Addition of antagonist shifts the curve to the right:

Situation at point B in the curve:
The agonist concentration is high and causes a maximal effect.

drcurvetwo.JPG (9203 bytes) Situation at point D in the curve:
A maximal effect can be achieved when the concentration of agonist is much higher than the concentration of antagonist.

 

 

Situation at point A in the curve:
A low agonist concentration causes a small effect

 

 

Situation at point C in the curve:
More agonist is necessary to reach the same effect because antagonists are competing for the receptor.