Humane societies and animal control agencies and the staff of those agencies may not purchase, possess, or administer controlled substances or legend drugs except sodium pentobarbital and approved legend drugs as provided herein. Provided, staff may administer legend drugs and controlled substances which have been prescribed by a licensed veterinarian for a specific animal and which drugs have been dispensed by a pharmacy or a veterinarian and are properly labeled in accordance with either RCW 18.64.246 or 69.41.050.