Cefalexin is an antibacterial agent of the cephalosporin class. Like other cephalosporin, cefalexin exerts antibacterial activity by binding to and inhibiting the action of penicillin-binding proteins involved in the synthesis of bacterial cell walls. This leads to bacterial cell lysis and cell death.

Cefalexin is acid stable and is readily absorbed after oral administration. Food may delay absorption. Intramuscular doses of the lysine salt are well absorbed but up to 15% of a dose is bound to plasma proteins. The half-life had been reported to range from 0.5 to 2 hours and this increases with reduced renal function. Cefalexin is widely distributed in the body but does not enter the cerebrospinal fluid in significant quantities. It diffuses across the placenta and small quantities are found in the milk of nursing mothers. About 80% or more of a dose is excreted unchanged in the urine in the first 6 hours by glomerular filtration and tubular secretion.