Referred to as among the deadlier poisons known to man.
Iocane powder is noted as being one of the deadliest poisons known to man. It has no odor, no taste, and will dissolve instantly when poured into liquid.
However, one can build up an immunity to iocane by gradually ingesting trace doses over time.
In the story
Westley, under the guise of the Dread Pirate Roberts, when challenging Vizzini to a battle of wits presents him with iocane powder. He seemingly poisons one of two wine goblets with iocane, and challenges Vizzini to figure out which one is poisoned.
Vizzini dies after they both drink, and Westley reveals to Buttercup that Vizzini would have died no matter which goblet he drank from: he himself had successfully built up an immunity to iocane over the last 5 years, so he poisoned both goblets.
Iocane is noted as coming from Australia, though it is never mentioned what is the poison's source.
The author states that Iocane powder is a fictional substance, invented for the novel [cited in As You Wish: Inconceivable Tales from the Making of The Princess Bride by Cary Elwes].