Veterinary prescription legend drugs are those which, by regulation, can only be used by or on the order of a licensed veterinarian since adequate directions for their use by a layman cannot be written. Generally, the prescription drugs are quite potent or otherwise hazardous, having a narrow range of tolerance in terms of safety for use.
Eckhoff, Gary A.
"Prescription Writing for the Veterinarian,"
Iowa State University Veterinarian: Vol. 38
, Article 2.
Available at: https://lib.dr.iastate.edu/iowastate_veterinarian/vol38/iss2/2