The magnitude of response produced by a drug is a function of the dose administered. There is a systemic relationship between the dose of a drug and the magnitude of response it elicits. The occupancy of receptors by a drug is proportional to the concentration of the drug and the concentration of free unoccupied receptors.
The graphical representation of the mathematical expression of the relationship between dose and the response is referred to as Dose–Response curves.
There are two basic forms of dose-response curve
- Graded dose–response curves: In this kind of curve, there is a progressive increase in effect with increasing concentration of the drug. The maximum response corresponds to the point at which receptor occupancy is almost 100%. An example is the action of histamine on the contraction of guinea pig ileum. The response increases in a gradual manner with increasing amounts of drug once a threshold concentration is reached, and rises to a maximum level. In general it is convex upwards if plotted on arithmetic scale, but if plotted logarithmically, the convexity diminishes and the line becomes approximately straight over the center part of the curve (sigmoid). Drugs that produce the same effect by the same mechanism generally yield log dose-effect curves whose slopes are parallel to each other.
- Quantal dose-response curves: It is also called ‘all or none’ dose response curve. They either occur or do not occur. An example is in determining the relationship the dose of barbiturate and the propensity to cause sleep, the effect could only be measured as an all or none response; either sleep is induced by a particular dose or not. Another example is the relationship between dose of a drug and its toxicity. A drug will either cause toxicity /death or not. This type of curve demonstrates the relationship between the dose of a drug and the proportion of the population showing the effect.
