Recently the press has reported on a piece of research linking the use of cannabis with schizophrenia. The study published in the Lancet (a respectable medical journal) was quoted by the daily mail as indicating a 40% increase in risk of developing schizophrenia from only one joint. Other newspapers reported the findings in less sensationalist terms but few that I came across pointed out two very salient facts (or if they did they failed to give them the prominence they deserved):
1) The actual risk factors that the study attributes to cannabis (for heavy users 200%) are smaller than the risk factors attributed to alcohol in other similar studies (Between 300% [women] and 800% [men] for heavy alcohol users). The greater apparent risk from alcohol may well also hold true for lighter use.
2) There has been no observed increase in the incidence of schizophrenia to match that expected from such levels of risk identified by the study. A fact that the authors of the study noted (the daily mail does mention this although it fails to give it due prominence).
“Despite a thorough review the authors admit that there is no conclusive evidence that cannabis use causes psychotic illness. Their prediction that 14 per cent of psychotic outcomes in young adults in the UK may be due to cannabis use is not supported by the fact that the incidence of schizophrenia has not shown any significant change in the past 30 years.” - Leslie Iverson
This is really indicative of a more general tendency of the media to seriously misrepresent the evidence with regards to cannabis.
It is well known within the medical community that schizophrenia causes people to behave erratically. In particular it makes people more likely to take drugs. In addition the active compound in cannabis (titled THC) binds to many of the same activation sites that certain natural hormones do. These hormones are notable in their low levels in schizophrenics. So schizophrenics (or those at risk of developing schizophrenia) are more likely to take cannabis as a form of self medication. All this adds up to a very powerful causative link from being at risk of schizophrenia to taking cannabis.
This is important because any study that implicates cannabis as a cause of schizophrenia must take this information into account.
The media typically quote the results of studies showing correlation between schizophrenia and cannabis usage as if that gives some support to the idea of cannabis usage causing the disease. It does not.
The media typically quote physicians who've encountered schizophrenics who believe in the links. Anecdotes are useful when you have no or few scientific studies. That is no longer the case.
In short the media as a whole really suck on matters cannabis related. Indeed I'd say with regard to other drugs too but thats another story.
On the other hand I don't want to denigrate the authors of this study. Where they have been quoted their statements tend to be measured and sensible for instance:
“We believe there is now enough evidence to inform people that using cannabis could increase their risk of developing a psychotic illness later in life.” - Hardly sensationalism!
"That risk is still relatively low, but it might be enough to prompt some users to cut back or stop smoking." (Stanley Zammit) - Just the sort of measured response I would like to see from the media...