Professor of the Practice of Law University of Denver Sturm College of Law

Cannabis & Cognition

Marijuana is legal for recreational use in 15 states and Washington, D.C.  A scarcity of cannabis research has made potential harm difficult to assess, but several recent studies have made more data available. 

Cannabis impacts numerous parts of the brain and body.  There are cannabinoid receptors in the cerebral cortex (executive function), hippocampus (memory processing), and cerebellum (balance, coordination).  We also have cannabinoid receptors in our central nervous system, immune system, GI tract, bones, and fat cells.

Approximately 1,000 New Zealanders were recruited for neuropsychological testing at age 13 prior to cannabis use, and then tested again at age 38.  At age 38, the amount of cannabis use and cannabis dependence were significantly associated with cognitive impairment, including lower IQ, even after controlling for education level.  Research has shown that cannabis interferes with all aspects of memory processing: encoding information, consolidation of information into long-term memory, and recall.  Executive function, which includes planning, considering options, and resisting inappropriate behavior, is impaired in long-term users for 2-3 weeks after cessation. 

A recent meta-analysis of seven studies examined cannabis use in teenagers in the UK, USA, Canada, and New Zealand.  There were 808 teens who started using cannabis before they were 18 and who had used it weekly for at least 6 months who were compared to 5,308 non-users of the same age.  All participants took an IQ test prior to any cannabis use and the follow up IQ testing was done in their late teens, while brain development is still occurring.  The cannabis users suffered a nearly 2-point IQ decline compared to the non-users in just a few years of adolescence. 

A life expectancy study of 6.9 million people living in Denmark from 2000 to 2015 showed that cannabis use disorder shortened men’s lives by 15.7 years and women’s lives by 12.2 years.  The same study showed alcohol use disorder shortened men’s lives by 14.4 years and women’s lives by 13.5 years.

Takeaway: Marijuana may be considered a fairly harmless substance, but research indicates that it activates receptors all over our brain and our body.  Research has revealed that cannabis use impairs cognition, lowers IQ, and shortens lifespan.


Suzette Gomez, All the States that Legalized Marijuana, Addiction Center, Nov 20, 2020,  The 15 states that have legalized cannabis are AK, AZ, CA, CO, IL, MA, ME, MI, MT, NJ, NV, OR, SD, VT, WA.

Debra S. Austin, Drink Like a Lawyer: The Neuroscience of Substance Use and its Impact on Cognitive Wellness, 15 NEV. L.J. 826, 853-857 (2015),

Emmet Power, Sophie Sabherwal, Colm Healy, Aisling O’Neill, David Cotter, and Mary Cannon, Intelligence Quotient Decline Following Frequent or Dependent Cannabis Use in Youth: A Systematic Review and Meta-Analysis of Longitudinal Studies, Psychological Medicine 1-7, Jan 27, 2021,

Nanna Weye, Natalie C. Momen, Maria K. Christensen, et al., Association of Specific Mental Disorders With Premature Mortality in the Danish Population Using Alternative Measurement Methods, JAMA Network, June 3, 2020, (data in Table 3)

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