dose–response curve
A dose–response curve is a picture of the basic rule of pharmacology: more drug, more effect — up to a point. It plots the dose or concentration given against the size of the response produced, capturing a relationship at a single glance.
When concentration is plotted on a logarithmic scale, this graded curve typically becomes a smooth S-shape (sigmoid). Its position along the horizontal axis tells you about potency (a leftward curve is more potent, with a lower EC50), and the height of its plateau tells you about efficacy (the Emax). The steep middle region, around the EC50, is where the response changes most sharply with dose.
Dose–response curves also reveal how drugs interact. A competitive antagonist shifts the agonist curve rightward in parallel without lowering the plateau (surmountable antagonism), whereas a non-competitive antagonist depresses the plateau itself. A separate kind of curve, the quantal dose–response curve, plots the fraction of a population responding rather than the graded effect in one subject.
Plotting muscle relaxation against rising acetylcholine concentration gives a sigmoid curve whose midpoint marks the EC50 and whose plateau marks the Emax.
Dose on one axis, response on the other.
Distinguish the graded curve (response magnitude in one subject) from the quantal curve (proportion of a population that crosses a defined threshold); they look similar but mean different things.