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Marijuana risks for teens - as a parent, you may wonder about the true dangers associated with adolescent marijuana use - how harmful is it, really?

On the one hand, you don’t want to see your children taking any drugs - but on the other hand, if they’re going to experiment with drugs (and about half of all students will by the end of high-school*) maybe we should feel grateful that it’s only marijuana, after all:

  • You may have used it (or use it) and experienced few negative consequences.
  • It seems increasingly accepted and commonplace within popular culture.
  • Medical marijuana advocates have raised awareness about the drug’s medicinal qualities.
  • Some states have moved toward decriminalization.

But here's the thing - though many adults can use marijuana without significant adverse effects (though some will develop addiction) teens, with their still-developing brains, may experience lasting (lifelong!) adverse consequences.

  1. Marijuana is not as toxic as meth (or alcohol for that matter) or as addictive as cigarettes, but it is still a powerful mind-altering substance. Research shows that using it regularly during important developmental stages causes lasting brain changes and functional impairments that can last a lifetime.
  2. Some potential brain-change consequences include diminished cognitive capacity (lowered I.Q.), an increased risk of mental illness, lowered academic success and an increased lifetime risk of substance abuse.
  3. However – if you can keep your child from developing a marijuana habit until young-adulthood – say until the age of 21 – you help him/her bypass virtually all of the risks associated with early marijuana use. You don’t have to keep them from marijuana forever – just help them delay initiation to when it won’t do so much harm.

In this article you will find out how marijuana affects the still-developing teen brain and why a younger age of regular use increases the risks of whole-life problems. This article will present research findings from major studies which illustrate the increased risks associated with earlier marijuana use across a range of domains, such as:

  • The cognitive consequences
  • The mental health consequences
  • The academic consequences
  • Addiction Issues

*According to the Monitoring the Future Survey Study, as of 2012, 49.1% of high school seniors had tried at least some kind of illicit drug at least once and 45.2% of grade 12 students had tried marijuana at least once.1 

The Cognitive Consequences of Marijuana Use

The research summaries below demonstrate that adolescent marijuana users experience thinking and impulse control deficits that may diminish academic performance and increase the likelihood of bad-decision making.

Regular users will experience these deficits at all times (not only while high) but these deficits are reversible with sustained abstinence – so after a month or so of avoiding marijuana, thinking powers return to normal.

However, regular users who start smoking at a young age do not demonstrate this same recovery with abstinence. If you get heavily into marijuana in young adolescence, you can cause lasting brain damage that may limit your whole-life potential - even after you stop using.

The Effects on Current Thinking Abilities

If your son or daughter smokes a lot of marijuana as a teen, will they be as sharp during their high school years?

Possibly not. This is likely dose-dependent (the more you smoke, the greater the impact) but research shows that adolescents who smoke marijuana do not perform as well on tests of short term memory and impulse control as teens who do not use marijuana. These deficits could easily affect school performance.2 Other studies have shown that teen marijuana users may also show deficits in verbal and non-verbal short term memory, processing speed, attention and executive functions.3

Marijuana's Effects on Lifetime Thinking Abilities

If you smoke marijuana as a teen, will it affect your potential as an adult?

Unfortunately, research suggests that adolescent marijuana use can have a lasting influence, especially with a young age of initiation.

The University of Oregon Study

Adolescents who smoke heavily (defined as 4 days a week or more) may do irreversible brain damage and shave-off a significant chunk of potential I.Q.4

University of Oregon Researchers examined data from a pool of over a thousand people who were followed from birth in 1973/74 to age 38. All subjects were assessed for I.Q. at age 13 and each subject was interviewed about marijuana use patterns at ages 18, 21, 26, 32 and 38.

The Results:

At age 38:

  • Subjects who started smoking as adolescents were more likely to become heavy persistent smokers into adulthood than those who started in young adulthood.
  • Heavy marijuana smokers scored worse on measures of I.Q., memory and concentration than those who abstained from marijuana.
  • Subjects who began smoking as adults but then cut back as they aged recovered from the cognitive deficits associated with marijuana use, but those who smoked heavily in young adolescence did not show the same cognitive recovery, even after cutting back with age.
  • Subjects who smoked heavily as teens lost an average of 8 I.Q. points over the ages of 13 to 38. This is considered a significant decline.

As an associational study the authors cannot prove that marijuana use causes irreversible I.Q. deficits, nevertheless, the study authors postulate that marijuana may be toxic to the adolescent brain in a way that it is not to the adult brain.

