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.

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 volume of distribution is the apparent volume in which the drug can be distributed in. It is the factor that relates the amount of drug in the body to the concentration of the drug in the sampling site. It is n. an actual volume, however it is a hypothetical volume determined by the dg distribution behavior.

The volume of distribution ranges from 3-5 liters to more than 25 L/kg. Drugs with higher affinity for tissues, have higher volume of distribution.

The steady state is the condition when the rate of drug administration is equal to the rate of drug elimination.

When the drug is administered as a constant rate IV infusion, there will be one steady state concentration as long as the rate of administration and the rate of elimination do not change.

The time required to achieve steady state is dependent on the drug half life. It takes about 5-6 half lives to achieve steady state. The steady state concentration during constant rate iv infusion is directly proportional to the infusion rate and inversely proportional to the total body clearance of the drug.