Drug antagonists are drugs that compete for the available receptors. They may be noncompetitive and have no pharmacological effect of their own, or competitive in that they are capable of reversing or altering an effect already achieved.

Competitive antagonism is the antagonism that blocks or reverses the effects of an agonist, provided that the antagonist is given at an appropriate dosage. The antagonism is completely reversible, and an increase in the biophasic concentration of the agonist will overcome the effect of the antagonist.
Noncompetitive antagonism is when the antagonist removes the receptor or its response potential from the system; this may be by preventing the agonist from producing its effect at a receptor site, by irreversible change to the receptor or its capacity to respond. The antagonism is not reversible by increasing the concentration of the agonist.

The free medical dictionary by Farlex. Internet. Accessed on January 23, 2016.