Merck Manual

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Pet Owner Version

Introduction to Drugs and Vaccines


Dawn Merton Boothe

, DVM, PhD, Department of Anatomy, Physiology, and Pharmacology, College of Veterinary Medicine, Auburn University

Reviewed/Revised Jul 2011 | Modified Nov 2022

If your pet has been diagnosed with a condition or disease that can be managed or cured, your veterinarian will discuss treatment options. He or she will recommend drugs that are necessary, safe, and effective for both the individual animal and the specific disorder. For many conditions, there are a variety of drugs that might be considered as an appropriate treatment. When selecting one to prescribe, veterinarians also consider the dose (amount), the method of action, how often to give the chosen drugs, the best way to give the drug, the particular form (for example, pills, liquid, or ointment) to be used, any public health or environmental effects, and local and state regulations.

This chapter provides some basic information on the types of formulations and application methods commonly used to treat diseases or disorders in animals, as well as a general discussion of the types of drugs used to treat disorders within certain body systems (such as the circulatory or digestive system). Specific drug therapies for individual conditions are not discussed here, however. For more detailed information on the treatment for a specific disease or disorder, refer to other chapters, as available, where the condition is described in detail.

Many chemicals and drugs can be used to combat disease-causing agents. Treatment with any drug involves a relationship between the host animal (your pet), the agent that causes disease, and the drug. The drug may be toxic only to the invading organism, but in many cases, it may also cause adverse effects in the host animal. Included in this category are drugs that are active against bacteria, viruses, fungi, parasites, and especially tumors.

Your pet’s natural defense mechanisms should be maximized when treating an infectious or parasitic disease in order to prevent an extended recovery time or a relapse. Minimizing stress, providing quality nutrition, and boosting the immune system are all approaches that may be used. An effective drug treatment program involves the veterinarian and the pet owner making informed decisions and carefully following all instructions.

As a pet owner, you should thoroughly discuss with your veterinarian any and all drugs—whether prescription, over-the-counter, or other medications (for example, herbal or other alternative preparations)—that you are thinking about giving to your pet. Inform yourself about how the drug works and any possible negative effects it may have. Even for drugs that seem easy to use or that you have given your pet before, it is important to carefully read the label and administer the correct dose as directed.

Extralabel Use of Drugs

Many of the drugs used in animals in the United States have not been formally approved by the Food and Drug Administration (FDA) for use in a particular species. However, veterinarians have the legal authority to prescribe drugs that have been approved for use in animals or humans for purposes beyond those for which they have been approved. This is referred to as extralabel (or off-label) use. The studies needed to gain FDA approval for a drug to treat a particular disease in a particular species usually require 7 or more years and millions of dollars. Because of these factors, the projected sales may not be large enough for a drug company to justify seeking approval in multiple species. Once a drug is approved for one use, however, it may also be prescribed by licensed professionals for other circumstances.

When deciding which drug to use to treat an animal, veterinarians rely on their training and experience, published reports in professional veterinary journals, and continuing education received at professional meetings. In order to prescribe a drug for your pet, your veterinarian must have what is called a valid veterinarian-client-patient relationship. This means that the veterinarian has agreed with you to serve your pet’s medical needs, has recently examined your pet, and is available for followup care.

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