Action of drug on the body
The action of a drug on the body can be described in terms of its half-life, the time interval required for the body's elimination process to reduce the concentration of the drug in the body by one half. For e.g. if a drugs half-life is 8 hours, then the amount of drug in the body is as follows:
· Initially 100%
· After 8 hrs. 50%
· After 16 hrs. 25 %
· After 24 hrs. 12.5%
· After 32 hrs. 6.25%
Because the purpose of most drug therapy is to maintain a constant drug level in the body, repeated doses are required to maintain that level. When an orally administered drug is absorbed from the GI tract in to the blood plasma, its concentration in the plasma increases until the elimination rate equals the rate of absorption. This point is known as the peak plasma level. Unless the client receives another dose of the drug, the concentration steadily decreases.
Onset of action: the time after administration when the body initially responds to the drug.
Peak plasma level: the highest plasma level achieved by a single dose when the elimination rate of a drug equals the absorption rate.
Drug half-life (elimination half- life): the time required for the elimination process to reduce the concentration of the drug to one-half what it was at initial administration.
Plateau: a maintained concentration of a drug in the plasma during a series of scheduled doses.