(in pharmacology) an antagonist for which the rate of dissociation from its receptor is relatively high, such that by increasing the concentration of an agonist the receptor occupancy (def. 2) of the antagonist falls. At sufficiently high concentrations of agonist a maximal tissue response can be obtained, even in the presence of the antagonist, such that the extent of antagonism is determined by the relative concentrations of agonist and antagonist.