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Pharmacodynamics is defined as the science of drug action on the body and pharmacokinetics is defined as the study of what the body does to a drug, namely the study of drug absorption, distribution, metabolism and elimination. A basic understanding of pharmacokinetics and pharmacodynamics is important, particularly in the discipline of anaesthesia. It allows known and unfamiliar drugs to be used more safely and efficiently and ensures a greater understanding of clinical effect. It allows predictions to be made about the clinical characteristics of anaesthetic and analgesic drugs that may be unfamiliar, increasing understanding about how the drug performs in both healthy and unhealthy patients. Pharmacokinetic properties will also inform about whether individual drugs are suitable for administration by continuous rate infusion. A second article focussing on the principles of pharmacodynamics will appear in a future issue of The Veterinary Nurse.

In order to use drugs safely in animals it is important to understand individual drug pharmacodynamics, or the physiological effects of the drug on the body. This article describes the principles of pharmacodynamics, starting with the different mechanisms by which drugs bound to a receptor can elicit an effect as well as the receptor actions of agonist, partial agonist and antagonist drugs. The terms efficacy and potency in terms of drug action are defined. Different pathways of drug metabolism and elimination are identified and the importance of understanding the relationship between drug pharmacokinetics and pharmacodynamics explained.