Cephalexin. Computer model of a molecule of the drug cephalexin. It is sold under the brand name Keflex. It is a cephalosporin antibiotic drug, used to treat bacterial infections. This drug is sold by Eli Lilly. The molecule's chemical formula is C16.H17.N3.O4.S. Atoms (spheres) are colour- coded: carbon (grey), hydrogen (light blue), nitrogen (dark blue), oxygen (orange), sulphur (yellow).