Cannabis and Psychosis: a Critical Overview of the Relationship by Charles Ksir & Carl L. Hart


Interest in the relationship between cannabis use
and psychosis has increased dramatically in recent years, in
part because of concerns related to the growing availability of
cannabis and potential risks to health and human functioning.
There now exists a plethora of scientific articles addressing
this issue, but few provide a clear verdict about the causal
nature of the cannabis-psychosis association. Here, we review
recent research reports on cannabis and psychosis, giving particular attention to how each report provides evidence relating
to two hypotheses: (1) cannabis as a contributing cause and (2)
shared vulnerability. Two primary kinds of data are brought to
bear on this issue: studies done with schizophrenic patients
and studies of first-episode psychosis. Evidence reviewed here
suggests that cannabis does not in itself cause a psychosis
disorder. Rather, the evidence leads us to conclude that both
early use and heavy use of cannabis are more likely in individuals with a vulnerability to psychosis. The role of early and
heavy cannabis use as a prodromal sign merits further
examination, along with a variety of other problem behaviors
(e.g., early or heavy use of cigarettes or alcohol and poor
school performance). Future research studies that focus exclusively on the cannabis-psychosis association will therefore be
of little value in our quest to better understand psychosis and
how and why it occurs.