A first-order process is a process that has a rate proportional to the amount of the reactant involved in this process.

For example, if the rate of the process is proportional to the amount of the drug available for the process, this means that this process follows first-order process. The proportionality constant is the first-order rate constant for the process. e.g.

Elimination rate = Elimination rate constant x drug amount

Transfer rate = Transfer rate constant x drug amount

Distribution rate = Distribution rate constant x drug amount

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 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.