Cannabis and Psychosis – More ‘Hand Waving’

Summary: Lancet published study claimed that smoking cannabis, even infrequently, increased chances of schizophrenia by 40%. Tons of media coverage. When doing percentage math, this means an increased percentage of people getting schizophrenia in the population is slightly more than .4% of the population, as onlyt 1.1% has schizophrenia.

Cannabis and Psychosis – More ‘Hand Waving’

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by ScienceDave | July 30, 2007 at 07:52 am

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  • revelation
  • Who says marijuana is a victimless crime?

“A fifteen-year-old lad apprehended in the act of staging a holdup – fifteen years old and a marijuana addict. Here is a most tragic case.

Yes. I remember. Just a young boy… under the influence of drugs… who killed his entire family with an axe.”[source]

When this story came out, I was immediately reminded of the 1936 propaganda film, Reefer Madness, produced by a small church group intending to “scare the living bejeezus out of every parent who watched it” [source]
What is Psychosis?
I found myself asking this question when I heard about this story.  I always imagined “psychoses” and “psychotics” as being an arcane method of categorizing mental disorders, adopted by the then-adolescent field of psychology of the 1950’s.  However, I was able to find a number of definitions, all of which typically dealt with a lack of contact with reality, i.e. delusions, hallucinations, a disconnect with reality, and the like.

Here’s a definition from the King’s Medical Library Engine Medical Dictionary

A severe mental disorder, with or without organic damage, characterized by derangement of personality and loss of contact with reality and causing deterioration of normal social functioning.


According to, psychoses can be categorized as follows:

  • Organic Brain Disorders: commonly observed as dementia
  • Psychosis Due to Drug Use: LSD, PCP, mescaline, etc.
  • Mania: i.e. manic depression
  • Schizophrenia: often associated with hearing voices and unfounded paranoia


There has been plenty of hoopla surrounding a recently published article (Cannabis use and risk of psychotic or aff ective mental health  outcomes: a systematic review, 2007 The Lancet (370): 319-328) claiming an increased incidence of psychoses in cannabis users.

After reading a number of news articles, all of which emphasized a 40% increased risk of psychosis in one-time users, and up to a 200% risk in chronic smokers, I decided to read the article myself.

Their analysis, based on a review of 35 previously published articles, did find the above numbers to be true.  Individuals who self medicate, use other illegal drugs, or are prisoners were not included in the study.  Although data was adjusted and sifted through with great care, they suthors do recognize that there could be significant unrealised bias in the data they coalesced.

But what are these figures really claiming?  According to the National Institute of Mental Health, 1.1% of the US population suffers from schizophrenia, the most recognizable form of psychosis [source].  So, lets try this:

A 40% increase sounds like plenty, however let me put this into perspective: 40% of a 40% likelihood is 16%, therefore resulting in a 56% likelihood of psychosis;  however, 40% of 1% is only 0.4%, resulting in a 1.4% likelihood.

In this example, there is a very big difference between 16% and 0.4%.  However, I believe much of the buzz surrounding this topic is due to a simply mathematical misunderstanding – percents are always in reference to a specific value, they are not absolute values in themselves.

So would you be concerned if your chances increased from 1% to 1.4%?  Personally, I would not.  However, from a policy-related perspective concerning the mitigation of drug-related crime and illness this is a big deal.  Take a population of 10 million pot-toking youth, and increase the rate of illness by 0.4% and you have an additional 40,000 cases of psychosis in the future.  That said, this is indeed not a

As the authors said, “Although individual lifetime risk of chronic psychotic disorders  such as schizophrenia, even in people who use cannabis regularly, is likely to be low (less than 3%), cannabis use can be expected to have a substantial effect on psychotic disorders at a population level because exposure to this drug is so common.”

Nonetheless, empathetic claims and blanket statements such as the following cannot help the hard science behind the study. If anything, it appears alarmist.

“Young people who otherwise would have been very unlikely to developed psychosis will, as a result of their early cannabis use, be affected by a life-long and severely disabling mental illness that will markedly narrow their life choices and quality of life,” said Professor Drummond.

Here is another example from Dr. Linda Harris…

“From a mental health perspective we do need to look at the reasons behind a society that’s drifting towards depressing and anxiety – cannabis could be a factor in this.”

If there was ever arm waving on a subject, this is a perfect example – an overly enthusiastic ground air traffic controller on their first day of the job, after a pot of coffee and a sixer of Red Bull would be competing with this sort of arm waving.
Professionals in the field have weighed in, according to an article published yesterday in the Independent.  Fifty of the top leading experts in drugs and mental health are convinced that pot is not as benign as it is often portrayed. However, not everyone is convinced.

Professor Tim Kirkham, a psychologist at Liverpool University, argued: “Cannabis has been used safely for many thousands of years,” and says there have been “concerted efforts to demonise the drug’s use.” Dr Trevor Turner, former vice president of the Royal College of Psychiatrists, says: “I don’t think it causes mental illness. I have never seen a case of so-called cannabis psychosis.

Dame Ruth Runciman, the chair of UK Drug Policy Centre who set in motion the downgrading of cannabis, disputes that the drug of today is any different to the weed that Ms Smith would have toked back in early 1980s.

“How do you know it’s stronger?” she said, adding: “There is indubitably some skunk that is stronger about the place, but the evidence has been hugely exaggerated and does not support such an alarmist view… Cannabis as Class C is exactly where it should be.”

I think it is very important to remember that no biochemical mechanism has been provided to link psychosis with cannabis use.  Although the author’s speculate that since cannabis alters the brain’s dopamine levels, a necesary neurotransmitter, there very well could be a causal relationship between cannabis use and psychosis, they have yet to actually determine any link at all How might this, if at all, influence governmental policy on the legalization of cannabis?



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