Well....you start to think or perceive things, which aren't real (call it hallucinations) o you start to believe things....which simply aren't true (delusions).
I sat and viewed a podcast recently, and came to realize....probably a quarter of the American population....fits into this problem.
How you notice these folks?
Typically, it goes to five things: first, you start to screw up at work. Second, you start to lose your concentration. Third, you are both uneasy and skeptical of just about everything around you. Fourth, you only chat with certain people....who mostly identify with your version of reality. Finally, you start to notice that you have maximum emotions on things which would usually be one-star events.
Drugs a part of this? I'd say it contributes, but a lot of folks go off on this tangent....without any drug-help.