Here's a quote from the MAIL (!) that you can use on those that believe cannabis causes mental illness:
"The study flies in the in the face of previous research, which has linked marijuana to increased rates of depression, anxiety, psychosis and schizophrenia
Can cannabis help TREAT depression? Compounds in marijuana 'may help balance brain chemical linked to emotions'
Cannabis and mental health: Royal College of Psychiatrsists, Leaflet, November 2011
The rate of general schizophrenia has stayed the same over recent years with 38,517 in 1998-99, going down a bit in the years in between and up
to 37,736 in 2002-2003. Likewise regular schizophrenia has stayed pretty much the same with 25,967 in 98-99 and 25,269 in 2002-03. The
rate of cannabis related mental health disorders has risen slightly between 98-99 and 2002-03 going from 669 to 788. Within that figure
psychosis has gone from 296 to 380. (source Department of Health 2004
Link between cannabis use and mental illness negligible experts sayBMH, November 2009
The report, CANNABIS AND PSYCHOSIS was prepared by Prof. Wayne Hall of the National Drug and Alcohol Research Centre, Australia.
Prof. Hall concludes ;
"This report reviews evidence on two hypotheses about the relationship between cannabis use and psychosis. The first hypothesis is that heavy cannabis use may cause a "cannabis psychosis" - a psychosis that would not occur in the absence of cannabis use, the symptoms of which are preceded by heavy cannabis use and remit after abstinence. The second hypothesis is that cannabis use may precipitate schizophrenia, or exacerbate its symptoms."
"There is some clinical support for the first hypothesis. If these disorders exist they seem to be rare, because they require very high doses of THC, the prolonged use of highly potent forms of cannabis, or a pre-existing (but as yet unspecified) vulnerability. There is more support for the second hypothesis in that a large prospective study has shown a linear relationship between the frequency with which cannabis had been used by age 18 and the risks over the subsequent 15 years of a diagnosis of schizophrenia. It is still unclear whether this means that cannabis use precipitates schizophrenia, whether it is a form of "self-medication", or whether the association is due to other drugs, such as amphetamines, which heavy cannabis users are more likely to use. There is better evidence that cannabis use can exacerbate the symptoms of schizophrenia. Mental health services should identify patients with schizophrenia who use alcohol, cannabis and other drugs and advise them to abstain or to greatly reduce their drug usage.'
"The treated incidence of schizophrenia , and particularly early onset, acute cases, has declined (or remained stable) during the 1970's and 1980's when cannabis use increased among young adults in Australia and North America. Although there are complications in interpreting such trends a large reduction in treated incidence has been observed in a number of countries and it cannot be explained as a diagnostic artefact."
Source: Technical Report No. 55, Presented Feb. 1998
National Drug and Alcohol Research Centre, Australia
Phone (612) 9398 9333