During the drug approval or indexing process, FDA is responsible for determining the marketing status (prescription, over-the-counter, or VFD) of animal drug products. To be sold over-the-counter (OTC), it must be possible to prepare "adequate directions for use" under which a layperson can use the drugs safely and effectively. Safe use includes safety to the animal, safety of food products derived from the animal, and safety to the persons associated with the animal.

Effective use of a drug product assumes that an accurate diagnosis can be made with a reasonable degree of certainty, that the drug can be properly administered, and that the course of the disease can be followed so that the success or lack of success of the product can be observed. Prescription (Rx) products can be dispensed only by or upon the lawful written order of a licensed veterinarian.

The same drug substances can be marketed in a number of different dosage forms, intended for use by different routes of administration, and in different species of animals. Thus, these drug products may be appropriately labeled Rx in some cases and OTC in others. Rx products must bear the legend:

"Caution: Federal law restricts this drug to use by or on the order of a licensed veterinarian." 

plastic injection vial

30ml dial a dose syringe

PLASTIC LIQUID BOTTLE

Source from FDA