Peer-reviewed journal finds no link between cannabis and schizophrenia

Last edited Sat Dec 17, 2011, 07:17 PM - Edit history (1)

http://www.ncbi.nlm.nih.gov/pubmed/19560900

This paper investigates whether this (a predicted increase in schizophrenia and/or psychosis among cannabis users) has occurred in the UK by examining trends in the annual prevalence and incidence of schizophrenia and psychoses, as measured by diagnosed cases from 1996 to 2005.

Retrospective analysis of the General Practice Research Database (GPRD) was conducted for 183 practices in England, Wales, Scotland and Northern Ireland. The study cohort comprised almost 600,000 patients each year, representing approximately 2.3% of the UK population aged 16 to 44.

Between 1996 and 2005 the incidence and prevalence of schizophrenia and psychoses were either stable or declining. Explanations other than a genuine stability or decline were considered, but appeared less plausible. In conclusion, this study did not find any evidence of increasing schizophrenia or psychoses in the general population from 1996 to 2005.

Studies that claimed schizophrenia cast in doubt (Sept. 2009):
http://www.abc.net.au/science/articles/2009/09/01/2673334.htm

Previous research has suggested cannabis use increases the risk of being diagnosed with schizophrenia or other psychotic disorders.

This latest study, led by Dr Martin Frisher of Keele University, examined the records of 600,000 patients aged between 16 and 44, but failed to find a similar link. Frisher and colleagues compared the trends of cannabis use with general practitioner records of schizophrenia.

They argue if cannabis use does cause schizophrenia, then an increase in cannabis use should be followed by an increase in the incidence of schizophrenia. According to the study, cannabis use in the UK between 1972 and 2002 has increased four-fold in the general population, and 18-fold among under-18s.

...But the researchers found no increase in the diagnosis of schizophrenia or other psychotic disorders during that period. In fact some of the data suggested the incidence of these conditions had decreased.

"This study does not therefore support the specific causal link between cannabis use and the incidence of psychotic disorders," the authors say. "This concurs with other reports indicating that increases in population cannabis use have not been followed by increases in psychotic incidence."

Minimal Link Between Psychosis and Marijuana (Oct. 2009):
http://www.sciencedaily.com/releases/2009/10/091022101538.htm

Scientists from Bristol, Cambridge and the London School of Hygiene and Tropical Medicine took the latest information on numbers of cannabis users, the risk of developing schizophrenia, and the risk that cannabis use causes schizophrenia to estimate how many cannabis users may need to be stopped to prevent one case of schizophrenia. The study found it would be necessary to stop 2800 heavy cannabis users in young men and over 5000 heavy cannabis users in young women to prevent a single case of schizophrenia. Among light cannabis users, those numbers rise to over 10,000 young men and nearly 30,000 young women to prevent one case of schizophrenia.

Cannabis-induced schizophrenia is merely schizophrenia (Nov. 2008)
http://www.reuters.com/article/2008/11/03/us-pot-induced-psychosis-idUSTRE4A26JV20081103?feedType=RSS&feedName=healthNews&rpc=22&sp=true

In a previous study, Arendt and colleagues found that nearly half of people who had an episode of cannabis-induced psychosis went on to develop schizophrenia within the next six years. In the current study, the researchers looked at the genetic roots of both conditions by comparing the family histories of 609 people treated for cannabis-induced psychosis and 6,476 who had been treated for schizophrenia or a related psychiatric condition.

They found that individuals treated for post-pot smoking psychotic episodes had the same likelihood of having a mother, sister or other "first-degree" relative with schizophrenia as did the individuals who had actually been treated for schizophrenia themselves. This suggests that cannabis-induced psychosis and schizophrenia are one and the same, the researchers note. "These people would have developed schizophrenia whether or not they used cannabis," Arendt explained in comments to Reuters Health.

Based on the findings, the researcher says, "cannabis-induced psychosis is probably not a valid diagnosis. It should be considered schizophrenia."

Roger Pertwee, leading British pharmacologist, says cannabis is not a threat to the general population in regard to schizophrenia. Only those with existing risk factors are at risk.
http://www.britishscienceassociation.org/web/News/FestivalNews/_Rethinkingcannabis.htm

you could identify people who might be at risk of developing schizophrenia. Cannabis is one factor which increases the risk of schizophrenia, but only if it's mixed with a bad childhood environment or a genetic predisposition to schizophrenia.

...and no indication of brain damage with heavy usage (July 2003):
http://www.webmd.com/mental-health/news/20030701/heavy-marijuana-use-doesnt-damage-brain

Long-term and even daily marijuana use doesn't appear to cause permanent brain damage, adding to evidence that it can be a safe and effective treatment for a wide range of diseases, say researchers.

The researchers found only a "very small" impairment in memory and learning among long-term marijuana users. Otherwise, scores on thinking tests were similar to those who don't smoke marijuana, according to a new analysis of 15 previous studies.

In those studies, some 700 regular marijuana users were compared with 484 non-users on various aspects of brain function -- including reaction time, language and motor skills, reasoning ability, memory, and the ability to learn new information.