The relationship between the dose (amount) of a drug and the response observed can often be extremely complex, depending on a variety of factors including the absorption, metabolism, and elimination of the drug; the site of action of the drug in the body; and the presence of other drugs or disease. In general, however, at relatively low doses, the response to a drug generally increases in direct proportion to increases in the dose. At higher doses of the drug, the amount of change in response to an increase in the dose gradually decreases until a dose is reached that produces no further increase in the observed response (i.e., a plateau). The relationship between the concentration of the drug and the observed effect can therefore be graphically represented as a hyperbolic curve (see Figure 1).

Figure 1 Representative Dose-Effect Curve, with Its Four Characteristics

Often, however, the response...