Cephalexin is an antibiotic used for dogs and cats. Cephalexin belongs to the cephalosporin class of drugs and is related to the penicillin drugs in how it kills bacteria. Cephalosporins have a much broader range of activity against bacteria than penicillin. It is is used in both dogs and cats to treat a variety of bacterial infections, including skin infections (pyoderma), wound infections, bone infections (osteomyelitis), pneumonia, upper respiratory tract infections, mastitis, and urinary tract (bladder) infections.