Cefalexin

Lore & Background

It is available as a generic medication and is on the World Health Organization's List of Essential Medicines. In Canada, it was the fifth most common antibiotic used in 2013. Cefalexin is a β-lactam antibiotic that acts by inhibiting synthesis of the peptidoglycan layer of the bacterial cell wall. It is most active against gram-positive cocci and has moderate activity against some gram-negative bacilli. Resistance can occur via bacterial β-lactamase enzymes that hydrolyze the β-lactam ring. The drug is rapidly and almost completely absorbed from the gastrointestinal tract, with peak levels occurring about 1 hour after administration. More than 90% of cefalexin is excreted unchanged in the urine within 8 hours. Cefalexin can treat otitis media, streptococcal pharyngitis, bone and joint infections, pneumonia, cellulitis, and urinary tract infections. It may also be used to prevent bacterial endocarditis and recurrent urinary-tract infections. It is not effective against methicillin-resistant Staphylococcus aureus (MRSA), most Enterococcus, or Pseudomonas. It can be used in those with mild or moderate allergies to penicillin, but is not recommended in those with severe penicillin allergies.

Reader's Guide

Cefalexin remains a widely used oral antibiotic in the first-generation cephalosporin class, valued for its effectiveness against common gram-positive and some gram-negative infections. Its significance lies in its oral bioavailability, allowing outpatient treatment for conditions such as skin infections, urinary tract infections, and strep throat. As an alternative to penicillins in patients with penicillin intolerance, it fills an important therapeutic niche, though cross-sensitivity occurs in up to 10% of penicillin-allergic patients. The drug's legacy is reflected in its inclusion on the World Health Organization's List of Essential Medicines and its sustained high prescription rates in multiple countries. Its safety profile—including use during pregnancy and breastfeeding, and in children and older adults—contributes to its continued clinical utility. However, its limitations against MRSA, Enterococcus, and Pseudomonas, as well as the potential for Clostridioides difficile infection, underscore the need for appropriate prescribing. The development of bacterial resistance via β-lactamase enzymes remains a concern, but cefalexin's role as a first-line oral agent for many common infections endures.

Did You Know?

• It is on the World Health Organization's List of Essential Medicines.
• More than 90% of cefalexin is excreted unchanged in the urine within 8 hours.
• Cefalexin can be used in those with mild or moderate allergies to penicillin, but is not recommended in those with severe penicillin allergies.