Teen Use of Marijuana Can Lower IQ: Studies Show Pot Damage Permanent


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Medical Marijuana By Coaster420 (California) , via Wikimedia Commons

Using pot, legally or illegally, permanently changes the adolescent brain. Image by Coaster420 (California)

Is the change in mental ability caused by pot use significant enough to cause day to day problems?

Family and friends of study participants noted higher incidences of attention and memory problems, the more marijuana was used.  A six point IQ loss is enough to take a person from average intelligence to below average intelligence.

How are adolescents affected differently than adult marijuana users?

Those who started using marijuana as teens showed a much higher IQ loss than the adults. The adults did not show any clinically relevant decline in IQ.  This indicates that a teen using marijuana can lose mental ability for the rest of their life, whereas an adult may not suffer from any permanent change in their intelligence.

If someone stops using marijuana will their brain function return?

No. This study demonstrated that if you start using marijuana as a teen and then stop, those IQ points are gone for good.

How Does Pot Affect a Teen’s Brain?

In a study by Andrew Zalesky and Nadia Solowij at the University of Wollongong School of Psychology in Melbourne, Australia, the researchers attempted to determine the effect of long term marijuana use on the brain. The developing adolescent brain has a large number of cannabinoid receptors in the white matter neuronal pathways. Studying both long term marijuana users and non-users, the white matter pathways in the hippocampus were only affected in those who used in adolescence, not in those who started using later.

These finding match the results of the Dunedin study.

The researchers found that this structural difference in the brain, the result of adolescent pot users, leads to higher levels of anxiety, depression and cigarette smoking.

Delaying use into adulthood minimizes these effects. The changes in brain chemistry in adolescent smokers leads to mental and psychological problems.

Marijuana Use: What Do These Findings Mean?

Andrew Zalesky, in his study of the affects of cannabis use on axonal connectivity, said “The age at which regular cannabis use begins is a key factor in determining the severity of any microstructural white matter alteration.”   This has an impact on the US.  Legalization of marijuana use could lead to a significant mental decline in the population if use is legalized in adolescents.

In an interview with DecodedScience, Mary S. Schwartz, a consultant pharmacist in South Florida spoke about participating with at-risk youth and spending time at drug-rehab meetings.  During this time she has seen the the truth of the lower IQ in adolescent marijuana users.  “They (the teens) talked about their problems with recall, how they had trouble remembering.  Many of the youth themselves discussed how they felt dumber.”  She went on to say  “There will have to be restrictions on legalization of pot, hopefully some type of age limit.  Now the studies need to determine the safe age to legalize marijuana and help provide guidelines on who can participate in the use of medical marijuana, who it will help, and who it will hinder.  Studies like this will be important on determining these things.”


Meier, M. Caspi, Avshalom, et al.  Persistent cannabis users show neuropsychological decline from childhood to midlife. (2012).Proceedings of the National Academy of Sciences of the United States of America. Accessed August 9, 2013.

National Institute on Drug Abuse.  Messages From the Director.  Marijuana’s Lasting Effects on the Brain.  (2013).  Accessed August 9, 2013.

Zalesky, A. Solowij, N. et al.  Effect of long-term cannabis use on axonal fibre connectivity. (2013). Brain, a Journal of Neurology.  Oxford Journals.  Accessed August 9, 2013.

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