Pet medications are available either by prescription or over the counter. "Over the counter" means that anyone may purchase the pet medication without the advice and consent of a veterinary doctor. No prescription is needed. A prescription medication is one that the FDA (Food and Drug Administration) regulations allow to be obtained "Only by or on the order of a licensed veterinarian".
Commonly, pet owners will ask a veterinarian for a written prescription for medications in order to procure those medications from a source other than the examining veterinarian. Veterinarians are ethically bound to provide a written prescription upon request and to document the data in the pet's medical record. The veterinarian may ethically charge a fee for writing the prescription and that prescription writing fee can be whatever the veterinarian chooses. The bottom line is that the client does not have to obtain or purchase the pet medication from the examining veterinarian. Keep in mind that the veterinarian is not required to call, fax or correspond with the pet owner's supplier; but is ethically obligated to provide a written prescription so that the client can obtain the medication at a supplier of their choosing.
Some non-prescription medications are "sold only to licensed veterinarians". This is a company policy and not mandated by law. It is not illegal for a veterinarian to re-sell non-prescription medications to other pharmacies or outlets; nor is it illegal for another business to sell these non-prescription products.
All pet medications should be treated with respect. Keep in mind that individual animals may have an adverse reaction to just about any medication. Your veterinarian should always be your first source of information about pet health care. Years of special training and a commitment to keeping your pet healthy will work to your advantage whenever your pet needs pet care or pet medications.
Call your veterinarian first before using any medication in your pet!