dose–effect relationship
The single most fundamental idea in pharmacology is almost a truism: how much drug you give changes how much happens. The dose–effect relationship is the systematic link between the amount of drug administered (or its concentration) and the size of the biological response it produces.
Characterizing this relationship lets us extract the numbers that describe a drug — its potency (where on the dose axis the curve sits), its efficacy (how high the response can climb), and the steepness of the curve. Whether the response is graded in one system or quantal across a population, the dose–effect relationship underlies dosing decisions, therapeutic indices, and toxicology alike.
The relationship is usually positive and graded but not always simple: it can have a threshold at the bottom, a ceiling at the top, and occasionally a biphasic shape as in hormesis. The famous principle that 'the dose makes the poison' — attributed to Paracelsus — is just the dose–effect relationship applied to toxicity: substance and harm are linked through dose, not by the substance alone.
Correlation here is causal by design: the dose is what the experimenter sets, and the effect is what is measured, so a well-controlled dose–effect study is among the cleanest demonstrations of cause and effect in biology.