Review: use of cannabis is associated with increased risk of psychotic outcomes later in life
Dr S Zammit, Cardiff University, Cardiff, UK;
What is the relation between use of cannabis and psychotic or affective mental health outcomes?
Studies selected evaluated the association between use of cannabis and risk of psychosis or affective mental health outcomes. Studies of patients with known mental illness or substance use related problems, prison inmates, and randomised controlled trials of medicinal use of cannabis were excluded. Outcomes were psychotic (including schizophrenia, schizophreniform, schizoaffective, or psychotic disorders; non-affective or affective psychoses; delusions; hallucinations; or thought disorders) or affective (including affective, mood, or bipolar disorders; depression, suicidal ideation or suicide attempts; anxiety; neurosis; or mania) mental health outcomes.
Medline, CINAHL, EMBASE/Excerpta Medica, PsycINFO, ISI Web of Knowledge, ISI Proceedings, ZETOC, BIOSIS, LILACS, Caribbean Health Sciences Literature (MEDCARIB) (all to Sep 2006); and bibliographies of relevant studies were searched for population-based prospective or nested case–control studies. Experts were consulted. 7 cohort studies (n = 61 485) met the selection criteria and reported psychotic outcomes; 15 cohort studies (n = 51 502) reported affective outcomes.
Meta-analysis showed that people who had ever used cannabis had higher risk of psychotic outcomes than those who had never used cannabis (table). People who used cannabis most frequently had an even greater risk of psychotic outcomes (table). Use of cannabis increased risk of depression in 5 of 10 studies, increased risk of suicidal ideation or suicide attempts in 4 of 7 studies, and increased risk of anxiety outcomes in 2 of 7 studies.
Use of cannabis is associated with increased risk of psychotic outcomes later in life. Association between use of cannabis and affective mental health outcomes is less clear.
Moore TH, Zammit S, Lingford-Hughes A, et al. Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review. Lancet 2007;370:319–28.
Clinical impact ratings: Family/general practice 5/7; Psychiatry 6/7; Substance misuse 5/7
The systematic review by Moore et al on cannabis use and psychosis was comprehensive and used meta-analysis and narrative review methodologies effectively. The review shows that risk of psychosis increases by about 40% in people who have previously used cannabis. As well, a dose–response effect exists, with risk increases of 50–200% in the most frequent users. Moore et al adjusted for the effects of imminent psychotic symptoms and ensured that psychoses were not related to transitory intoxication effects. Additionally, the careful selection of studies that controlled for or eliminated study samples with baseline psychosis also adds strength to the causal associations.
In the USA, primary mental disorders tend to predict subsequent onset of substance use disorders, with a median interval ⩾5 years between onset of primary mental disorders and onset of secondary substance use disorders.1 People with childhood or adolescent mental disorders are at highest risk of severe secondary substance use disorders.1 These figures may lead one to disregard the findings of the review by Moore et al and to believe that mental health problems lead to cannabis use. However, most of the primary studies for psychoses excluded people with psychoses and adjusted for markers of premorbid psychoses at baseline.
Cannabis use increases risk of psychotic symptoms in young people, and the effect is stronger in those predisposed to psychosis.2 Mental health promotion campaigns need to draw attention to the long-term risks of cannabis use as part of a comprehensive programme for young people.