17 year old smoking weed

Delaying marijuana smoking to age 17 cuts risks to teens’ brains, new study suggests

The more teenagers delay smoking marijuana until they’re older, the better it is for their brains, but there may be little ill effect if they start after age 17, says a new Université de Montréal study.

Adolescents who smoke pot as early as 14 do worse by 20 on some cognitive tests and drop out of school at a higher rate than non-smokers, confirms the study, published Dec. 29 in Development and Psychopathology, a Cambridge University Press journal.

“Overall, these results suggest that, in addition to academic failure, fundamental life skills necessary for problem-solving and daily adaptation [. ] may be affected by early cannabis exposure,” the study says.

However, the cognitive declines associated with cannabis do not seem to be global or widespread, cautioned the study’s lead author, Natalie Castellanos-Ryan, an assistant professor at UdeM’s School of Psychoeducation.

Her study found links between cannabis use and brain impairment only in the areas of verbal IQ and specific cognitive abilities related to frontal parts of the brain, particularly those that require learning by trial-and-error.

In addition, if teenagers hold off until age 17 before smoking their first joint, those impairments are no longer discernible. “We found that adolescents who started using cannabis at 17 or older performed equally well as adolescents who did not use cannabis,” said Castellanos-Ryan.

In the study, she and her team of researchers at UdeM and CHU Saint-Justine, the university’s affiliated children’s hospital, looked at 294 teenagers who were part of the Montreal Longitudinal and Experimental Study, a well-known cohort of 1,037 white French-speaking males from some of the city’s poorer neighbourhoods. The teenagers completed a variety of cognitive tests at ages 13, 14 and 20 and filled out a questionnaire once a year from ages 13 to 17 and again at 20, between 1991 and 1998.

Roughly half — 43 per cent — reported smoking pot at some point during that time, most of them only a few times a year. At 20 years of age, 51 per cent said they still used the drug. In general, those who started early already had poor short-term memory and poor working memory (that is, the ability to store information such as a phone number long enough to use it, or follow an instruction shortly after it was given). Conversely, the early users also had good verbal skills and vocabulary; Castellanos-Ryan suggested one possible explanation: “It takes quite a lot of skills for a young adolescent to get hold of drugs; they’re not easy-access.

She and her team found smoking cannabis during adolescence was only linked to later difficulties with verbal abilities and cognitive abilities of learning by trial-and-error, and those abilities declined faster in teens who started smoking early than teens who started smoking later. The early adopters also tended to drop out of school sooner, which helped explain the decrease in their verbal abilities. “The results of this study suggest that the effects of cannabis use on verbal intelligence are explained not by neurotoxic effects on the brain, but rather by a possible social mechanism: Adolescents who use cannabis are less likely to attend school and graduate, which may then have an impact on the opportunities to further develop verbal intelligence,” said Castellanos-Ryan.

Besides filling out questionnaires about their use of drugs and alcohol over the previous year, the boys participated in a number of tests to measure their cognitive development. For example, they were given words and numbers to remember and repeat in various configurations, were asked to learn new associations between various images, played a card game to gauge their response to winning or losing money, and, in a test of their vocabulary, had to name objects and describe similarities between words. In general, those who performed poorly in language tests and tests that required learning by trial-and-error, either to make associations between images or to detect a shift in the ratio of gains to losses during the card game, reported smoking pot in their young teens.

What should be the takeaway from the study?

“I think that we should focus on delaying onset (of marijuana use),” said Castellanos-Ryan, who next intends to study whether these results can be replicated in other samples of adolescents and to see if cannabis use is associated with other problems, such as drug abuse, later in life. Prevention is especially important now, she added, since marijuana is much more potent than it was in the 1990s, and because teenagers today have a more favourable attitude to its use, viewing marijuana as much less harmful than other recreational drugs.

“But it is important to stick to the evidence we have and not exaggerate the negatives of cannabis,” she cautioned. “We can’t tell children, ‘If you smoke cannabis you’re going to damage your brain massively and ruin your life.’ We have to be realistic and say, ‘We are finding evidence that there are some negative effects related to cannabis use, especially if you start early, and so, if you can hold off as long as you can — at least until you’re 17 — then it’s less likely there’ll be an impact on your brain.'”

Adolescents who smoke marijuana as early as 14 do worse by 20 points on some cognitive tests and drop out of school at a higher rate than non-smokers. But if they hold off until age 17, they’re less at risk.

Why Marijuana Use in Teens Is Harmful

Aron Janssen, MD is board certified in child, adolescent, and adult psychiatry and is the vice chair of child and adolescent psychiatry Northwestern University.

The stigma of marijuana use has declined since the early 2000s, with some states legalizing it for medical and even recreational uses. While it may not be as problematic for adult use as once believed, there are still plenty of good reasons to discourage pot smoking in teenagers.

First of all, it’s illegal. And research by the National Institute on Drug Abuse (NIDA) indicates that childhood marijuana use can affect learning ability, attention, memory, coordination, balance, judgment, and decision-making.  

