Marijuana – Its negative impact on adolescent development, Journal of Abnormal Psychology

Recreational marijuana use remains very popular among U.S. teens and young adults.  This is nothing new, as pot has been a prominent part of popular culture for decades.  But some important things have changed.  First, the levels of THC (the primary psychoactive ingredient in marijuana) in confiscated marijuana have risen markedly over the years, from about 1-2% in the late 1970s, to roughly 10% in 2009.  Although more potent strains of marijuana have always been available, they are far more common now. This means that today’s pot smokers are exposed to a far stronger version of the drug, and this adds to concerns about both the immediate effects of a single dose and the long-term effects of repeated use.  Secondly, we now know that there are specific receptors in the brain to which THC molecules can bind and change the functioning of nerve cells.  This means that there is a natural, internal (“endogenous”), and delicate cannabinoid system in the brain that can be powerfully stimulated by marijuana.

Importantly, we also now know that the adolescent brain is far more sensitive than the adult brain to some of the effects of THC.  Most of the recent research has focused on the ability of THC to impair learning and learning-related brain function. In our laboratory, we have shown that THC disrupts learning performance in adolescent animals far more potently than it does in adults, and that this difference in sensitivity can be traced all the way down to the level of individual neurons in the hippocampus.  These are striking findings because they indicate some very fundamental differences in how the adolescent and adult brain function and react to THC.  Like with alcohol, it is now possible to have a conversation with an adolescent about marijuana that is based on facts and not emotions or power struggles.  It might not seem fair, but for better or worse the adolescent brain is simply more susceptible to learning impairments by marijuana.  And in studies of adolescent marijuana users, higher levels of use are associated with more significant learning deficits.While the impairments that marijuana causes in learning and memory appear to improve some with abstinence, deficits in attention, a fundamental ability involved in nearly every facet of daily life, are still evident for at least three weeks after a teen stops using. Further, like with alcohol, it appears that the earlier one initiates use of marijuana the bigger the deficits produced by the drug. In one recent study comparing adolescent chronic marijuana users who began before or after the age of 15, those who began earlier showed larger deficits in attention and memory.

Another serious consideration is that recent research indicates that some adolescents may be at risk for serious psychiatric problems, including delusions, hallucinations, and the emergence of schizophrenia later in life if they smoke marijuana regularly during their teens.  The research is new and there are many details to be worked out, but it does raise a red flag for caution.  We’ve known for some years that marijuana use by young people conveys a moderate statistical risk for the development of psychiatric illness, and the younger the age at which marijuana use is initiated, the greater the risk.  But the new work suggests that whether or not a person is at risk depends on the expression profile of a particular gene that gives rise to an enzyme in the brain called COMT (catechol-o-methyl-transferase) that regulates the activity of dopamine and several other neurochemicals that are important for mood and other functions.  People with a particular version of the gene may be far more likely to develop psychiatric illness than others.
Fortunately, marijuana does not share the profound impact alcohol has on vital reflexes, like gagging and breathing, which makes marijuana less deadly after one time use. However, marijuana can bring about death via its effects on the cardiovascular system, particularly in the first half hour or so after smoking it. It can also lead to death by causing people to do stupid things that seem really smart at the moment. According to data from the Drug Abuse Warning Network (DAWN), which tracks mentions of drug use recorded during visits to Emergency Rooms, marijuana-related visits have been on the rise over the last few decades, surpassing the number of Emergency Room visits attributed to heroin.

There is widespread misconception regarding whether marijuana use can lead to dependence.  An abundance of data from recent studies leaves little doubt that marijuana dependence is real and that quitting often brings about a very uncomfortable syndrome of withdrawal symptoms. In one recent study of 104 adolescents in treatment for marijuana dependence, 9 out of 10 reported experiencing withdrawal symptoms characterized by craving, irritability, restlessness, anxiety and depression. As with alcohol, there appears to be a genetic component to marijuana dependence. A recent study examined the DNA of kids with and without dependency on the drug, and found that there is a particular type of gene associated with dependence as well as another associated with use but no dependence. Around 12% of the sample had the putative dependence-risk-enhancing gene.

While research regarding the impact of marijuana on adolescent brain development and psychological development is, in many ways, still in its infancy, there is sufficient evidence to conclude that marijuana use during adolescents can lead to lingering deficits in basic cognitive abilities, like learning and attention, and that regular use of marijuana can lead to physical dependence and withdrawal. Studies regarding the direct effects of marijuana on the adolescent  braindo not yet paint a compelling story about changes marijuana produces, but do indicate that a long list of changes occurs. Several studies suggest reduced thickness of cortical gray matter layers in the cerebellum, which is involved in learning, balance and driving skills, in adolescent marijuana abusers. Other studies suggest increased thickness of gray matter layers in some areas of the frontal lobes and decreased thickness in others among chronic users. While we await a more definitive picture of the impact of marijuana on the teen brain, the existing research tells us that marijuana use during adolescence impairs cognitive skills, increases the risk for schizophrenia and other psychotic disorders, is associated with depression, and can lead to dependence and withdrawal. These reasons alone should be sufficient to motivate adults to do everything they can to delay or prevent the use of marijuana by teens.

Impact of adolescent drug use and social support on problems of young adults: A longitudinal study.  Newcomb, Michael D.; Bentler, Peter M.  Journal of Abnormal Psychology, Vol 97(1), Feb 1988, 64-75. doi: 10.1037/0021-843X.97.1.64

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