Does Cannabis Use Cause Schizophrenia?

Over the past few years, the Australian media have promoted a new version of  “Reefer Madness”. According to this myth, cannabis use causes schizophrenia. The “proof” for this is to exhibit a few schizophrenics who use cannabis. The fact that there are millions of Australians who use cannabis and who are not schizophrenics is ignored.

Because schizophrenics tend to have extremely high rates of illicit drug use, it is relatively easy to find schizophrenics who use cannabis. Table 1 (below) shows the relative amount of psychoactive drug use among Australian schizophrenics versus the Australian population in general.


Psychoactive drug Use Australian Pop. Aust Schizo.
Tobacco (M) 27.3% 73.2%
(F) 20.3% 56.3%
Cannabis 18% 35%
Tranquillisers 4% 13.5%
Amphetamines 4% 12%
LSD 3% 10%
Heroin 1% 8%
Table 1: Recent Use of Psychoactive Drugs in the Australian Population (over 14) Versus Australian Schizophrenics (1998). (The schizophrenia data is from People Living with Psychotic Illness: An Australian Study 1997-98; while the figures for the population in general are from the National Drug Strategy Household Survey for 1998).

You could argue that the table supports the Reefer Madness hypothesis: Schizophrenics use cannabis at a significantly higher rate than the population in general, therefore cannabis must cause schizophrenia.

However, the totality of the evidence shows a different story. Although there are high rates of cannabis use among schizophrenics, the bigger picture shows that the level of drug taking by schizophrenics is significantly higher across the whole range of illicit and licit substances.

Unlike the population in general, schizophrenics are sick people who suffer from a mysterious and life-threatening disease. They are mad, and are treated as such; locked up, restrained, forcibly medicated. Taking drugs is the way they learn to treat their illness; and when their disease makes them depressed and suicidal, they seek out drugs to alleviate these moods. When their prescribed drugs don’t work, they use drugs from the street that are euphoriant and mood-altering. No doubt there are many occasions when these feel-good drugs work, and help those suffering from schizophrenia feel better. Likewise, there are no doubt occasions when the effects of illicit drugs compound the problems of schizophrenia and contribute to a breakdown.

You can regard the drugs in the list above either, as drugs that may cause schizophrenia (in which case, tobacco clearly causes schizophrenia) or alternatively, as drugs that schizophrenics find useful in alleviating the symptoms of their disease.

My suspicion is that the drugs in the table above are the ones that schizophrenics choose to self-medicate with. They are supplementary medication for a group of ill people who lack an effective therapeutic agent. Indeed, before they were banned, LSD, ecstasy, cannabis and amphetamines were all used at various times in the treatment of mental illness. In a post-prohibition world, the psychiatric use of these drugs (under medical supervision) might well reemerge.

Imposing the puritan ideal of a drug-free lifestyle on the schizophrenic population is itself rather crazy.


Australia’s Cannabis Plague

An important feature of the table above is the high level of cannabis use across the entire Australian population. Australians smoke more pot than nearly any other nationality on the planet.

Although Australia has many puritans who support drug prohibition and see drug-taking as evil, a great proportion of the Australian population is hedonistic and pleasure-seeking and has a long tradition of disregarding puritans, whom they contemptuously refer to as “wowsers”. Manning Clark saw Australian history as a struggle between “the enlargers of life” and “the straighteners” and tribal conflicts between “wowsers” and “larrikins”, “bodgies”, “surfies” and “hippies”—as young Australians have been variously known—are a recurring feature of life “down-under”.

Both pot smoking and the twenty-something lifestyle began in Australia with the Baby-Boomer generation of the 1960s. Since then, smoking pot became a rite-of-passage for Australian teenagers into the desired twenty-something world.

By 1998, recent use of marijuana among Australians aged 20-29 stood at 36%. Thirty years previous it stood at 0%. Decades of tougher and tougher drug laws, culminating in a US-style “War on Drugs”, caused the rate of cannabis use among twenty-somethings to fall by minus 36%!

By forbidding cannabis use, Australian puritans made it almost compulsory among Australia’s twenty-somethings. Partly this can be explained as “larrikin” rebellion, and partly as the “forbidden fruit” syndrome.

Table 2 (below) distributes Australia’s recent cannabis users into cohorts based on sex and age to demonstrate the high levels of cannabis use in the twenty-something age group. The 1998 group of twenty-somethings are the children of the Baby Boomers who make up the 40-49 cohort. The low level of use in the 60+ age group reflects the low cannabis use in the pre-Baby-Boomer generation.