Understand Why Teens Smoke Pot

It would be nice if there was one clear reason that children and young teenagers begin using marijuana, but there are actually many reasons teens choose to begin smoking weed. If you want to keep your children from smoking weed at an early age, you will have to be diligent in several areas of influence in their lives.

Pressure From Peers and Others

Children who have family members who smoke or express approval of marijuana are much more likely to begin using marijuana than those with no family involvement with the drug. Similarly, if they live in a neighborhood where they see drug activity going on, teens are less likely to express disapproval of drug use and become more likely to try drugs themselves.

Peer pressure to smoke pot remains a strong influence as well.   If they have friends who are using marijuana, they are more likely to try it themselves. There’s a tendency to adopt the attitude that “everyone is doing it” and it’s part of the normal teenage experience. But research shows that the majority of teens make it all the way through high school without ever using marijuana.  

Drug and alcohol use is often promoted in the music they download, and the videos they watch online. Those influences can be significant for young teens.

Self-Medicating and Escape

Many teens turn to marijuana in an effort to self-medicate, to make themselves feel better. They use marijuana to try to cope with depression, anxiety, and anger.

Teens will also begin using weed as an escape. Boredom is one of the main reasons some teens report that they use marijuana.

Children who have been physically or sexually abused are at greater risk than other teens for using marijuana and other drugs. They turn to drugs to escape the fear and pain of abuse.

Know the Risks of Weed

Some kids begin using marijuana because they don’t understand the harm it can do to them while their brains and minds are still not fully developed. Or more often, they make a decision to begin using based on misinformation.

The marijuana legalization movement has played a role in sending a mixed message to young people. Teens might believe “if it’s medicine, it must be safe” or “if it’s legal, it must be OK.”

In no state where marijuana has been made legal for medical or recreational use has it been made legal for anyone under that age of 21. Not even the most adamant legalization advocates propose making it legal for children to use marijuana.

If you are a parent who wants to protect your children from the dangers associated with early marijuana use, educate them with the facts so that they can make an informed decision about the risks. Marijuana use in adolescence can have these short-term consequences:  

  • Difficulty learning and retaining information
  • Injury
  • Car accidents
  • Risky sexual behavior (leading to sexually transmitted disease)
  • Bronchitis (can become chronic if marijuana use continues)


Addiction to marijuana is possible and it’s even more likely if you begin to use the drug prior to age 18.   In fact, marijuana use disorder accounts for nearly 50% of admission for those ages 12 to 17 years who are receiving substance use disorder treatment, according to the NIDA.

Poorer Quality of Life

Not only are students who misuse marijuana more likely than their non-using peers to drop out of high school, but they have been found to have lower salaries, less career success, and diminished life satisfaction later in life.   This may be associated with altered brain development and cognitive impairment.

Mental Illness

Research has found that high doses of marijuana can result in panic attack or even acute psychosis, including hallucinations, paranoia, and disorganized thinking in those who already have severe mental illness. The risk of psychosis is even greater if someone has genetic risk factors. Some research has even linked chronic cannabis use in adolescents to an increased risk of schizophrenia.  

Auto Accidents

When you use marijuana, your judgment, alertness, concentration, coordination, and reaction time are impaired. Driving while under the influence of marijuana can double a young driver’s risk of a car accident, and the risk is even higher if they consume even small amounts of alcohol.  

Talk to Kids About Marijuana

It is important to talk to your teen about marijuana use,   not only if you feel they may be smoking pot, but to help prevent future use. If you’ve found evidence of your teen using, share with your teen what you have found and express why you believe they are using drugs. Don’t be surprised if your teen gets defensive or denies it.

Pick the Right Time

If you think your teen is high, it’s best to wait until they are sober to have a conversation so they communicate properly. Instead of arranging a sit-down meeting, which might be met with resistance, try to make the talk more spontaneous and casual during a time when you’re both physically and mentally present.

Be Positive and Hold Judgment

If you judge or condemn your child, they will likely deny use and be less receptive to your message. Instead, think about when you were a teen and how you’d want your parents to talk to you with respect, understanding, and curiosity.

Stay Clear-Headed and Calm

While you want to get your point across, you don’t want to approach the conversation with anger or panic. This may prevent your teen from really listening to your concerns.

A Word From Verywell

If you think your child is misusing marijuana, help is available. Reach out to your pediatrician, who could talk to your child and/or recommend an addiction or mental health professional. Since it is possible to become addicted to marijuana, your child may need outpatient counseling or perhaps even residential treatment.

If your child is struggling with substance use or addiction, contact the Substance Abuse and Mental Health Services Administration (SAMHSA) National Helpline at 1-800-662-4357 for information on support and treatment facilities in your area.

For more mental health resources, see our National Helpline Database.

With marijuana being legalized in more and more states, it's no wonder that teenagers may believe it's safe for them to use. But evidence shows it's not.