Pharmacokinetics is the study of the fate of a pharmaceutical product (drug) when administered to a living organism. The word is derived from the term "pharmacon", meaning drug or the science of preparing and dispensing medicines, and "kinetics", meaning motion.
The study of the action of a drug in living cells or organisms can include the rate of absorption, which organs it migrates to, whether or not it is metabolized or simply excreted/eliminated. This is the main focus of Phase I clinical trials, which use healthy volunteer test subjects to study the pharmacokinetics of a new drug. These studies also evaluate the consequences of ingestion, injection, absorption or other modes of exposure to the drug, since some pharmaceuticals can interfere with normal metabolic processes, through various modes of action that include inducing or inhibiting biochemical reactions, competing with enzymes for active sites, or binding to DNA to initiate or prevent transcription.
The possible effects of a drug on an organism, whether it is a single cell, or a full grown human, are countless, making the study of pharmacokinetics very complex. Sometimes individual differences in genetic makeup, or those with certain conditions react differently to the drug than the rest of a population. This explains why sometimes new drugs that passed a certain phase of clinical trial later turn out to have undesirable side effects that went previously unnoticed.
