To understand the journey, we need to understand the term “drugs.”  The Federal Food, Drug, and Cosmetic Act (FFDCA) defines the term “drugs” to include, among other things, “articles intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease in man or other animals” and “articles (other than food) intended to affect the structure or any function of the body of man or other animals.” (See Section 201(g)(1)(B) & (C) of the Federal Food, Drug, and Cosmetic Act [21 U.S.C. 321(g)(1)(B) & (C)].)

The intended use of a product determines if it's a drug. Here are a few examples to illustrate this concept:

When a company sells bottled water for people to drink as a beverage, the water is not a drug. But if the company sells those same bottles of water as a cure for cancer in dogs, then the water is a drug under the FFDCA because the intended use is to cure a disease (cancer) in dogs.

When a company sells formaldehyde for a car manufacturer to use to make automotive parts, it's not a drug. But when a company sells formalin—a solution of formaldehyde—for a fish biologist to use to kill external parasites on finfish, it's a drug under the FFDCA because the intended use is to treat a disease (parasitism) in fish.

When a company sells a product claiming it makes cows ovulate at the same time, the product is a drug. Although it's not treating or preventing a disease in the cows, the product's intended use is to change how their bodies function, which makes it a drug under the FFDCA.

The FFDCA gives the U.S. Food and Drug Administration (FDA) the legal authority to approve and regulate drugs for both people and animals.  A drug intended for use in animals is called a new animal drug.  FDA’s Center for Veterinary Medicine (CVM) approves and regulates new animal drugs.

CVM is made up of six offices that work together to approve new animal drugs and monitor the drugs after they are on the market.  The Office of New Animal Drug Evaluation (ONADE) is the “pre-approval office,” meaning that it is the lead office for reviewing the information about a new animal drug before it is approved. 

Source from FDA