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Cannabis and Mental Illness

Systematic review

There is considerable evidence linking cannabis and psychosis, from case studies to large observational studies. What it might mean is debated, because this sort of evidence cannot be not conclusive regarding causation. But that is perhaps getting ahead of ourselves. How strong is the association between cannabis use and psychosis in the first place? A new systematic review pulls it all together [1].

Systematic review

Authors sought studies in several databases, one specific for mental health, as well as bibliographies and reviews. For inclusion, studies had to contain original data on cannabis exposure and either schizophrenia or psychotic symptoms. Diagnosis of mental illness used established criteria. In case-control studies exposure to cannabis had to precede the onset of schizophrenia. In cohort studies subjects had to be recruited before the median age of onset of the illness, and cannabis use was determined prospectively and blind to diagnosis.


Eleven case control studies examined the relationship between cannabis use and psychosis in about 50,000 subjects, though some studies were later follow ups of earlier studies. Size, methods of ascertaining cannabis use, and diagnostic criteria varied, but there was consistency in the relationship between the seven original studies contributing information (Figure 1). Overall, the pooled odds ratio was 2.9 (95% CI 2.4 to 3.6), meaning that cannabis use was about three times more likely to lead to schizophrenia-like psychosis than cannabis non-use. Figure 1 also contains an eighth study published recently [2] which had an odds ratio of 2.8.

Figure 1: Association between cannabis use and developing psychosis

Six more case-control studies rated psychotic symptoms in cannabis users compared with non-users, with about 23,000 subjects in all. All except the smallest study (with only 100 patients) found a significant relationship, with odds ratios of 2-4 for cannabis use. The most recent study [2] also found significant increase for psychotic symptoms.


What the review does well is establish the link between cannabis use and psychosis or psychotic symptoms. There is much more to it than this, of course. First, there is a dose response, with more likelihood of schizophrenia with increased ever-use of cannabis. The large Swedish study that followed up over 40,000 conscripts [3] showed this dose response, both at any time (Figure 2), and within five years or after five years from conscription.

Figure 2: Use of cannabis and development of schizophrenia

Does this mean that cannabis causes schizophrenia or psychosis. Well, it might do, and certainly chronic cannabis use results in persistent changes to blood flow in the brain [4], and that is in people, not rats. But we also need to recognise that cannabis users tend to be different from non-users. Table 1 shows some results from the Swedish conscript cohort, looking at just some of the differences.

Table 1: Differences between conscripts who were ever users or non-users of cannabis

Ever users
Psychiatric illness on conscription (%)
Disturbed behaviour (%)
City dewller (%)
Cigarette smoker (%)

The trouble is that we just don't know, and probably at the moment we can't know whether there is a causal link rather than an association. People taking proton pump inhibitors go to hospital with gastrointestinal bleeding more often than those who do not, but that doesn't mean that they cause gastrointestinal bleeding. But even if there is no causal link, that doesn't mean that cannabis is a good thing.

As Bandolier 141 indicated, cannabis makes it more difficult to drive on simulators, is associated with car accidents, and that alcohol and cannabis combined is an explosive mix that produces severe impairment of cognitive, psychomotor, and actual driving performance. A new study from France [5] confirms the link between cannabis and road accidents.


  1. DM Semple et al. Cannabis as a risk factor for psychosis: systematic review. Journal of Psychopharmacology 2005 19: 187-194.
  2. RF Ferdinand et al. Cannabis use predicts future psychotic symptoms, and vice versa. Addiction 2005 100: 612-618.
  3. S Zammit et al. Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study. BMJ 2002 325: 1199-1204.
  4. RI Herning et al. Cerebrovascular perfusion in marijuana users during a month of monitored abstinence. Neurology 2005 64: 488-493.
  5. B Laumon et al. Cannabis intoxication and fatal road crashes in France: population based case-control study. BMJ, doi:10.1136/bmj.38648.1F (published 2 December 2005)

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