I once had a single, very high dose Psilocybin experience which lead to a lasting change that was very much for the worse.

Having had my perception of reality profoundly changed by the experience I began to question my senses on a regular basis, with associated anxiety and panic. Then there were also the nightmares where I was tripping, would wake up, have something weird / trippy happen have a "world is ending" anxiety attack complete with ears ringing so loudly its painful. Then I'd wake up from that, with similar results, only to wake up from that. Never really sure if I was fully awake or not.

Thankfully after 18 months or so, it went away.

Of course, your mileage my vary :-)

I personally am in favour of this type of research but just wanted to share my experience. Its not all beer and skittles!