Antagonists are substances that inhibit receptor stimulation by agonists or, they are substances that inhibit the effects of receptor stimulation by agonists. In chemical antagonism the antagonist directly interacts with the agonist to render the agonist inactive. There is no involvement of the receptor in this type of antagonism. Chemical antagonism has the effect of decreasing. When two independent agonists stimulate two independent receptor systems to cause counteracting effects on two independent effector systems we have physical antagonism. Functional antagonism is similar to physical antagonism. Two independent agonists stimulate two independent receptor systems to produce opposing effects on the same effector system. An example of functional antagonism is the G protein-regulated adenylate cyclase system. Irreversible antagonists are substances that interact with the receptor to inhibit it permanently. A simple competitive antagonist is a ligand that binds to the same site on a receptor as the agonist, but is incapable of stimulating the receptor. Simple competitive antagonists have zero intrinsic activity.