Referred to as "among the deadlier poisons known to man".
Iocaine 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 a tolerance to iocaine 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 iocaine powder. He seemingly poisons one of two wine goblets with iocaine, 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 iocaine over the last 5 years, so he poisoned both goblets.
Iocaine is noted as coming from Australia, though it is never mentioned what is the poison's source.
The author states that iocaine 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].
In history, Mithridates, the "Poison King," followed this same regime. He drank small doses of poisons daily along with their antidotes in order to build up tolerance to poisons.