, Pseudomonas, or ESBL-producing Enterobacteriaceae. 1
• If Melody has risk factors for complicated UTI (diabetes, immunosuppression, recent instrumentation, recurrent infections), obtain urine culture before initiating therapy. 6, 1

Comparative Effectiveness

• Cefdinir should be avoided—it has nearly twice the treatment failure rate (23.4% vs 12.5%) compared to cephalexin due to poor urinary penetration and bioavailability. 8
• If local E. coli resistance to trimethoprim-sulfamethoxazole is <20% and she weren't allergic, Bactrim would be preferred first-line. 6, 4

Common Pitfalls to Avoid

• Do not prescribe prophylactic antibiotics for someone without active symptoms just because they've had UTIs before. 1
• Do not rely on dipstick testing alone in patients with catheters or recurrent UTIs, as pyuria doesn't differentiate symptomatic infection from colonization. 1
• Do not use cephalexin empirically if she has had recent cultures growing resistant organisms or ESBL producers—obtain culture and sensitivities first. 1
• If symptoms don't resolve within 48-72 hours or worsen, reevaluate and consider culture-directed therapy. 4

Algorithm for This Patient

1. Confirm current UTI symptoms (dysuria, frequency, urgency, suprapubic pain)
2. If symptomatic: Prescribe cephalexin 500 mg PO BID × 5-7 days
3. If asymptomatic: Do not prescribe antibiotics; educate on when to seek care for future symptoms
4. If complicated features present (fever, flank pain, diabetes, immunosuppression): Obtain urine culture before treatment and consider broader-spectrum therapy
5. If no improvement in 48-72 hours: Obtain culture and adjust based on sensitivities

6, 1, 4, 2, 3