 Pharmacodynamics

Pharmacodynamics describes how a drug changes a biological system as a function of exposure. It connects target binding to a measured response, whereas pharmacokinetics describes the time course of exposure. A simple illustrative model iswhere $E_{max}$ is the maximum increment above baseline and $E{C}_{50}$ is the concentration producing half that increment. Doubling $C$ does not generally double the response. ^[ref]

Potency and maximal effect differ: two drugs can have different $E{C}_{50}$ values but the same $E_{max}$, or the same $E{C}_{50}$ but different maximal effects. Moreover, $E{C}_{50}$ need not equal a binding dissociation constant, because downstream amplification and measurement time also shape the response.