Antagonism
It is pharmacological opposite effect of drug Agonist. Drug Antagonist are drugs that decrease the actions of another drug or endogenous ligand. Antagonists, however, have no intrinsic activity and, therefore, produce no effect by themselves. Although antagonists have no intrinsic activity, they are able to bind avidly to target receptors because they possess strong affinity. If both the antagonist and the agonist bind to the same site on the receptor, they are said to be "competitive." For example, the antihypertensive drug prazosin competes with the endogenous ligand, norepinephrine, at a1-adrenoceptors, decreasing vascular smooth muscle tone and reducing blood pressure.