ABSTRACT
Drugs exert their effects by binding to target proteins at the site of action (in the target organ). Generally, an increased concentration of the drug provides an increased effect. There is generally a good correlation between the concentration of a drug in the target organ and the effect, as there is between the concentration of a drug in the blood and the concentration of that drug in the target organ when distribution equilibrium is achieved. Consequently, there is usually a correlation between the concentration of drugs in the blood and effects. This is the rationale for monitoring the concentration of drug in the blood in order to decide if the dose is to be changed. The value is usually expressed as the plasma concentration of the drug, as once the blood sample has been collected, it is the amount in the plasma that is measured.
