This very "debate" proves that if cannabis causes psychosis, it does not cause very much of it, very often (see references below). According to widely published expert Professor Fergusson of Christchurch, the jury is still �out� on causation, but there probably is a small group whose numbers are hard to measure considering schizophrenia is relatively common and cannabis smoking is rife in our society.
Stringent laws against cannabis have never been shown to reduce the use of cannabis or its complications. Even in places like Jamaica and Nimbin (NSW) where cannabis is used rather heavily, no evidence has ever been produced that schizophrenia is more commonly diagnosed. Comparisons of areas with decriminalisation such as Canberra, Adelaide and Amsterdam against places using severe law enforcement (eg. Bali and San Francisco) has never shown either higher rates of cannabis use, nor higher rates of psychosis to my best knowledge.
An important finding in recent years is that the younger one starts using cannabis (or probably any other drug), the more likely one is to develop serious and perhaps permanent complications. A strong reliance on law enforcement in drug control may well encourage more children to become involved as happened in the American alcohol prohibition period last century.
I hope these references are useful for readers.
References
Reinarman C, Cohen PDA, Kaal HL. The Limited Relevance of Drug Policy: Cannabis in Amsterdam and in San Francisco. Am J Public Health. (2004) 94:836 842
Fergusson DM, Horwood LJ, Swain-Campbell N. Cannabis use and psychological adjustment in adolescence and young adulthood. Addiction (2002) 97:1123-35
Lynskey MT, Heath AC, Bucholz KK, Slutske WS, Madden PAF, Nelson EC, Statham DJ, Martin NG. Escalation of Drug Use in Early-Onset Cannabis Users vs Co-twin Controls. JAMA (2003) 289:427-433
van Os J, Bak M, Hanssen M, Bijl V, de Graaf R, Verdoux H. Cannabis Use and Psychosis: A Longitudinal Population-based Study. Am J Epidemiol 2002; 156:319-327
Rey JM, Sawyer MG, Rafael B, Patton GC, Lynskey M. Mental health of teenagers who use cannabis: Results of an Australian survey The British Journal of Psychiatry (2002) 180: 216-221
