Secondary Schizophrenia (70 page)

use and symptoms of psychosis and depression while
Jablensky and colleagues
[57]
, and Martinez-Arevalo
adjusting for serial dependence, medication com-and colleagues
[58]
have provided supportive evi-pliance, and other demographic and clinical vari-

174

dence. Cleghorn and colleagues
[56]
compared the
ables. Cannabis use predicted a small but statistically
Chapter 11 – Schizophrenia secondary to cannabis use

significant increase in symptoms of psychosis but not
evidence for a causal role of cannabis in the vulnera-of depression after controlling for other differences
ble. It would also potentially provide a way of identi-between cannabis users and nonusers. Symptoms of
fying young people who are at high risk of developing
depression and psychosis, by contrast, did not predict
psychosis if they use cannabis.

cannabis use.

Second, replications and extension of studies of the
Most but not all (e.g., Zisook and colleagues
[60])

effects of cannabis use on the course of psychosis are
of the evidence is consistent with the hypothesis
needed. These studies might also look at the interac-that cannabis use exacerbates psychotic symptoms in
tion between the COMT allele and psychosis. It would
patients with schizophrenia. The major cause of uncer-be useful to conduct these studies as part of interven-tainty is assessing the contribution of confounding fac-tion studies to reduce cannabis use among persons
tors in some of these studies. It is possible, for example,
with schizophrenia.

that the difference in the rates of psychotic symptoms
Third, we need more human studies of the possi-between schizophrenia patients who do and do not use
ble mechanism for interactions between cannabinoids
cannabis is due to differences in premorbid personal-and the dopaminergic system. This may include neu-ity, family history, and other characteristics
[61].
This
roimaging studies of the effects that cannabinoids have
is least likely in the WHO schizophrenia study
[57]
and
on human brain function.

the recent study of Linszen and colleagues
[59],
both of
Finally, we need to consider new and innovative
which used multivariate statistical methods to adjust
ways to reduce cannabis use among persons with psy-for many of these confounders.

chosis. The effective treatment of drug use is a chal-The other difficult issue is separating the contribu-lenging task and one that is not made easier by comor-tions that cannabis, alcohol, and other drug use made
bid mental health problems such as schizophrenia.

to exacerbations of schizophrenic symptoms. It is rare
Effective pharmacological and psychological interven-for a schizophrenic patient to only use cannabis
[62].

tions, appropriate for this group, are required.

The concurrent use of alcohol is common, and the
heavier their cannabis use, the more likely they are to
Conclusion

use psychostimulants and hallucinogens. The Linszen
There is strong evidence from longitudinal studies
and colleagues
[59]
and Degenhardt and colleagues
that cannabis use may precipitate schizophrenia in
[9]
studies statistically adjusted for the effects of con-vulnerable individuals. There is also reasonable evi-current alcohol and drug use and found that the rela-dence that cannabis use exacerbates symptoms of psy-tionship persisted. Our confidence that the effect is
chosis in persons with a psychosis who continue to
attributable to cannabis would be increased by repli-use cannabis. These are biologically plausible relation-cations of these findings.

ships given the known effects of cannabis on dopaminergic and other brain neurotransmitter systems and
Future research directions

the results of recent provocation studies of THC in
schizophrenia.

The following are a number of priorities for research
that will assist in sorting out the nature of the relationship between cannabis use and psychosis that has been
Acknowledgments

consistently observed in observational epidemiologi-This chapter is an updated version of two previous
cal studies. First, there needs to be replication of the
reviews of the evidence on cannabis and psychosis
cannabis-genotype interactions reported by Caspi and
published in 2001 and 2004. Thanks to Emma Black
colleagues
[54]
between a COMT allele and cannabis
and Amanda Roxburgh for assisting with compilation
use. If replicated, these findings would provide strong
of references and proofreading the paper.

175

Organic Syndromes of Schizophrenia – Section 3

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