cannabis and mental illness

Last reviewed 01/2018

Marijuana and mental illness

• there is now considerable clinical evidence showing an association between cannabis use and mental illness, especially schizophrenia, although it is unclear whether this association is causal
• patients with recent onset of psychosis are twice as likely to have used cannabis (but not alcohol or other illicit drugs) when they are compared with a population without psychosis
  • a study linking cannabis use to mental illness (1) involved 50,000 Swedish conscripts. Those who admitted at age 18 to having taken cannabis on more than 50 occasions were nearly seven times more likely to develop schizophrenia in the next 15 years
  • more recent evidence concerning the possible causal association between cannabis and mental illness is mixed:
    • a systemic review (2) has questioned any causal relationship between schizophrenia and cannabis use
      • incidence of schizophrenia seems to be strongly associated with cannabis exposure over a fairly short period (four-fold to five-fold relative risks over follow-up of 10–30 years)
        • however cannabis use appears to have increased substantially amongst young people over the past 30 years, from around 10% reporting ever use in 1969–70, to around 50% reporting ever use in 2001, in Britain and Sweden
        • if the relation between use and schizophrenia were truly causal and if the relative risk was around five-fold then the incidence of schizophrenia should have more than doubled since 1970 - this has not occurred and population trends in schizophrenia incidence suggest that incidence has either been stable or slightly decreased over the relevant time period
      • the study authors concluded that "..Available evidence does not strongly support an important causal relation between cannabis use by young people and psychosocial harm, but cannot exclude the possibility that such a relation exists. The lack of evidence of robust causal relations prevents the attribution of public health detriments to illicit drug use"
    • a cross-sectional survey in the context of an ongoing cohort study (3) has examined the relationship between cannabis and psychosis
      • the authors concluded that the study results add credence to the hypothesis that cannabis contributes to the population level of expression of psychosis. In particular, exposure early in adolescence may increase the risk for the subclinical positive and negative dimensions of psychosis, but not for depression
    • an analysis in the BMJ states that (4) "epidemiological evidence suggests a persistent association between cannabis use and psychosis that is robust to methological challenges"

Reference:

1. The Lancet 1987; 8,574: 1,483–6.
2. Lancet. 2004;363(9421):1579-88.
3. Addiction. 2004 Oct;99(10):1333-41
4. BMJ. 2006 Jan 21;332(7534):172-5.