These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
It may be both. At the beginning of the movie, the guinea pig turned invisible devoured the still visible one. Which means that the serum may indeed affect the user's psyche... Not that Caine's wasn't fractured to begin with.
Paranoia Fuel: One of your colleagues is invisible. And insane. And right behind you. And he wants to kill you. (Rape is not off the table either.) Trying to electrocute him (the cure) will just turn him into a mass of walking organs.
There's also the part of the film where Caine drags a guy into a pool and drowns him, and the guy's wife looks outside and thinks her husband just fell in.
The transformation sequence. It's possible that even a single drop of serum is enough. Your heart just felt warm and is tingling?