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.
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