Cannabis and psychosis: Search of a causal link through a critical and systematic review | Cannabis et psychose: Recherche d’un lien de causalité à partir d’une revue critique systématique de la littérature
L'Encéphale. 2009-09-01; 35(4): 377-385
DOI: 10.1016/j.encep.2008.02.012
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1. Encephale. 2009 Sep;35(4):377-85. doi: 10.1016/j.encep.2008.02.012. Epub 2008 Jul
9.
[Cannabis and psychosis: search of a causal link through a critical and
systematic review].
[Article in French]
Le Bec PY(1), Fatséas M, Denis C, Lavie E, Auriacombe M.
Author information:
(1)Laboratoire de psychiatrie, EA4139, faculté de médecine Victor-Pachon,
institut fédératif de recherche en santé publique, Inserm-IFR no 99, université
Victor-Segalen Bordeaux-2, Bordeaux, France.
INTRODUCTION: Although cannabis use may be involved in the aetiology of acute
psychosis, there has been considerable debate about the association observed
between cannabis use and chronic psychosis. In particular, because of the
frequent co-occurrence between schizophrenia and cannabis use, the question has
been raised of a causal link between exposure to cannabis as a risk factor and
the development of psychosis or psychotic symptoms.
OBJECTIVE: The aim of this article was to examine the evidence that cannabis use
causes chronic psychotic disorders by using established criteria of causality.
These criteria were defined by: biologic plausibility, strength of the
interaction between the risk factor and the disease, reprieability of the
results, temporal sequence between the exposure to the risk factor and the
beginning of the disease and existence of a dose-effect relationship.
METHODS: The selected studies were found in Medline using the keywords « cannabis »
and « psychosis », « cannabis » and « schizophrenia », « cannabis » and « psychotic
symptoms » and « prospective » or « cohort » or « longitudinal ». The selected studies
were all prospective studies assessing the temporal sequence between cannabis use
and emergence of psychosis or psychotic symptoms. The search strategies resulted
in 60 records that were screened by reading both titles and abstracts. Seventeen
studies were considered eligible, and then, after reading the full text, seven
met the inclusion criteria.
RESULTS: Together, the seven studies were all prospective cohorts and represented
50,275 human subjects. There were three European studies (from Sweden, Holland
and Germany), one from New Zealand and one from Australia. Only one study of the
seven did not show a significant association between cannabis consumption and
increase of the risk of developing a psychosis. However, this study had some
bias, such as low level of cannabis use and the lack of evaluation of cannabis
use after inclusion. For the six other studies, data show the existence of a
significant association between cannabis use and psychotic disorders (with an
increased risk between 1.2 and 2.8 in Zammit et al.’s study), particularly among
vulnerable individuals (that is with a prepsychotic state at the time of
inclusion). Therefore, all the studies that assessed a dose-effect relationship
showed this link between cannabis use and the emergence of psychosis or psychotic
symptoms. The fact that all causal criteria were present in the studies suggests
that cannabis use may be an independent risk factor for the development of
psychosis. Results seem to be more consistent for vulnerable individuals with the
hypothesis that cannabis use may precipitate psychosis, notably among vulnerable
subjects. In particular, early onset of cannabis use during adolescence should be
an environmental stressor that interacts with a genetic predisposition to induce
a psychotic disorder.
CONCLUSION: The objective of this article was to examine whether cannabis use can
be an independent risk factor for chronic psychotic disorders, by using
established criteria of causality. Data extracted from the selected studies
showed that cannabis use may be an independent risk factor for the development of
psychotic disorders. Early screening of the vulnerability to psychotic disorder
should permit improved focus on prevention and information about the specific
risks related to cannabis use among this population.
DOI: 10.1016/j.encep.2008.02.012
PMID: 19748375 [Indexed for MEDLINE]