Testing hypotheses about the relationship between cannabis use and psychosis

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Abstract

Aim: To model the impact of rising rates of cannabis use on the incidence and prevalence of psychosis under four hypotheses about the relationship between cannabis use and psychosis. Methods: The study modelled the effects on the prevalence of schizophrenia over the lifespan of cannabis in eight birth cohorts: 1940–1944, 1945–1949, 1950–1954, 1955–1959, 1960–1964, 1965–1969, 1970–1974, 1975–1979. It derived predictions as to the number of cases of schizophrenia that would be observed in these birth cohorts, given the following four hypotheses: (1) that there is a causal relationship between cannabis use and schizophrenia; (2) that cannabis use precipitates schizophrenia in vulnerable persons; (3) that cannabis use exacerbates schizophrenia; and (4) that persons with schizophrenia are more liable to become regular cannabis users. Results: There was a steep rise in the prevalence of cannabis use in Australia over the past 30 years and a corresponding decrease in the age of initiation of cannabis use. There was no evidence of a significant increase in the incidence of schizophrenia over the past 30 years. Data on trends the age of onset of schizophrenia did not show a clear pattern. Cannabis use among persons with schizophrenia has consistently been found to be more common than in the general population. Conclusions: Cannabis use does not appear to be causally related to the incidence of schizophrenia, but its use may precipitate disorders in persons who are vulnerable to developing psychosis and worsen the course of the disorder among those who have already developed it.

Introduction

Clinical research has shown that high proportions of persons with schizophrenia report regular cannabis use and meet criteria for cannabis use disorders (Fowler et al., 1998, Mueser et al., 1990, Ziedonis and Trudeau, 1997). Epidemiological studies have also found an association between cannabis use and psychosis in the general population (Anthony and Helzer, 1991, Cuffel et al., 1993, Degenhardt and Hall, 2001, Tien and Anthony, 1990).

There has been considerable debate about the reasons for this association (Batel, 2000, Blanchard et al., 2000, Gruber and Pope, 1994, Hall, 1998, Hall and Degenhardt, 2000, McKay and Tennant, 2000, Mueser et al., 1998, Rosenthal, 1998, Thornicroft, 1990). Depending upon the nature of the relationship between cannabis use and psychosis, changes in the prevalence of cannabis use may potentially lead to changes in the incidence, prevalence or age of onset of psychosis.

In Australia, there has been a dramatic increase in the prevalence of cannabis use since the early 1970s (Degenhardt et al., 2000, Donnelly and Hall, 1994, Makkai and McAllister, 1998, McCoy, 1980). The present report assesses the evidence for four hypothesised relationships between cannabis use and psychosis, which would each predict different effects of increased cannabis use on the incidence, prevalence and age of onset of schizophrenia and the prevalence of chronic cannabis use among persons with the disorder.

According to this hypothesis there is a causal link between cannabis use and schizophrenia in the sense that cannabis use causes cases of the disorder that would not otherwise have occurred. This hypothesis has arisen from reports of ‘cannabis psychoses’ (Basu et al., 1999, Bernardson and Gunne, 1972, Carney et al., 1984, Chopra and Smith, 1974, Eva, 1992, Solomons et al., 1990, Tennant and Groesbeck, 1972, Wylie et al., 1995) and there is some evidence that cannabis users are more likely to report psychotic symptoms (e.g. Andreasson et al., 1987, Degenhardt and Hall, 2001, Tien and Anthony, 1990).

If this hypothesis is correct, then an increase in prevalence of cannabis use among young adults should increase the incidence and ultimately the prevalence of schizophrenia. Since there has been a dramatic rise in the prevalence of cannabis use in Australia, this hypothesis predicts an increase in the greater number of cases of schizophrenia among recent birth cohorts. Since the age of cannabis initiation has also declined, this hypothesis also predicts that the age of onset of schizophrenia would decline in recent birth cohorts. This hypothesis also predicts a rising prevalence of cannabis use among persons with schizophrenia.

