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.

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.

Some drugs produce their pharmacological effects by stimulation or inhibition of a factor that will lead to the production of the pharmacological effect.

For example oral anticoagulants inhibit the synthesis of clotting factors. However because of the existing clotting factors the anticoagulant effect is not immediate. The effect is observed when the level of the existing clotting factors declines due to the normal turnover of the these factors. In this case the effect will be delayed relative to the drug conc.

The indirect pharmacodynamic response models described by Sharma and Jusco assume that the initiation of effect is zero-order and the loss of effect is first-order. The drug can inhibit or stimulate either the initiation or loss of effect. The inhibition or stimulation of effect is related to the drug conc by hyperbolic (or sigmoid) function.