Is Keflex a Broad-Spectrum Antibiotic?
No, Keflex (cephalexin) is not a broad-spectrum antibiotic—it is a narrow-spectrum first-generation cephalosporin with limited activity primarily against gram-positive organisms and select gram-negative bacteria, but notably lacks coverage against important pathogens like Haemophilus influenzae and Pseudomonas aeruginosa. 1, 2
Antimicrobial Spectrum of Cephalexin
Cephalexin's spectrum includes:
Gram-positive coverage: Effective against methicillin-sensitive Staphylococcus aureus (MSSA) and Streptococcus species, making it useful for streptococcal and staphylococcal skin infections with cure rates of 90% or higher 3
Limited gram-negative coverage: Active against some common urinary tract pathogens and certain Enterobacteriaceae, but this is significantly restricted compared to broader-spectrum agents 4
Critical gaps in coverage: Notably ineffective against Haemophilus influenzae, with failure rates of 50% in H. influenzae otitis media 2, 5. Also lacks activity against Pseudomonas aeruginosa, anaerobes (requiring combination with metronidazole when anaerobic coverage is needed), and extended-spectrum beta-lactamase (ESBL)-producing organisms 6
Comparison to Broad-Spectrum Agents
The evidence clearly distinguishes cephalexin from broad-spectrum antibiotics:
Narrow-spectrum designation: Clinical guidelines specifically classify cephalexin alongside cefadroxil as "narrow-spectrum cephalosporins," in contrast to "broad-spectrum cephalosporins" like cefixime, cefuroxime, cefdinir, and cefpodoxime 7
Preferred for narrow coverage: Guidelines recommend narrow-spectrum cephalosporins like cephalexin over broad-spectrum alternatives specifically because of their narrower antimicrobial spectrum and lower risk of selecting for antibiotic-resistant flora 7
Contrast with true broad-spectrum agents: Broad-spectrum antibiotics listed in guidelines include piperacillin-tazobactam (with anti-Pseudomonas and anaerobic coverage), carbapenems (active against ESBL-producing pathogens and anaerobes), and fourth-generation cephalosporins like cefepime (effective against AmpC-producing organisms) 6
Clinical Implications
Appropriate uses: Cephalexin is effective for uncomplicated skin and soft tissue infections caused by streptococci and staphylococci, urinary tract infections caused by susceptible organisms, and streptococcal pharyngitis in penicillin-allergic patients 6, 7, 4, 3
Inappropriate uses: Should not be relied upon for respiratory infections where H. influenzae is suspected (particularly in pediatric populations), infections requiring anaerobic coverage, or any infection where Pseudomonas or ESBL-producing organisms are concerns 2, 5
Resistance considerations: The narrow spectrum is actually advantageous in appropriate clinical scenarios, as it minimizes disruption of normal flora and reduces selection pressure for resistant organisms compared to broad-spectrum alternatives 7