What the Body Does to A Drug: Pharmacokinetics

Abstract

Pharmacokinetics may be defined as the study of the dynamic movements of foreign chemicals (xenobiotics) during their passage through the body and as such encompass the kinetics of absorption, distribution, biotransformation/metabolism and excretion. Absorption is the process that brings a drug from the administration, e.g., tablet, capsule, into the systemic circulation. Bioavailability is the fraction of the originally administered drug that arrives in systemic circulation and depends on the properties of the substance and the mode of administration. It can be a direct reflection of medication absorption. Distribution describes how a substance is spread throughout the body. This varies based on the biochemical properties of the drug as well as the physiology of the individual taking that medication. In the body, a drug may be protein-bound or free. Only free drug can act at its pharmacologically active sites, e.g., receptors, cross into other fluid compartments, or be eliminated. Metabolism is the processing of the drug by the body into subsequent compounds. Excretion is the process by which the drug is eliminated from the body. The pharmacokinetic term half-life (t1/2) refers to the time taken for half the initial dose of medicine administered to be eliminated from the body. After three to five half-lives the drug is considered undetectable and unable to exert a pharmacodynamic effect.