A second hypothesis is that regular cannabis use precipitates schizophrenia among vulnerable individuals, that is, among persons who would have developed the disorder regardless of whether they used cannabis or not (Hall, 1998). This is supported by evidence that: (a) persons with first-episode schizophrenia who use cannabis are younger than those who do not (Linszen et al., 1994, Mathers et al., 1991, Rolfe et al., 1993); (b) cannabis use usually precedes the development of psychotic symptoms (Allebeck et al., 1993; Hambrecht and Haefner, 2000, Linszen et al., 1994); and (c) among first-episode cases of psychosis, those who used cannabis were more likely to have a family history of psychosis (McGuire et al., 1995).

According to this hypothesis, an increase in regular cannabis use in the general population would not affect the incidence of schizophrenia but it would reduce the age of onset of psychotic illness among those who used cannabis. That is, the incidence rates of persons using cannabis would be ‘brought forward’. If this led to more chronic psychotic disorders (e.g. because earlier onset cases are more likely to relapse) the prevalence of chronic cases of psychosis would increase. This would increase the prevalence of regular cannabis use among persons with schizophrenia.

According to this hypothesis, cannabis use would worsen the prognosis of schizophrenic persons by increasing relapse to schizophrenia. It is supported by evidence that persons with schizophrenia who use cannabis are more likely to suffer a relapse (Jablensky et al., 1991, Linszen et al., 1994). This hypothesis does not predict an increased incidence of schizophrenia among regular cannabis users. Instead, it predicts that persons with schizophrenia who are regular cannabis users will be more likely to have a relapse after their initial episode. This could increase the number of persons in the population with chronic schizophrenia. It would not affect the age of onset of psychosis. The prevalence of cannabis use among persons with schizophrenia would increase because there would be more cannabis users among chronic cases.

According to this hypothesis, persons with schizophrenia are more likely to become regular cannabis users, if they use the drug (Mueser et al., 1998). There is no causal relationship between cannabis use and psychosis, so increasing rates of cannabis use will have no effect upon the incidence or prevalence of schizophrenia and there would not be a change in age of onset. There would be an increased prevalence of cannabis use among persons with psychosis.

The predictions generated from each of these four hypotheses are summarised in Table 1; notably, each hypothesis has a unique set of predictions. We used mathematical modelling to assess the plausibility of these four hypotheses. We combined empirically derived information about the epidemiology of cannabis use and psychosis to predict trends in incidence, prevalence and age of onset of schizophrenia according to each of these hypotheses. These predicted trends were compared with evidence on observed trends in schizophrenia and cannabis use.

Section snippets

Incidence

It was assumed that schizophrenia does not develop before the age of 15 years (Goldstein et al., 1984), and that new cases do not occur after the age of 54 years (Goldstein et al., 1984). Separate specific incidence rates were used for males and females because males have an earlier onset of schizophrenia on average than females (Jablensky et al., 1991, Jones and Cannon, 1998). Estimates of the average incidence rate of schizophrenia per 100 000 population per year by age and gender were

Modelling the natural history of cannabis use

Fig. 2 shows the estimated natural history of cannabis use in each of the birth cohorts. The peak prevalence of regular cannabis use occurs earlier in recent birth cohorts while peak prevalence of weekly use was higher for earlier birth cohorts.

Modelling the prevalence of schizophrenia

Fig. 3 shows the estimated prevalence of schizophrenia among Australian males and females according to age. The prevalence of schizophrenia by age 54 was 1.17% for males, and 1.08% for females. This is at the higher end of the estimated prevalence of

Does cannabis use cause psychosis?

The hypothesis that cannabis causes schizophrenia was not supported by the data on trends in the incidence of this psychosis in Australia. There was no evidence that there has been an increase in incidence over the past 30 years of the magnitude predicted by the hypothesis. This suggests that cannabis use has not caused cases of psychosis that would not otherwise have occurred. Even if regular cannabis use did double the risk of users developing schizophrenia (the ‘doubling’ of risk being the

Conclusions

This study has used modelling (incorporating data-based parameters) to predict what changes we would expect to see in the incidence and prevalence of schizophrenia if each of four hypotheses about the relationships between cannabis use and psychosis were true. The claim about cannabis and psychosis is widely understood in the popular media and public debate in Australia to imply that cannabis use has increased the number of cases of psychosis in the population (in the sense of causing cases of

References (72)

  • P. Allebeck et al.

