Bioavailability
·
Drugs are absorbed from the site of
administration into the systemic circulation.
·
In simple words, the fraction of drug that
reaches systemic circulation in unchanged form, from the site of administration is the
bioavailability. It not only indicates the rate of absorption but also the extent of absorption.
·
In intravenous route, all of the drug directly
enters into systemic circulation. Therefore, the bioavailability is 100%
·
Example-
1000 mg of Drug A is given by oral route. Out of 1000 mg, 800 mg is
absorbed from the gut. 200 mg undergoes Hepatic First pass metabolism. So the
amount of drug that reaches systemic circulation is 800-200= 600 mg. So
bioavailability is (600/1000)x 100= 60 %.
·
In pharmacokinetic, bioavailability of an orally
administered drug is determined using the plasma-concentration time curve (see
the graph below)
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