This is a sort of act related question. I have an important drug that causes the protagonist to encounter people and things he normally wouldn't, and it is the foundation of a convoluted plan by the antagonist. The drug is fantastical, causing others to experience what the protag hallucinates, essentially making it real as more and more people believe it is. Otherwise, the world is 'real life.'
Do I need to introduce the fantasy concepts in the first act outright, or is hinting at their existence sufficient? I want the reveal that he's been poisoned to come out of nowhere at the end of the story and turn it all around, but even suggesting such a drug could exist in acts 1 and 2 would instantly tip the audience off that the mystery isn't genuine.