    Cannabis and schizophrenia: a longitudinal study of cases treated in Stockholm County [published erratum appears in Acta Psychiatr. Scand. 1993 Oct; 88 (4) 304]

    Acta Psychiatr. Scand.

    (1993)
  • S. Andreasson et al.

    Cannabis and mortality among young men: a longitudinal study of Swedish conscripts

    Scand. J. Soc. Med.

    (1990)
  • J.C. Anthony et al.

    Syndromes of drug abuse and dependence

  • J. Bamrah et al.

    Epidemiology of schizophrenia in Salford, 1974–84: changes in an urban community over 10 years

    Br. J. Psychiatry

    (1992)
  • D. Basu et al.

    Cannabis psychosis and acute schizophrenia—a case-control study from India

    Eur. Addict. Res.

    (1999)
  • J. Berkson

    Limitations of the application of fourfold table analysis to hospital data

    Biometr. Bull.

    (1946)
  • G. Bernardson et al.

    Forty-six cases of psychosis in cannabis abusers

    Int. J. Addict.

    (1972)
  • J. Brewin et al.

    Incidence of schizophrenia in Nottingham: a comparison of two cohorts, 1978–1980 and 1992–1994

    Br. J. Psychiatry

    (1997)
  • S. Brown

    Excess mortality of schizophrenia. A meta-analysis

    Br. J. Psychiatry

    (1997)
  • M.W.P. Carney et al.

    Psychosis after cannabis use

    Br. Med. J.

    (1984)
  • C. Caron et al.

    Comorbidity in child psychopathology: concepts, issues and research strategies

    J. Child. Psychol. Psychiatry

    (1991)
  • B. Carone et al.

    Posthospital course and outcome in schizophrenia

    Arch. Gen. Psychiatry

    (1991)
  • D. Castle et al.

    The incidence of operationally defined schizophrenia in Camberwell, 1965–84

    Br. J. Psychiatry

    (1991)
  • K. Chen et al.

    The natural history of drug use from adolescence to the mid-thirties in a general population sample [see comments]

    Am. J. Public Health

    (1995)
  • G.S. Chopra et al.

    Psychotic reactions following cannabis use in East Indians

    Arch. Gen. Psychiatry

    (1974)
  • B.J. Cuffel et al.

    Correlates of patterns of substance abuse among patients with schizophrenia

    Hospital Community Psychiatry

    (1993)
  • L. Degenhardt et al.

    The association between psychosis and problematical drug use among Australian adults: Findings from the National Survey of Mental Health and Well-Being

    Psychol. Med.

    (2001)
  • C. DiMaggio et al.

    Evidence of a cohort effect for age at onset of schizohprenia

    Am. J. Psychiatry

    (2001)
  • N. Donnelly et al.

    Patterns of cannabis use in Australia. NCADA Monograph Series No. 27

    (1994)
  • J. Eagles

    Is schizophrenia disappearing?

    Br. J. Psychiatry

    (1991)
  • J. Eagles et al.

    Decline in the diagnosis of schizophrenia among first admissions to Scottish mental hospitals from 1969 to 1978

    Br. J. Psychiatry

    (1985)
  • W.W. Eaton et al.

    Long-term course of hospitalization for schizophrenia: Part I. Risk for rehospitalization

    Schizophr. Bull.

    (1992)
  • W.W. Eaton et al.

    Long-term course of hospitalization for schizophrenia: Part II. Change with passage of time

    Schizophr. Bull.

    (1992)
  • J. Eva

    Cannabis psychosis

    Psychiatr. Bull.

    (1992)
  • I. Fowler et al.

    Patterns of current and lifetime substance use in schizophrenia

    Schizophr. Bull.

    (1998)
  • G. Galbaud Du Fort et al.

    Psychiatric comorbidity and treatment seeking: sources of selection bias in the study of clinical populations

    J. Nerv. Ment. Dis.

    (1993)
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