Half-life of Drugs
Experimentally the half life can be determined by giving a single dose, usually intravenously, and then the concentration of the drug in the plasma is measured at regular intervals. The concentration of the drug will reach a peak value in the plasma and will then fall as the drug is broken down and cleared from the blood.
The time taken for the plasma concentration to halve is the half life of that drug. Some drugs like ibuprofen have very short half lives, others like warfarin and digoxin, take much longer to eliminate from the plasma resulting in a long half life. So drugs like ibuprofen that are cleared from the blood more rapidly than others need to be given in regular doses to build up and maintain a high enough concentration in the blood to be therapeutically effective.