Excessive cannabis use can lead to psychosis as explained, for example, in 2007
here and
here, in
2005,
1994,
1993, and
1982.
... scientists found a more disturbing outlook for "heavy users" of pot, those who used it daily or weekly: Their risk for psychosis jumped to a range of 50 percent to 200 percent.
As well, combining antidepressants with cannabis can yield acute psychosis in which confusion, amnesia, delusions, hallucinations, anxiety, agitation and hypomanic symptoms predominate; discussed
here and
here.
Excessive cannabis use can lead to psychosis as explained, for example, in 2007 here and here, in 2005, 1994, 1993, and 1982. 
