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 results in a shift of the line to the right without change in slope or maximum (green line) : in other words in the presence of antagonist, much more ligand is required to achieve the same percentage of binding as without antagonist present. In the presence of a high antagonist concentration (red line), extremely high ligand concentrations are required to reach maximal binding to the receptor.
Therefore: a competitive antagonist reduces agonist potency.