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Shame on you, David! (pot and schizophrenia)

By admin | February 12, 2010

Dr. David Suzuki

219-2211 West 4th Avenue

Vancouver BC

V6K 4S2

Dear Dr. David Suzuki:

I watched your show, The Nature of Things, on Thursday, January 28, 2010, concerning the relationship between marijuana consumption and schizophrenia. I know you were not the writer/director (Bruce Mohun was), but you are responsible for evaluating the science of documentaries you present on your show. I was shocked by the weakness of the studies in the program, the wild misinterpretation of data, and the almost nonsensical conclusion generated.

I have been monitoring marijuana research for 40 years, and I have been watching the pot-schizophrenia research closely for about 10. The conclusion I have reached is the opposite of that reached by The Downside of High: Marijuana does not increase the risk of schizophrenia. It does relieve the symptoms of schizophrenia, however, making it an attractive drug for those who suffer from this disease.

Whether you agree with my conclusion is not the point. The point is, rather, that The Downside of High is a scientific mess, its most significant fault being that it did not examine alternative hypotheses. When you’re examining a disorder with an onset between the ages of 15 and 25 and linking it with behaviours (smoking pot) that begin about the same time, behaviours that without question relieve symptoms of the disorder, you are bound to find high correlations between the behaviours and the disorder. As a scientist, you know that this is dangerous territory and that any interpretations of the correlations must made with great caution. Alternative explanations MUST be examined as well.

In my opinion, the research in The Downside of High is essentially worthless. Without exception, the data were generated by what philosophers of science call “quasi-experimental designs.” They are not true experiments, which require randomization of subjects into groups: the groups are then treated differently and outcomes assessed. A true experiment, in this case, would require a large sample of several thousand teenagers, split by randomization into two groups, one of which would smoke five joints a day for ten years, the other refraining from cannabis for the same time. This kind of study, of course, is unethical, which is why medical research, especially epidemiology, is fraught with interpretative perils.

The groups to be compared in The Downside of High are not formed randomly. They are defined by pre-experimental differences, in most cases as “schizophrenic” or “not schizophrenic.” These groups differ, in addition to mental disorder, in an almost infinite number of ways. Any difference in a dependent variable could be attributed to any one of these millions of variables — to a single one only with great risk of invalidity. I’m sure I needn’t explain quasi-experimental designs, and their perils, to you, one of the premier scientists of our generation.

The alternative hypothesis, that marijuana use does not increase the risk of schizophrenia, was not mentioned in The Downside of High, although it was refuted, poorly. Especially troubling to the pot-schizophrenia hypothesis are data showing absolutely no relationship between increases in marijuana use among teenagers between 1970 and the present and increases in diagnosed schizophrenia. Two such studies are attached at the bottom of this letter. In The Downside of High , the scientists tried to explain such results by claiming “reporting variations,” an explanation totally without empirical support.

All in all, the science in The Downside of High is shabby indeed, the more probable alternative hypothesis is not discussed at all, and the conclusions, in my opinion, are incorrect. It is not a significant contribution to the debate of a serious issue. I am surprised, therefore, that such a knowledgeable scientist as yourself would let this program slip through your scientific filters.


James Geiwitz, Ph.D.


Science: The development of the number of new schizophrenia

cases in the UK does not support the hypothesis that cannabis use

increases schizophrenia risk

According to research of scientists at Keele University in

Staffordshire, UK, the incidence (the number of new diagnosed

cases) of schizophrenia in the years 1996 to 2005 does not support

the hypothesis that cannabis use increases the risk for the

development of schizophrenia or psychosis. For this study an

analysis of data from 183 practices in England, Wales, Scotland

and Northern Ireland was conducted. The study cohort comprised

almost 600,000 patients each year, representing approximately 2.3

per cent of the UK population aged 16 to 44. Between 1996 and

2005 the incidence of schizophrenia and psychoses were either

stable or declining.

A recently published study found that cannabis use increased

fourfold between 1972 and 2002 and 18-fold among people under

18 years of age. If the risk of schizophrenia is elevated 1.8-fold

among light users and 3.1-fold among heavy users as suggested

by another study, and assuming an elevated risk for 20 years, an

increase in schizophrenia incidence of 29 per cent would have

been expected between 1996 and 2005. Researches concluded

that “the causal models linking cannabis with

schizophrenia/psychoses are not supported by this study” and that

“the underlying causes of schizophrenia/psychoses remained

stable/declined over the study period.”

(Source: Frisher M, Crome I, Martino O, Croft P. Assessing the

impact of cannabis use on trends in diagnosed schizophrenia in the

United Kingdom from 1996 to 2005. Schizophr Res. 2009 Jun 26.)

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