Iocaine powder

A colorless, odorless, and deadly poison from Australia. Westley spent two years building up a tolerance to iocaine powder. He uses it to trick Vizzini in their battle of wits.

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.