The Harvard Study

Researchers at Harvard tested the cognitive performance of three groups of adult study subjects. Subjects in the first group were non-users. Subjects in group 2 were heavy using adults who had started using marijuana after the age of 17 (late onset) and subjects in group 3 were heavy using adults who had started using prior to age 17 (early onset). All marijuana users had been abstinent for at least 28 days at the time of cognitive testing.

  • After 28 days of abstinence, adults who had started smoking marijuana after the age of 17 showed no differences in cognitive performance to non-using adults. Subjects from the early onset group, however, performed significantly worse in cognitive tests, particularly tests of verbal intelligence, even after 28 days of abstinence.5

Brazilian Study

Cognitive testing on treatment-seeking chronic adult marijuana users shows that those who started with marijuana prior to the age of 15 perform significantly worse on measures of executive functioning (working memory, problem solving, cognitive flexibility and planning) than those who started after the age of 15.6

Attention Dysfunction Study

The brain’s visual scanning system matures greatly during a developmental stage that occurs between 12 and 15 years of age.

When testing adult study subjects on a quick-reaction computer task that involved visual scanning ability, subjects who had initated regular marijuana use prior to age 16 (early onset) performed significantly worse than subjects who initiated use after age 16 (late onset). Late onset marijuana users performed equally well to control subjects, indicating that marijuana is toxic to the visual system during certain developmental stages.7

Observable Brain Changes

Beyond observing differences in thinking, researchers can also see how an early age of marijuana onset causes MRI visible brain alterations. Earlier ages of onset are associated with progressively decreased white matter integrity8 and decreased cortical thickness.9

Academic Consequences of Teen Marijuana Use

While there are always exceptions, in general, students who use marijuana get lower grades than students who abstain. The Washington State Healthy Youth Survey Study revealed that marijuana using high school students were more likely to get Cs, Ds and Fs than non-users.10

Given that current marijuana use impairs attention, working memory and impulse control it’s hardly surprising to find an association between use and lower school performance – but are there unique academic risks associated with earlier age of regular use?

Yes, as with cognitive consequences, earlier age of regular use is associated with greater declines in school performance.

  • Australian researchers found that teens who started smoking marijuana at least once a week by the age of 15 were far more likely to drop out of school prior to completion than students who started with weekly use at an older age or those who abstained from marijuana - the younger the age of regular use – the greater the risk of school drop-out prior to completion.11

Postulated explanations for why marijuana use - especially early onset use – is associated with academic underachievement include:

  • Marijuana-caused learning difficulties
  • Greater involvement with non-academically oriented peers

Or, it could be that young people attracted to early marijuana use are also more exposed to other risk factors that also predict school non-completion.12

Mental Health Consequences

Adolescents who smoke marijuana raise their risk of developing a psychotic disorder in adulthood and smoking earlier in adolescence increases the dangers.13

  • A  major Swedish study found that smoking heavily by the age of 18 raises your risk of adult schizophrenia by 600%.
  • A follow-up study from New Zealand replicated the increased risk findings. In this study, the researchers found that people who had smoked marijuana by the age of 15 were 4 times more likely than control subjects to be diagnosed with schizophrenia by the age of 26 and that those who started at 15 were at greater risk to develop schizophrenia than subjects who started after the age of 18.14

Who’s Most at Risk?

Not every teenager using marijuana has the same schizophrenia risk. Teens more at risk include:15

  • Those with a close family member with psychosis or another serious mental illness.
  • Those who have already had an unusual psychological experience after using marijuana (such as a break from reality).

Addiction Consequences

  • The younger a person starts using marijuana, the greater the risks of lifetime addiction.
  • The earlier in adolescence a person starts using, the more rapidly addiction develops.16

Take-Home Message

Whatever your opinion on marijuana for adults, it seems clear that initiating regular use before the age of 18 puts an adolescent at risk of serious and lasting neurocognitive deficits. In fact, given that the brain continues to mature and develop to the age of 25, it seems likely that delaying marijuana use even beyond the age of 18 would be prudent – to that end, the California Society of Addiction Medicine advocates helping young adults delay first use to the age of 21.17

If Concerned about a Teen Not Currently Using

  • Talk to your children about marijuana. Don’t scaremonger (you’ll lose credibility if you do) and don’t stray from the facts, but make sure you explain that marijuana is dangerous to teens in a way that it is not to adults - and that starting young could have lasting or even irreversible consequences.

If Concerned about a Teen already Using

  • Communicate the facts, and consider interventions (counseling, etc.) if necessary, to help this person achieve behavioral change.
  • Ask the person to take a quick adolescent marijuana addiction self test.
 
References
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Page last updated Oct 10, 2014

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