The image below shows the effect of agonists and antagonists on a biological reaction. In figure “A” the agonist alone causes a reaction but add the agonist and competitive antagonist this changes the reaction. You now need more agonist to get the same effect (this is competitive antagonist). In figure “B” you see again the effect an agonist has on a biological reaction but when we add the agonist together with a non-competitive irreversible antagonist we see the effect is to reduce the rate of the reaction.

Courtesy: Pharmacology Corner