Abstract
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Twenty years ago cannabis was generally perceived to
be a benign drug with few significant adverse effects.
As outlined elsewhere in this book, evidence has since
mounted in the scientific literature for a range of harms
associated with the use of cannabis, including the
development of dependence and health-related harms
(see also Hall and Solowij, 1998 ; Hall and Degenhardt,
2009 ). As the overall theme of this book indicates, an
association between cannabis use and the development
of psychotic symptoms or overt psychosis has grown
to be recognized as a significant potential harm, and
investigating the mechanisms by which cannabis may
trigger psychosis is a priority. This includes understanding
the effects of cannabis on brain structure,
biology and function. We recently highlighted a similarity
between the cognitive impairment that has been
reported in cannabis users and the deficits observed
in schizophrenia (Solowij and Michie, 2007 ), suggesting
common underlying neuropathology. Few would
argue that cognition is not impaired to some degree
during acute intoxication with cannabis. That impaired
cognition persists beyond the period of acute intoxication
is more contentious. Despite objective appraisals
of the literature in interpreting the evidence, it is inevitable
that researchers will be influenced by the weight
of their own data in formulating scientific opinion.
Accordingly, and on the basis of the accumulating evidence,
this review will come to some rather different
conclusions from those made in the first edition of this
book (Pope and Yurgelun-Todd, 2004 ).
The goal of this chapter is to update our knowledge
of the short- and long-term effects of cannabis
on cognition based on integrating evidence from the
most recent literature on this topic. We acknowledge
the weight of evidence from our own studies that must
inevitably guide us to the conclusions that we draw,
while also aiming objectively to assess the evidence
from multiple sources. We consider evidence from
preclinical research, studies of acute administration
of cannabinoids to humans, studies of long-term or
heavy cannabis users tested in the unintoxicated state,
including adults and adolescents and patients with
schizophrenia, and we evaluate the evidence for recovery
of function after prolonged abstinence.