There is no poison! Or there is!
Basically, there is no such poison. You just make everyone think there is.
Think about it. You have a poison that never works if you don't know you were poisoned, but always works when you have been poisoned. The only way to know FOR SURE that you were poisoned is to be dead.
So, how does someone get this myth going. Simply, you claim that you have such a poison, and then use some real poison of some individuals. Make sure, after they have been poisoned, that they realize it (make it easy, or obvious, or use a whole lot of discrete suggestion), then get these events some publicity.
If anyone later thinks that they were poisoned, just tell them that you didn't poison them, and they therefore survived.
When you want to use this poison, simply make sure that it is noticed.
So, basically, IT IS A SHAM. There is not a spider in every cup. Those who never realize they were poisoned never were. You just have to make sure that every real victim gets enough publicity to the fact that they realized they were poisoned for your fantastic story to continue. Ultimately, dead men tell no tales, live men must not have realized or not have been poisoned. It is up to you, as the man behind the curtain, to decide which!