When the relation between drug dose (X-axis) and drug response (Y-axis) is plotted on a linear scale, the resulting curve is usually hyperbolic. Clinical responses that might be plotted in this way include change in heart rate, blood pressure, gastric pH or blood glucose. Non-clinical (biochemical) responses can also be plotted in this way including enzyme activity, accumulation of an intracellular second messenger, membrane potential, secretion of a hormone, or contraction of a muscle.
If the drug dose is plotted on a base 10 logarithmic scale, this produces a sigmoidal dose-response curve. This representation is more useful because it expands the dose scale in the region where drug response is changing rapidly and compresses the scale at higher doses where large changes have little effect on response. Note that, in reality, it is ligand concentration (and resulting receptor occupation) that affects response - the term ‘dose-response curve’ assumes that the drug dose and ligand concentration are closely linked.