A "valid prescription" means having a doctor's order to get medication from a pharmacy. It ensures that the medicine is given for a specific reason and in the right dosage. Full definition
Dispensing of these medications can only be done if the animal has been seen at the clinic at some time or has valid prescription from another clinic (langleyanimalclinic.ca)
Large pharmacies or hospitals require valid prescriptions. (anywhere.com)
Drivers must not drive while sick or tired and may not drive while under the influence of alcohol, illegal drugs or other intoxicating substances (even those for which the driver has an otherwise valid prescription). (breslinandbreslin.com)