From what I understand, the drugs awaken the power of Persona in a potential. The only problem is, because it's being forced out, it retaliates against its host. Meaning, the host has to keep taking the drug for the rest of their life, so that their Persona won't kill them. In essence, for people like Takaya and the others, their Personas are just a hair's breadth away from returning to Shadows because they are just barely being suppressed by their host's ego.