Age Group M F Total
14-19 35% 34.2% 34.6%
20-29 43.7% 29.3% 36.5%
30-39 24.1% 16.3% 20.2%
40-49 16.6% 6.3% 11.3%
50-59 5.6% 7.6% 6.6%
60+ 1.1% 1.2% 1.1%
All ages 21.3% 14.7% 17.9%
Table 2: Recent Use of Cannabis in Australia by Sex and Age (1998


The Reefer Madness hypothesis

The extent of Australia’s cannabis plague and its relatively recent history make Australia an ideal country to study the relationship between cannabis and schizophrenia.

Let us accept the Reefer Madness hypothesis and assume that cannabis use causes schizophrenia. What this means is that if we can find a population where cannabis use is recent and has increased enormously over the last decades, we should be able to trace a corresponding “Madness” plague in that country, accompanying the “Reefer” plague.  Because “Reefer” causes “Madness”, we would predict a schizophrenic epidemic as many new cases of schizophrenia caused by the cannabis plague increased the natural, background level of schizophrenia in the population.

Australia is ideal for this purpose.

Because of our recent high levels of cannabis use, Australia should, over the last decades, have developed rates of schizophrenia higher than anywhere in the world.

If we were to chart Australia’s “Reefer” plague and compare it to our schizophrenia levels, the Reefer Madness hypothesis should be proven by the discovery of a corresponding “Madness” plague.

Table 3 (below) gives the dimension of Australia’s Reefer plague and the polls on which these estimates are based.


Year No. Cannabis Users Survey
1973 500,000 McNair
1977 675,000 McNair
1979 750,000 Morgan
1982 975,000 Morgan
1984 1,175,000 Morgan / NCADA
1988 1,500,000 Morgan / NCADA
1991 1,625,000 NCADA
1993 1,666,000 NDS
1995 1,850,000 NDS
1998 2,700,000 NDS
Table 3: The Australian Reefer Plague: Recent Users of cannabis in Australia, 1973-1998

As the table shows, the potential army of recruits for schizophrenia among Australia’s cannabis users is enormous. Even a prevalence rate of 1 in 100 (barely twice the background level) would add tens of thousands to the schizophrenic population.

If cannabis caused schizophrenia, the past two generations of Australians would have seen an unrivalled schizophrenia plague, matching our cannabis plague.

The popular press would be full of news of this extraordinary increase in madness in Australia, and some suitably “dumbed-down” name like “Mad Aussie Syndrome” would be given to the phenomena. The investigation of this disease —Mad Aussie Probe—and the determination of its cause —Mad Aussie Shock—would be front page news.

Eventually some researcher would correlate the extraordinary increase in schizophrenia and match it with the extraordinary increase in cannabis use. This would constitute credible scientific proof of the proposition that cannabis caused schizophrenia.

The fact that nothing like this happened is compelling proof that cannabis does NOT cause schizophrenia.

The evidence that schizophrenia levels have remained constant is overwhelming, not just in Australia, but right throughout the world.

People Living with Psychotic Illness: An Australian Study 1997-98, published as part of the National Survey of Mental Health and Wellbeing, found that the prevalence for schizophrenia and schizoaffective disorders in urban Australia was about 3 per 1000 in 1998. This was in the middle range of a world-wide prevalence of 2 to 5 per 1000.

The report commented:

“Epidemiological studies estimating the prevalence of schizophrenic and related psychotic disorders have been conducted since the early decade of this century [i.e.Twentieth Century]. The results of studies carried out at different times and different parts of the world … suggest that, notwithstanding cultural and biological differences between populations, and the different methods used to ascertain morbidity, the prevalence of schizophrenia is remarkably similar around the world (in the range between 2 and 5 per 1000 according to the majority of studies) and stable over time.”

This conclusion—that schizophrenia levels have been stable over place and time for as long as they have been measured—was based on an examination of 28 surveys into the incidence of schizophrenia conducted throughout the world over several decades. One of these surveys was a 1993 World Health Organisation study on the incidence of schizophrenia in which standardised methods of case detection, interviewing and diagnosis were applied simultaneously in 7 countries. Like the other surveys, this found little variation in the incidence rates of schizophrenia across the different populations and cultures.

Countries which had extremely high cannabis use (Jamaica in 1995] had the same schizophrenia rates per 1000 as countries with zero cannabis use (Norway in 1926-1935).

Q: Does cannabis use cause schizophrenia?

A: No. Over the past four decades most countries in the Western world have experienced rapidly increasing cannabis use. None of these countries has experienced any accompanying “Madness” plague. Countries like Australia and Jamaica which have the highest levels of cannabis use have levels of schizophrenia similar to the rest of the world.

We’ve smoked the “Reefer” for four decades, and “Madness” levels have remained steady, not just in Australia, but all over the world.

Where does that leave the “Reefer Madness” theory??

Dr John Jiggens.

“Does Cannabis Use Cause Schizophrenia”, StickyPoint Magazine Issue 01 (2006)
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