In competitive antagonism a ligand (agonist) and an antagonist compete for binding sites on the receptors. The graph shows the specific binding of the ligand alone, and in presence of a low, intermediate and high concentration of the antagonist. In the absence of antagonist, the ligand is able to bind the receptor already at low concentrations (black line). The ligand can reach a maximum specific binding to the receptor at a certain concentration. The presence of antagonist
Therefore: a competitive antagonist reduces agonist potency.