Bioavailability and Half-Life

Bioavailability: the rate at which a drug makes it to its destination and becomes available to the site of action for which it is intended

• Describes the concentration of drug in systemic blood in relation to the amount of drug given

• An IV injection (given directly into the bloodstream) has 100% of bioavailability because the drug does not have to be absorbed

• Bioavailability of PO (by mouth) medication is rarely 100% and usually range from 10% to 50%

Half-Life: the amount of time it takes the body to break down and excrete one half of a drug

• Time taken for the plasma concentration of the drug to decrease by 50%

• Half-life is an important factor in the creation of drugs because it helps determine proper dosing intervals

• Clearance is the amount of plasma from which a substance is removed per unit time

First Pass Effect: all drugs absorbed by the GI tract enter the portal blood supply, and go directly to the liver

• Blood from the GI tract goes directly to the liver first

• The liver metabolizes many drugs before they enter the systemic blood supply