Cannabis and the Adolescent Brain

For some time, people have known that using cannabis during adolescence increases the risk of developing cognitive impairment and mental illness (e.g. depression, anxiety or schizophrenia) later in life. Importantly however, the mechanisms responsible for this vulnerability are not well understood. A new study, published in Brain, shows that long-term cannabis use that starts during adolescence damages the neural pathways connecting brain regions, and that this may cause the later development of cognitive and emotional problems.

The authors used diffusion tensor imaging (DTI), a MRI technique that measures water diffusion, to examine the microstructure of white matter in 59 heavy cannabis users, who used cannabis at least twice a month for three years or longer, as well as 33 non-users. In the human brain, white matter pathways are formed by bundles of axons, which carry the neural signals, and myelin, which coat the axons and speeds up signal transfer. These white matter pathways are crucial for normal brain function as they enable disparate regions of the brain to communicate, and act together.

When the authors investigated white matter microstructure in the cannabis users, they found damage in the white matter pathways of the hippocampus, crucial for memory, and the corpus callosum, which connects the brain’s two hemispheres. Both pathways are critical for normal brain function. The authors suggest that impaired connectivity due to damage in these pathways may be the cause of the cognitive impairment and vulnerability to schizophrenia, depression and anxiety seen in long-term users.

The authors also show an inverse relationship between the amount of white matter damage and the age of first use. That is, participants who started using cannabis younger had more white matter damage and showed poorer brain connectivity. Adolescence is a critical period in the development of white matter in the brain, when the neural connections we rely on in adulthood are being finally formed. The authors point out that white matter cells have cannabinoid receptors (those susceptible to cannabis) during adolescence, which disappear as the brain matures. This new study demonstrates a mechanism that may help explain how cannabis use in adolescence causes long-term changes in brain function. The cannabis users in the study had significantly higher levels of depression and anxiety compared to the non-users.

This important new study suggests that young people’s brains are at risk of white matter injury due to cannabis, and that cannabis exposure during adolescence may permanently damage white matter development. Future research must address the question; can white matter pathways and connectivity recover when a person quits using cannabis?


Zalesky A, Solowij N, Yücel M, Lubman DI, Takagi M, Harding IH, Lorenzetti V, Wang R, Searle K, Pantelis C, & Seal M (2012). Effect of long-term cannabis use on axonal fibre connectivity. Brain : a journal of neurology, 135 (Pt 7), 2245-55 PMID: 22669080

Image via Amihays / Shutterstock.

  • Vierotchka

    That’s not half as bad as the damaging effect of alcohol on adolescents’ brains.


      Alcohol isnt an alternative to weed ..its an entirely different topic. Cars also damage white matter when misused ..I dont see the corrulation.

      • Mark

        VIEROTCHKA’s point makes perfect sense being that alcohol is legally for sale at any convenience store despite being the cause of 1000’s of deaths PER MONTH. Even as a recreational method, alcohol was legalized because our country (US) couldn’t prevent it’s widespread use. We are no better at stopping cannabis use now than we are 70 years ago.
        Now, your point that cars damage white matter when misused is just matter of fact. How about combining alcohol and cars, how much white matter does that damage per year? Where’s that study? I’m sure it’s quite significant..

      • Malcolm Kyle

        Alcohol (United States) is a factor in the following:

        * 73% of all felonies * 73% of child beating cases * 41% of rape cases * 80% of wife battering cases * 72% of stabbings * 83% of homicides.

        According to the Australian National Drug Research Institute (2003): “Tobacco, alcohol and illicit drugs are prematurely killing around seven million people worldwide each year, and robbing tens of millions more of a healthy life. The research into the global burden of disease attributable to alcohol, tobacco and illicit drugs found that in 2000, tobacco use was responsible for 4.9 million deaths worldwide, equating to 71 percent of all drug-related deaths. Around 1.8 million deaths were attributable to the use of alcohol (26 percent of all drug-related deaths), and illicit drugs (heroin, cocaine and amphetamines) caused approximately 223,000 deaths (3 percent of all drug-related deaths).”

