Half life

 Elimination half-life, written as t_1/2, is the time needed for the concentration of a substance (usually a drug, metabolite or hormone) to half in the blood plasma.

• The t_1/2 or rate of decay (dC/dt) proportional to concentration is exponential so when trying to measure half-life graphs are usually plotted on a semi-logarithmic scale in order to get a straight line which is easier to manipulate. 
• Half-life is important in pharmacokinetics and depends on how quickly the substance is eliminated ie. the clearance as well as on volume distribution. 
• Half-life is important when determining how frequently drugs should be administered.