In the film "The Princess Bride," what kind of poison does Westley put in the wine during his battle of wits with Vizzini?

There are a number of battles that occur in the "The Princess Bride." One such battle is a "Battle of Wits" for the life of Buttercup. This is a battle between Westley (Man in Black) and Vizzini. Where the other battles are sporting events between individual men, this one concerning Vizzini is won by deception (a lie).

Westley puts a deadly undetectable poison (iocane) into two goblets of wine. He tells Vizzini that he has put it into only one goblet. He asks Vizzini to chose a goblet; they will both drink at the same time. But Vizzini lies to Westley. He distracts Westley with a lie (he sees something in the bushes); he changes the goblets. He thinks Westley will be mistaken about his choice. He has acted and gave Westley the goblet with the poison.

Vizzini's trick however is of no use. He drinks the poison and dies. Westley also drinks the poison, but he at that point in time tells Buttercup over Vizzini's dead body, "They were both poisoned. I spent the last few years building up an immunity to iocane powder."