        According to DrugRehabs.Org, national mortality figures for 2009 were: tobacco 435,000; poor diet and physical inactivity 365,000; alcohol 85,000; microbial agents 75,000; toxic agents 55,000; motor vehicle crashes 26,347; adverse reactions to prescription drugs 32,000; suicide 30,622; incidents involving firearms 29,000; homicide 20,308; sexual behaviors 20,000; all illicit drug use, direct and indirect 17,000; and marijuana 0.

        Researchers led by Professor David Nutt, a former chief drugs adviser to the British government, asked drug-harm experts to rank 20 drugs (legal and illegal) on 16 measures of harm to the user and to wider society, such as damage to health, drug dependency, economic costs and crime. Alcohol scored 72 out of a possible 100, far more damaging than heroin (55) or crack cocaine (54). It is the most harmful to others by a wide margin, and is ranked fourth behind heroin, crack, and methamphetamine (crystal meth) for harm to the individual.

        The American Journal of Preventive Medicine found that in the U.S. alone, an estimated 79,000 lives are lost annually due to “excessive” drinking. The study estimates that the overall cost of excessive drinking by Americans is $223.5 billion each year.

        Health-related costs per user are eight times higher for those who drink alcohol when compared to those who use marijuana, and are more than 40 times higher for tobacco smokers, according to a 2009 review published in the British Columbia Mental Health and Addictions Journal.
        It states, “In terms of [health-related] costs per user: tobacco-related health costs are over $800 per user, alcohol-related health costs are much lower at $165 per user, and cannabis-related health costs are the lowest at $20 per user.”

        Having three or more alcoholic drinks a day increased lung cancer risk by 30 percent.
        “Heavy drinking has multiple harmful effects, including cardiovascular complications and increased risk for lung cancer,”
        – lead researcher Stanton Siu, MD, of Kaiser Permanente

        Alcohol, when used alone, is “involved” in far more emergency department visits than every illegal drug combined. According to the Centers for Disease Control, “In the single year 2005, there were more than 1.6 million hospitalizations and more than 4 million emergency room visits for alcohol-related conditions.” A study published this year in the Annals of Emergency Medicine, meanwhile, suggests that as many as 50 percent of emergency room visits could be alcohol-related. And that number is only going up. In New York City, for instance, “nearly 74,000 people wound up in hospitals in 2009 for alcohol-related reasons, compared with just 22,000 in 2003.”

        Apart from the fact that legal drugs kill far more people than all the illegal drugs combined, debating whether a particular drug is harmless or not is missing the whole point. Are drugs like Heroin, Meth or Alcohol dangerous? It simply doesn’t matter, because if we prohibit them then we sure as hell know that it makes a bad situation far worse. If someone wants to attempt to enhance or destroy their lives with particular medicines or poisons, that should be their business, not anybody else’s. Their lives aren’t ours to direct. And anyway, who wants to give criminals, terrorists and corrupt law enforcement agents a huge un-taxed, endless revenue stream?

        • Mark

          Wow.. well that was kind of my point but thanks Malcolm. I think me and this guy Malcolm would get along quite well. I used to write essay after essay in college about cannabis citing various legal standings and medicinal benefits but I think everyone has gone through that phase at some point. Keep up the good fight my man! And consider looking into Ron Paul, I think you’d dig him..
          Ron Paul 2012 folks or else..

    • jacob stokley

      hey guys pot aint that bad as long as u control urself

  • nick

    Honestly, I’d concur with the findings. I’ve been using cannabis for 2 years, daily for about a year, and depression and anxiety has increased consistently in that period. I’m 16 btw. Thanks for the article. I think ill study cannabis’ effects on the brain when I’m old enough to at a college level.

    • Malcolm Kyle

      Kindly Google “Concerne Troll”

      Maju Mathew Koola (University of Maryland School of Medicine, Baltimore, USA) and team have found that Cannabis use is associated with reduced mortality risk in patients with psychotic disorders.

      Researchers studied 762 patients with schizophrenia, schizoaffective disorder or psychosis not otherwise specified, aged 18-55 years, who were followed up for more than 10 years. Analysis revealed that cannabis users were significantly less likely to die over the study period than nonusers, with 5- and 10-year overall mortality rates of 3.1% versus 7.5% and 5.5% versus 13.6%.

      “We observed a lower mortality risk in cannabis-using psychotic disorder patients compared to cannabis non-users despite subjects having similar symptoms and treatments.” … “Future research is warranted to replicate these findings and to shed light on the anti-inflammatory properties of the endocannabinoid system and its role in decreased mortality in people with psychotic disorders.”

