Cefalexin works by impersonating a building block of bacterial cell walls
First sold as Keflex in 1969, cefalexin is still among the most prescribed antibiotics, with over 7 million American prescriptions in 2023. Its trick is molecular mimicry: it resembles a piece of the bacterial wall so closely that it jams the enzyme that builds it, and the bacterium dies.
Chemically, cefalexin is a first-generation cephalosporin, part of the beta-lactam family that also includes penicillin. Bacteria build their walls from a mesh called peptidoglycan, and the enzymes that stitch it together, penicillin-binding proteins, grab onto a short end segment called d-alanyl-d-alanine. Cefalexin looks enough like that segment to bind irreversibly, halting construction. It is strongest against gram-positive cocci and has moderate reach against some gram-negative rods.
Resistance follows a familiar route. Some bacteria make beta-lactamase, an enzyme that splits the drug's ring and disables it. Cefalexin does not work against MRSA, most Enterococcus, or Pseudomonas, and like every antibiotic it does nothing for viral illnesses such as colds or flu. It is used for infections of the middle ear, throat, skin, bone and joints, and urinary tract, and to prevent bacterial endocarditis.
Its behaviour in the body is simple. Taken by mouth, it is absorbed rapidly and almost completely, peaks after about an hour, and is not broken down; more than 90 percent leaves unchanged in urine within 8 hours, with a half-life of 30 to 60 minutes in people whose kidneys work normally. Stomach upset and diarrhoea are the commonest side effects. True cefalexin allergy is rare, under 0.1 percent of patients, although cross-reactions occur in some people allergic to penicillin.
Developed in 1967, the drug is now generic and sits on the World Health Organization's List of Essential Medicines. It ranked as the fifth most used antibiotic in Canada in 2013 and among Australia's ten most prescribed medicines between 2017 and 2023. The US Food and Drug Administration has approved it for dogs as well as humans, where it treats skin infections, and the European Union licenses a version for cattle and cats too.
Source: Cefalexin
