Cannabis and Mental Illness
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.
Results
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.
Comment
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
| Number | ||
| 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.
References:
- DM Semple et al. Cannabis as a risk factor for psychosis: systematic review. Journal of Psychopharmacology 2005 19: 187-194.
- RF Ferdinand et al. Cannabis use predicts future psychotic symptoms, and vice versa. Addiction 2005 100: 612-618.
- 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.
- RI Herning et al. Cerebrovascular perfusion in marijuana users during a month of monitored abstinence. Neurology 2005 64: 488-493.
- 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)