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 dg elimination rate is proportional to the amount of the drug in the body, this means that this drug is eliminated by a first-order process. The proportionality constant is the first-order elimination rate constant. i.e.

Elimination rate = elimination rate constant x drug amount

The rate of drug elimination decreases with time because the amount of the drug remaining decreases with time.

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

This rate constant represents drug elimination through all routes of drug elimination such as metabolism, renal excretion, and other routes.

The first-order elimination rate constant has units of time^-1.

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.

A zero-order process is a process that has a constant rate. The rate a the zero-order process is the zero-order rate constant and it does not depend on the amount of the reactant involved in the process.

For example, when the drug elimination follows zero-order kinetics, the amount of the drug eliminated per unit time is constant and is equal to the zero-order rate constant.

In this case, the zero-order rate constant has units, of amount/time.

A zero-order process is a process that has a constant rate. The rate of the zero-order process does not depend on the amount of the reactant involved in the process.

If the elimination of a drug follows zero-order kinetics, this means that there is a constant amount of the drug eliminated per unit time, regardless of the administered dose.

Because the rate of drug elimination is constant (zero-order elimination) the fraction of the dose eliminated per unit time after administration of a large dose, is much smaller than the fraction of the dose eliminated per unit time after administration of smaller doses.