Introduction
After a drug has been administered it is immediately subject to absorption and distribution, both of which occur relatively quickly, followed by a longer period during which the drug is eliminated. These phenomena can be described by plotting the concentration of the drug in plasma against the time that has elapsed since administration.
These concentration-time relationships show some important differences between drugs. Some are eliminated by first order kinetics (a constant fraction of the drug is removed over a given time), and some are zero-order (a constant amount of drug is removed over a given time). Most drugs are eliminated by first-order kinetics and their concentration over time can be predicted by knowing their half-life, the period over which concentration halves.
The concentration-time relationship also varies depending on the route of administration. It demonstrates clearly how the exposure to the pharmacological effects of a drug over time (its bioavailability) is significantly less when it is given by the oral compared to the intravenous route.
Even when the dose, drug and route are the same, the concentration-time relationship (and therefore the exposure to the drug) will differ between patients.
All of these phenomena are of practical importance to prescribers when they select the most appropriate dose and route of administration, titrate drug doses, and try to understand the causes of adverse effects and treatment failure. The relationships described after single drug doses are also the basis of understanding the drug exposure that follows regular doses.