    • Vierotchka

      Just as in the case of alcohol, it should not be used by anyone before they have reached full physical maturity. You say you are sixteen and have used cannabis since you were fourteen – your brain, not being fully formed, can only have been harmed, just as it would have been harmed with alcohol use. Best you stop now and wait until after you are 21 before trying it again. Meanwhile, eat wholesome foods and plenty of “brain foods”, i.e. foods that are good for the brain and the nervous system. Vitamin B complex supplements as well as adding brewer’s yeast to your food in salads, yoghurts and breakfast cereals should now be your staple. Wheat germ too.

  • Jeremiah

    As people will most likely be using this as a reason to encourage people to stop using cannabis, it is only logical to point out other mind altering substances that are legal and used for entertainment, namely alcohol in any one of it’s many forms is likely worse than cannabis (see or yet somehow alcohol is legal and accepted, while if you use cannabis then you are given a social label as a criminal, a police record, and will likely have trouble getting aid for school, getting a good job, and being socially productive. This article only goes to provide further proof that although cannabis can be harmful, it is not nearly as harmful as alcohol which is socially acceptable, or overeating which is socially acceptable, but also very risky to your health. So really kids, don’t use cannabis because it’s better for you than drinking, smoking, or eating too much food, and in some cases, riding in a vehicle, but because it is slightly bad for your health, like skateboarding or getting a tattoo.

  • A passing reader

    It’s beautiful how nowadays on almost every article about cannabis, the comments are pro-cannabis and not ignorantly against it. That’s not something you saw some years back.

    • Depends how you think

      But what about those who are ignorantly in favor of cannabis? The thinking process of some of the most rabid pro-cannabis supporters is as devoid from reason as some of the most anti-cannabis folks.

  • Rull

    what about violence in video games?

  • david

    “Honestly, I’d concur with the findings. I’ve been using cannabis for 2 years, daily for about a year, and depression and anxiety has increased consistently in that period. I’m 16 btw”

    then why the hell are you doing it? i consume medicinally and it HELPS MY DEPRESSION AND ANXIETIES! im 30 and ive been a consumer off and on since 14.
    i disagree with these findings, since depression and anxiety can be caused by diet, such as high fats for depression and sugars and caffeine for anxiety. and the fact that schizophrenia cases rise with usage is the possibility of self-medication, since the endocannabinoid cannabidiol (CDB)has been shown through research to have anti-psychotic effects, which can cancel out the psychoactive effects of THC.

    • James

      The problem is that our Cannabis these days is not CBD rich. Most of it has been bred out over the years to produce high THC plants. Thankfully there is an effort around the globe including to change this so we can again utilize this unique medicine.

      • Vierotchka

        THC is the substance in cannabis that has the most and the best medicinal properties, especially in curing cancers (including glioma, the worst cancer of the brain).

  • Michael

    I think anyone that blindly accepts the assertions of flawed studies like this have deficits in cognitive functioning.

    I’ve been smoking cannabis- daily, for over 24 years since age 13. And I don’t feel as if I have suffered any “brain damage”, indeed I am as sharp as ever, and I lead a constructive and satisfying life.

    I know various other highly functional / intelligent people that have been using Marijuana since the same age as well (as well as a variety of other drugs without issue).

  • Pingback: Cannabis and the Adolescent Brain | Brain Blogger |()

  • Malcolm Kyle

    “Associated with” is not the same as “causation.”

    Schizophrenia affects approximately one percent of the population. That percentage has held steady since the disease was identified, while the percentage of people who have smoked marijuana has varied from about 5% to around 40% of the general population.

    Despite a massive increase in the number of Australians consuming the drug since the 1960s, Wayne Hall of the University of Queensland found no increase in the number of cases of schizophrenia in Australia. Mitch Earleywine of the University of Southern California similarly found the same with regard to the US population and Oxford’s Leslie Iversen found the same regard to the population in the UK. According to Dr. Alan Brown, a professor of psychiatry and epidemiology at Columbia University, “If anything, the studies seem to show a possible decline in schizophrenia from the ’40s and the ‘50s”.

