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.

  • Richard Kensinger, MSW

    The synthetic versions of MJ show much higher levels of THC than the versions available in the 60’s & 70’s. The Fed govt. classifies it as a controlled agent on Sched 1 along w/ the likes of Heroin & Meth.
    MJ is a very complicated agent upon biotransformation. Of its 400+ chemical derivatives, 2/3 show psychoactive properties. It readily binds w/ fat & can be stored for months at a time in fat cells. It primarily neuromodulates acetylcholine, responsibe for learning & memory.

    It is rapped as “gateway”, but so is nicotine & alcohol!

    As a clinican, I never recommend its recreational use.


    • voxpop

      Hey Richard- be honest.

      Do you “ever” use or condone the “recreational usage” of alcoholic substances?

      Be honest for these people on this site, please.

      Alcohol and OTC medications are toxic and DEADLY compared to cannabis.

      • onergk69


        As a clinician absolutely not! Only second to nicotine, alcohol destroys many lives. And no psychoactive agent is as highly associated w violence & aggression, inc. intimate violence! And those who are depressed & suicidal, are 30 X more likely to complete suicide.

        I thank you for your interest in my clinical opinion.


  • univ. of California did a study of 100.000 people and found the the earlier you start smoking pot the better!! it was proven to double your i.q.

    • Dr. Shefali Sabharanjak

      Hi Jim,
      Please provide more information about the Univ. of California study that you have quoted. I would like to review it and compare it against other studies.

      • Chris

        Geez… he was taking a piss, read again ;P

        • leo

          no bro. pot is a stimulant therefore, it helps in almost every task. plus, it is BOMB!!!!

    • jim thorp

      how’s it feel so called doctor to hear lies as big as yours? i’d fire your ass if i could .instead you have become an object of ridicule. what did you take to adversely affect your i.q so much?

      • Chris

        Once again: the guy was obviously joking. If you don’t like it, don’t laugh. But taking such comment seriously and bitching about it only speaks volumes about your IQ.

        • brainiac i beg to differ dingle nuts your lack of understanding is all too apparent to one and all. your grandiose misscharacterising of the situation speaks for itself .can’t you read?LOL jim 1 dgnts 0

  • Pingback: Marijuana Addiction Help « erecoverator77m()

  • David R

    If I understand the Gonzalez/Swanson paper correctly, then the cognitive effects were for people *who were essentially constantly high for twenty years*. That they are less fit cognitively might not be too surprising, nor should it on its own warrant making marijuana illegal. Lots of things are unhealthy if not consumed with moderation.

    From the paper:

    “Both adolescent onset and almost 2 decades of persistent cannabis use may be needed to obtain the magnitude and
    pervasiveness of long-term neuropsychological deficits reported by Meier et al.”


    raises questions about the proportion of cannabis users in the general population to whom these findings most readily apply
    and highlights the need to interpret the findings in the context of a specific pattern of cannabis use rather than to all cannabis
    use, per se.”

    On top of that there remain issues with potential confounding factors, e.g.:

    “The interconnectedness of mental health problems, persistent cannabis use, and neuropsychological functioning
    demands more careful attention, and the literature (12, 13) suggests the value of controlling for more than just schizophrenia (as did Meier et al.).”

    Overall, I don’t see a convincing argument here.

    • Dr. Shefali Sabharanjak

      Hi David,
      The paper does stress on both phenomena- early onset of cannabis abuse as well as persistence of the habit for 20 years or so. The two events have to occur to result in the kind of cognitive decline the subjects have shown. The timing of first exposure to the stimulant- adolescence vs. adulthood- is important. Initiation of all kinds of substance abuse- alcohol, tobacco, heroin and pot- in early adolescence is more likely to establish addiction in people than when the first encounters with these substances occur in adulthood. That is why the DunedIn study is important. Please also see the Gonzalez paper that says that subsequent de-addiction does not make the damaging effects of pot go away completely.
      Whether a person wants to piss away his/her life in a neurostimulant induced haze or not is a matter of individual choice. However, given that public acceptance of this particular addictive substance is increasing, these results may be something that teenagers and their parents might want to be aware of. With changes in policy, possession of marijuana will be legal and its availability can only increase. There will always be people who will fall into some addictive habit or the other- no matter what opinions are expressed in a blog or any other forum. The paper just points out a detrimental outcome of actions taken in adolescent years. Affinity towards risky behavior is high in these years but the ability to choose between short-term pleasure and long-term brain damage may not always be present.

