What others said about shattering your worldview, but that can be a good thing. Most bad trips, if they have a negative effect, basically cause PTSD. A "flashback" is actually just q panic attack where one feels the same fight or flight response incurred during the bad trip. That physiological reaction can then cause a dissociative psychological reaction similar to the effects of the LSD. So a person feels like they're "tripping" again land back in the same mental space of the LSD experience. If that experience involved delusions and psychotic symptoms, then those may also return.

Basically it's the same thing that happens to soldiers when they get a panic attack from a loud noise and feel like they're back in a conflict zone.

All that being said, some people do go off the deep end and are profoundly changed after a psychedelic experience. The general thinking on those people is that they had the underlying psychosis or mental condition and the psychedelic just brought it out and triggered the break.

There's some really interesting research on the use of periodic dosing of psilocybin as a treatment for clinical depression. They're doing some clinical trials with it at Johns Hopkins currently.

This post was edited on 7/6/17 at 5:20 pm