The half life is the time required for the amount of the drug or the plasma concentration of the drug to decrease by 50%. The half life has units of time.

The half life and the first order elimination rate constant are dependent on the total body clearance and the volume of distribution.

CLT

Vd = k =

0.693

t 1/2

The half life of a drug is constant within a patient (dose and concentration independent) when the drug elimination process follows first-order kinetics. However different patients may have different half lives for the same drug.

The half life is a very important parameter, because it indicates how fast the drug is eliminated from the body. Drugs with shorter half lives are eliminated faster than those with longer half lives.