Cannabis and cognition: short- and long-term effects

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.