      • Chris

        Please take the stick out of your ass. Most people who smoke marijuana do it occasionaly, they do it with friends to have a laugh or two and then move on with their lives when it wears off. You either lack basic understanding, or just love the drama. Relax. Smoke a joint.

      • voxpop

        Biased comment, professor: “….Whether a person wants to piss away his/her life in a neurostimulant induced haze or not is a matter of individual choice.”

        I lived in Palo Alto 5 years ago, surrounded by psychiatrists
        who would make money off treating “cannabis addiction” in teenagers.

        They would often be seen stumbling between their homes at night-
        but it was from drinking alcohol.

        It is interesting that those doctors who consume alcohol regularly were the most hostile to cannabis use, which would be fine,
        but they are not honest about their own usage of alcohol-
        which is a very toxic, and potentially deadly substance,
        unlike cannabis.

  • Dr. Shefali Sabharanjak

    @ Chris & @ Jim Thorp,
    My response is to David’s comments not to the ones made by you two super geniuses. Please keep your nose out of this.

    • Chris

      I saw you smoking pot today.

  • LoganB

    I been smoking nothing but high grade hydroponic marijuana since 13 I’m now 21. An I agree if your not a doctor and your commenting your opinion against one on some shit u herd from your stoner friend that our try in to back up. Please don’t. Try and be polite. People are trying to educate themselves by reading all this. An your non expert opinion on the matter is making u look dumb and dumbing everyone else down with the presence of your ignorance. Sorry.

  • Mike

    ^^ I’ve been smoking nothing buy high grade hydroponic marijuana since I was 13, I’m now 21. I agree if you’re not a doctor and you’re commenting your opinion against one another on some shit you heard from your stoner friend then back it up. Please don’t, try and be polite. People are trying to educate themselves by reading all of this. Your non expert opinion on the matter is making you look dumb and dumbing everyone else down with the presence of your ignorance, Sorry.

    Spell it out..

  • Christopher


    You can tell all the comments by stoners. Looks like DR. SHEFALI SABHARANJAK is right. LOL!

    • stoner

      I’m sorry what were you saying again?

    • voxpop

      Here’s something better than a comment- actual information you can research yourself, ignoramus:

      a free book on cannabis research in .pdf format:

      “Marijuana and the Cannabinoids”


    “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”

    What is the existence of neurological and psychological deficits?
    Is it not conforming to work? Living a sort of “alternative” life?
    This study also fails to mention the other factors involved…are these cannabis users also drinking, smoking cigarettes, watching TV, on other meds?
    Maybe this study only used people who are desperate enough to participate in such a sham – obviously they didn’t do to Harvard to get their subjects – if they did the results would be drastically different. This study os probably picking some lazy, booze hounds, and couch potatoes who are up to no good. 😉

    Studies like this welcome discrimination, bias and stereotyping. Cannabis can be a medical miracle for all people, of all ages, and this study is

    Walter Benjamin, George Washington, Michael Bloomberg, Montel Williams, Stephen King, Rick Steves, Terence Mckenna ,Carl Sagan…and your lawyer, doctor, and dentist might be doing it too.


      and this study is…sad, very sad. National Academy of Sciences is filled with political bias.

  • Dodo

    I’m an RN and I have to agree with the Dr Shefali Sabharanjak. Patients who are chronic marijuana users tend to be cognitively slower than your average person. However heavy drinking plays a very big role as well.

    The patient who only smokes pot are much more pleasant and easier to look after (most of the time) until withdrawal kicks in… Pot and alcohol on the other hand is worse, the withdrawals are worse.

    Everything in moderation!

    • voxpop

      This statement is nonsensical in context:

      “Patients who are chronic marijuana users tend to be cognitively slower than your average person. However heavy drinking plays a very big role as well.”

  • Queen Bead

    Wow, some so-called “professional” people will say anything to keep their jobs..getting kickbacks from the pharmaceutical companies and drug cartels are a good incentive to lie for the cause..

    • Chris

      I saw you with a blunt too. You looked stoned. Oh, there were some mexican drug cartel members smoking with you. They looked stoned too.

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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