This graphic shows a dose-response curve: the relationship between the dose or concentration (on a log-scale) of a ligand and the tissue response or effect (in %, with 100% as maximum). The graph shows the effect of different ligands. Full agonists cause a maximal response. The half maximal effect (Emax) is at 50%. The corresponding concentration is named EC50. Agonist 1 (black) is more potent than agonist 2 (purple), because it reaches its Emax at a lower concentration. In other words: more agonist 2 is required
to achieve a similar tissue response as agonist 1. Partial agonists have per definition a maximal response lower than 100%. And thus, the half Emax is always lower than 50%. Partial agonist 1 (blue) has a higher potency than partial agonist 2 (green), although its maximal effect is lower. And, as shown in the graph, a partial agonist can be more effective at lower concentrations than a full agonist. An antagonist (red) has no effect and is thus a line drawn over the x-axis. No EC50 and Emax can be determined.