Review: Clearing the smoke on cannabis

May 23, 2019

A research report released by the Canadian Centre on Substance Use and Addiction entitled “Clearing the Smoke on Cannabis – Regular Use and Cognitive Functioning” evaluates a large number of scientific studies on the effects of cannabis use on cognitive functions and intelligence. The report is the first in its series and reviews the effects of weekly, or more frequent cannabis use, and how it can affect human functioning and development. According to the report, a significant proportion of Canadians aged 15 or over (approximately 16% of the population, or about 4.6 million) report using cannabis at least once in the past three months. Some of the key findings of the report are summarized below.

Cognitive Functioning

The evidence reviewed in the report shows that chronic cannabis use does not seem to produce severe or grossly debilitating impairment of cognitive function. Specifically, the reviewed research shows that the effects of cannabis use seem subtle and disappear after days to weeks of abstinence. However, results of research studies examined also show that initiating regular cannabis use in early adolescence and continuing through young adulthood can lead to more pronounced and long-term cognitive deficits

Learning and Memory

According to the results of the report, the evidence examining association between frequent cannabis use and learning and memory deficits has been inconsistent. Although some research studies cited in the report have demonstrated that long-term cannabis use can contribute to a progressive decline in learning and memory capacity over time, evidence from a large longitudinal study also failed to find an association between frequent cannabis use and accelerated memory decline. The report stated that an almost equal number of cross-sectional studies found no significant differences in cognitive abilities between individuals who frequently consumed cannabis and those who did not.


The results of studies examining the effects of chronic cannabis use on attention and concentration have also been inconsistent. Several studies showed that young adults and adolescents who regularly used cannabis demonstrated poor performance across several tasks measuring attention. However, other studies involving adolescents and adults who frequently used cannabis showed that participants did not differ in attention and concentration from those who rarely or never used cannabis.

Executive Function

Executive functions are a group of cognitive processes required for the cognitive control of behaviour, and include working memory, inhibition and cognitive flexibility. According to the evidence presented in the report, frequent cannabis use is associated with mild to moderate deficits in executive functions. However, research examining the effects of cannabis on each of the cognitive functions has been limited, although correlational studies have shown an association between frequent cannabis use and deficits in executive functioning, causality of cannabis use on cognitive function impairment has not been established. The evidence presented in the report shows that effects of regular cannabis use on working memory seem to be short-term.

Moreover, the association between frequent cannabis use and decreased inhibition was not clear, and was more likely to be present among individuals who began regular cannabis use early in life.

The available evidence examining the link between cannabis use and cognitive flexibility is not consistent, and shows that chronic cannabis use initiated early in life could be associated with difficulties in some forms of cognitive flexibility.


The report discusses one longitudinal study which shows that early onset cannabis use was associated with a decline in intelligence demonstrated by reduced IQ. Two more studies mentioned in the report showed that regular cannabis use among adolescents was associated with a progressive decrease in intelligence and neuropsychological functioning even after a relatively short period of cannabis use. These findings are in contrast of two recent longitudinal studies, which showed different results, demonstrating little evidence that cannabis use was associated with a decline in IQ.


The findings of the report conclude that for most individuals, chronic cannabis use does not appear to lead to significant impairment in cognitive functioning, including major deficits in learning, memory and executive functions. However the evidence presented suggests that chronic cannabis use can result in mild cognitive difficulties, including memory, strategic planning, multitasking and decision-making. However, whether these difficulties are reversible upon cannabis use cessation remains unclear. According to the report findings, more research is required to determine predicting factors for cognitive development impairments among adolescents who regularly use cannabis. The authors of the report also highlight the evidence linking early cannabis use with the heightened risk for mental illnesses, such as psychosis and schizophrenia, especially among individuals with a family history of these conditions.