The New York Post reports a study from Denmark that finds schizophrenia associated with use of cannabis. I would caution that the study appears to be correlational, although the write-up attempts to see causation.
Correlations does not establish causation, it is as likely that those with damaged minds find weed more attractive as it is that use of weed can bring on schizophrenia. All we know for sure is that it appears that to a greater-than-random-chance extent the two coexist in Danish individuals.
I am inclined to believe, without specific scientific proof, that much persistent use of street drugs is individuals with damaged minds self-medicating. These individuals are mostly unaware, I believe, of the underlying reason they keep doing drugs.
