The Veterinary Practice Act limits dispensing prescription medication (and food) if a veterinarian has not seen your pet within a designated time frame (usually 6 or 12 months). This establishes a valid Veterinary-Client-Patient-Relationship (VCPR).
The American & Missouri Veterinary Medical Associations set the standards for a valid VCPR:
A VCPR is established only when your veterinarian examines your animal in person, and is maintained by regular veterinary visits as needed to monitor your animal's health.
If a VCPR is established but your veterinarian does not regularly see your pet afterward, the VCPR is no longer valid and it would be illegal and unethical for your veterinarian to dispense or prescribe medications or recommend treatment without recently examining your pet. A valid VCPR cannot be established online, via email, or over the phone.