In Act 5 Scene 1-2 of Romeo and Juliet:Why does Romeo call the poison a "cordial potion"?
I assume you are talking about these lines from Act V, Scene 1:
It is not exactly what you quote, but it seems to be the only place in the play that the word "cordial" is used.
The reason it is called this is that Shakespeare was using the word "cordial" to mean a medicine. What Romeo is saying is that what the apothecary sold him is not a poison but rather a medicine. In other words, it is good for him. He is saying that if Juliet is dead, then poison is what he needs because all he wants to do is die with her.