Dose–response curve

A dose–response curve plots the dose or concentration of a drug along the horizontal axis, usually on a logarithmic scale, against the effect it produces on the vertical axis. Drawn this way the curve is typically S-shaped: little happens at low doses, the response then climbs steeply, and at high doses it levels off at a ceiling where more drug adds no further effect.

The curve shows three key features. Its position along the dose axis reflects potency: a curve lying further to the left means less drug is needed, often summed up as the EC50 or ED50, the dose that gives half the maximum effect. Its height shows maximal efficacy, the largest effect the drug can produce, and its slope shows how sharply the response changes with dose. A more potent drug is not necessarily a more effective one.

A graded curve tracks the size of a response in one tissue or person, while a quantal curve shows the share of a population that responds at each dose. Comparing quantal curves for a wanted effect and a toxic effect gives the therapeutic index. Antagonists shift an agonist's curve to the right or flatten it.