Cannabis Abuse In Adolescence – Cognitive Decline In Later Life

How many people in the US would support the legalization and use of marijuana? The answer is a whopping 70% of respondents of a survey support legalization of marijuana for medical use and 62 % favor its legalization for recreational use. When young adults ranging in age from 18-29 years were asked about legalization of marijuana use, 62% of the respondents favored the decriminalization of marijuana as a recreational drug. Statistical analysis presented in these polls show that marijuana is increasingly being “accepted” as a substance of use (medical or otherwise) in American society and social stigma associated with marijuana use has eroded considerably.

Exposure to cannabis at a young age has been increasing steadily in the American population. Gonzalez and Swanson have reviewed evidence from longitudinal studies on abuse of cannabis in their paper to be published soon in the Proceedings of the National Academy of Sciences (PNAS). Data they have presented shows that there is a steady increase in the number of 8th-, 10th-, and 12th-graders who experiment with marijuana since the 1991-1997. In 2011, nearly 40% of 12th graders have reported to have tried out marijuana. Long term addiction is the obvious inherent risk in ‘trying out’ marijuana. However, the timing of first exposure and persistent use can have more debilitating effects than just dependence. In a longitudinal study, called the DunedIn study, 1073 participants have been tracked from birth to mid-life. Results of this long-term study have been published online in August 2012 in a paper authored by Meier and colleagues in PNAS. The lifestyle of participants in this study was gauged through interviews conducted in the intervening years as well as from reports provided by other informants. The study subjects were asked to take tests to assess their cognitive abilities at the age of 13 and subsequently at 38 years of age. The tests showed that early-onset use as well as persistent use of cannabis resulted in decline in neurological tasks performed by people. Independent reportage by the participant’s colleagues and family members also confirmed the existence of neurological as well as psychological deficits. People in this study who had never used marijuana had increased IQ at the age of 38 as compared to their performance in these tests at the onset of adolescence. However, people who consumed cannabis regularly over the period of 20 years, showed a definite decline in their IQ levels. This specific decline in mental capabilities was not buffered by other sociological factors like education indicating that persistent cannabis use resulted in long-term physiological changes in the brain. Observations recorded by third-party informants (informants were people whom the cannabis users nominated as “people who knew the subjects well”) suggested that cannabis users had reduced attention span and memory issues.

Although this data has come from people with long-term dependence on cannabis, the age of onset of cannabis usage is also important. The decline in IQ was prominent for cannabis users who began using it during adolescence as against those whose first exposure to this intoxicant was in adulthood. Significantly, the neurological deficits in adolescence-onset cannabis users were not restored by subsequent de-addiction or reduction in frequency of cannabis use. These findings are significant in light of increasing acceptance of pot as one of the rites of passage in the growing years. Another study by Van Ryzin and colleagues suggests that quality of family life and peer influence are the major predictors of cannabis usage in kids of age 12-23 years. In their study of 998 adolescents, good familial ties and absence of cannabis abusers from the peer network were found to be instrumental in helping kids avoid exposure to cannabis as well as alcohols and other intoxicants.

In Nova Scotia, a study of 976 high school students showed that cannabis users were at greater risk for depression than people who were into substance abuse of any other sort.

Notwithstanding general public opinion, medical evidence related to the effects of marijuana on brain and cognitive functions points towards abstinence from marijuana as a recreational drug.


Gonzalez R, & Swanson JM (2012). Long-term effects of adolescent-onset and persistent use of cannabis. Proceedings of the National Academy of Sciences of the United States of America, 109 (40), 15970-1 PMID: 23012451

Meier MH, Caspi A, Ambler A, Harrington H, Houts R, Keefe RS, McDonald K, Ward A, Poulton R, & Moffitt TE (2012). Persistent cannabis users show neuropsychological decline from childhood to midlife. Proceedings of the National Academy of Sciences of the United States of America, 109 (40) PMID: 22927402

Rasic D, Weerasinghe S, Asbridge M, & Langille DB (2012). Longitudinal associations of cannabis and illicit drug use with depression, suicidal ideation and suicidal attempts among Nova Scotia high school students. Drug and alcohol dependence PMID: 23041136

Van Ryzin MJ, Fosco GM, & Dishion TJ (2012). Family and peer predictors of substance use from early adolescence to early adulthood: an 11-year prospective analysis. Addictive behaviors, 37 (12), 1314-24 PMID: 22958864

Image via Nikita Starichenko / Shutterstock.

Shefali Sabharanjak, PhD

Shefali Sabharanjak, PhD, is a professional science and medical writer. She holds a doctorate in Cell Biology from the National Center for Biological Sciences. Clear and precise communication is her forte.
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