In 1994, Congress added provisions to an existing law that gave veterinarians the legal ability to use approved human and animal drugs in an extra label manner. Basically the law allows vets to use approved drugs in animals even if the intended use or species is not listed on the label.
However, it is not as simple as that as there are several provisions that must be met to do so. First, a vet must have recently examined the animal to make medical judgements, make a diagnosis, and be available for follow up. Then, there has to be no animal drug approved for the intended use available; the approved drug is not in the required dosage form or required concentration in your horse; or the approved drug is ineffective. In companion animals, a human drug can be used even if an animal drug is available.
This is not the case in food-producing animals. To use a drug off label in food animals, it is much more difficult to satisfy the requirements because the client must maintain identification of the treated animals and the vet must determine a withdrawal period by examining appropriate scientific information. The vet must also assure no illegal drug residues occur and if the vet cannot be sure, the drug cannot be used.
Another off label concern is compounded drugs and veterinarians use lots of these in companion animals. These drugs must be compounded from approved animal or human drugs and not from bulk drugs. And then there are some drugs that are completely banned from use in food-producing animals, including horses intended for food. This information is to give you an idea of the legal factors that play into a veterinarian’s choice of a drug when treating your animals and explain why limitations exist concerning the use of certain drugs.