Virginia R. Fajt, DVM, PhD
In most cases, we administer drugs at a different site than we want to drug to act. Understanding how drugs get to their site of action and how long they stay there is essential to making therapeutic decisions about which drug, what route, how much, how often, and for how long.
The Food and Drug Administration Center for Veterinary Medicine (FDA CVM) approves drug labels. The Environmental Protection Agency approves pesticides and products used on premises. State Boards of Pharmacy regulate the practice of pharmacy and drug dispensing. State Boards of Veterinary Medicine regulate the practice of medicine.
The science of how drugs work on the body (or the microorganism or parasite) is pharmacodymanics (its counterpart being pharmacokinetics, how the body works on the drug). In this section, the basic concepts of drug concentration and drug action are followed by a review of the mechanisms of action of the major drug groups used in food animal practice including NSAIDs, glucocorticoids, reproductive drugs, antimicrobials, and parasiticides.
At the time of this writing, the focus on farm animals by the media (and likely therefore consumer perception) seems to be on antimicrobial use in animal agriculture and on farm animal welfare.
Traditionally, we have assumed that if a bacterial pathogen is "susceptible" to an antimicrobial, we just use the dose on the bottle or in a formulary, and the infection will be eliminated. The increasing incidence of "resistant" pathogens, i.e., pathogens requiring high concentrations of antimicrobials such that they become untreatable, has focused attention on identifying ways to reduce the selection for resistant organisms.