One area that has not been discussed is the danger of the mentally ill using marijuana leading to psychotic states. I work as a counselor and I recently had a client to threatened to gouge out the eyes of myself and my co-workers and he admitted to me that he was a heavy marijuana user. He was very paranoid. I have worked with one client who ended up being arrested for making violent threats. I had another client was tricked into eating marjuana laced cookies and he developed horrifying hallucinations and he continued to be very haunted by the experience 6 months afterward. He described strange bodily sensations 8 months after having ingested the marijuana laced cookies.
It is interesting that the perpetrator of the recent Sagamihara massacre suspect tests positive for marijuana .
http://www.japantimes.co....
There are numerous well controlled laboratory studies that have shown that the administration of natural and synthetic cannibinoids do induce a psychotic state.
Biol Psychiatry. 2016 Apr 1;79(7):526-38. doi: 10.1016/j.biopsych.2016.01.011.
There is strong evidence that the use of marijuana in schizophrenics can induce psychotic episodes.

Cannabinoids and psychosis.Int Rev Psychiatry. 2009 Apr;21(2):152-62. doi:

Use of marijuana causes shrinkage of the subcortical structures in schizophrenic patients.
print]Relation between cannabis use and subcortical volumes in people at clinical high risk of psychosis.Psychiatry Res. 2016 Jun 2;254:3-9. doi: 10.1016/j.pscychresns.2016.06.001.
There is new evidence that there is a dysfunction in the CB-1 cannibanioid receptor in the brain of schizophrenics.