    Kindly Google any of the following combinations:

    Nicotine and Schizophrenia
    Alcohol and Schizophrenia
    Chocolate and Schizophrenia
    Sugar and Schizophrenia
    Gluten and Schizophrenia

    So should we hand the market in any of the above substances to criminals (which is what prohibition effectively does) because its use is “associated” with a certain minute part of the population? Many bipolar patients misuse caffeine and tobacco in an effort to bring on a manic state, thus becoming a danger to themselves or others. Should tobacco and caffeine or whatever works for each individual be prohibited to boost ratings or rhetoric also? Where does it end?

    Persons with chronic mental illness die 25 years earlier than the general population does, and smoking is the major contributor to that premature mortality. This population consumes 44% of all cigarettes.

    Cigarette smoking rates in the American population are approximately 23%, whereas rates of smoking in clinical and population studies of individuals with neuropsychiatric disorders are typically two- to four-fold higher.

    Caffeine is most certainly linked with mental illness; psychosis even.
    Broderick, P. & Benjamin, A.B. (2004). Caffeine and psychiatric symptoms: a review. Journal of the Oklahoma State Medical Association, 97(12), 538-542.

    Hedges, D.W., Woon, F.L. & Hoopes S.P. (2009). Caffeine-induced psychosis. CNS Spectrums, 14(3),127-129.

  • Mel

    I think a lot of people missed the point. Yes their are other drugs just as or even more harmful than cannabis. The problem here is not the government laws and regulation of alcohol or cannabis. We all know that we can find drugs anywhere regardless of any laws. The problem is the lack of education. Adolescents are consuming cannabis and alcohol without knowing the long term effects. The whole thing about cars also causing damage on white matter I don’t quite understand. Would anyone care to educate me or link me to an article. In any case, ANYTHING in excess is not healthy.

  • Nick

    I hope that I’m not the only one to see that this study has some major issues. While it may be true that this article only gives some of the story, there seem to be many conclusions based on data from not only a small sample, but also a sample that has no previous background medical history suggesting that these psychological disorders did not exist prior to the lack of proper brain development. I find that I have many questions that need to be answered before I can find any of this information credible, especially with the strong ambiguity when it comes to comorbidity of psychological disorders, including substance abuse.
    Thank you for the entertainment, though!

  • david
  • Pingback: CADFY Californians for Drug Free Youth » Cannabis and the Adolescent Brain()

  • Pingback: Cannabis and the Adolescent Brain « lederr()

  • Pingback: Cannabis Abuse In Adolescence – Cognitive Decline In Later Life | Top Medications Online | Top Medications Online()

  • Anonymous

    I noticed that the people who get depressed from cannabis are the same people who focus on only the bad things in life.

  • anon

    Is it the cannabis or the life style, … because we all know, our brain is full of chemical reactions based on our feelings, and our feelings reactions of our brain,… the emotional damage that cannabis is outlawed and the isolation in a society that accepts alcohol use but denies a cannabis user rights and a public life resulting in isolation out of fear for being caught,… that also is a damage to our brain,…

  • Pingback: Bilingualism May Be Neuroprotective()

  • just like alcohol isn’t allowed to be sold to minors i think cannabis shouldn’t be either but then isn’t it just common sense

  • Pete Thornton

    Oh my god, Cannabis is so harmful for our neurological system. I know there are some harmful effects of cannabis but it amazed me when I come to know from your blog that they are harmful up to such an extent. Thanks for your information.

  • momma bear

    Oh boy pete thornton, you need to do some research. There is an incrediable amount of benefits from cannibas. Go on youtube even and see the people it has saved, and with no harsh side effects not like all the pharmaceutical drugs they jam down our throats. Overdosing is impossible, and it helps with sleeping, pain, stopping seizures, cancer, etc. A plant that has been used for thousands of years to help with all sorts of ailments is considered a dangerous drug now because they can not patent it. There is no money in something anyone can grow in their backyard for free. Cures dont make money, keeping people sick do. Sad but true.

India Bohanna, PhD

India Bohanna, PhD, earned her Bachelor of Science from Monash University and Doctor of Philosophy from University of Melbourne. She is currently a mental health research fellow.

See All Posts By The Author

Do not miss out ever again. Subscribe to get our newsletter delivered to your inbox a few times a month.