Which of the following drugs is not classified as a penicillin?

Cephalexin is not classified as a penicillin; instead, it belongs to the class of antibiotics known as cephalosporins. Cephalosporins have a different chemical structure compared to penicillins, although they share a similar mechanism of action, which involves inhibiting bacterial cell wall synthesis.

In contrast, amoxicillin, ampicillin, and cloxacillin are indeed part of the penicillin family. These medications are derived from Penicillium fungi and are characterized by their beta-lactam ring structure, which is crucial for their antibacterial activity against a variety of bacterial infections. Understanding these classifications helps to discern the specific pharmacological profiles and uses of different antibiotics in veterinary medicine.