Teens who start using cannabis early face more health problems in life

A new study tracked 1,591 youth in Quebec from early adolescence to age 23. The results showed that teens who began using cannabis regularly before the age of 15 were more likely to need medical care for depression, anxiety, and various physical problems in young adulthood.

The researchers followed students at ages 12, 13, 15, and 17 – then linked their records to provincial health data through age 23.

The team found that adolescents who started using cannabis after the age of 15 had a greater higher risk for physical conditions, but no clear rise in mental health care use.

Effects of early cannabis use

The experts identified three patterns using trajectory modeling. One group never used cannabis, one began after 15 with occasional use, and one group began before 15 with frequent use.

Early and frequent users had higher odds of visits for common mental disorders and for injuries and other physical conditions. 

The adolescent brain is still reshaping circuits that support attention and learning. Those changes include executive function.

The study’s lead researcher, Massimiliano Orri, is an assistant professor of psychiatry at McGill University and clinician-scientist at the Douglas Research Centre.

“Youth under 15 are in a critical period of brain growth, which may make them more susceptible to cannabis’s effects on mental health,” said Professor Orri.

Teens and cannabis link

Across Canada, student survey data from 2023 to 2024 show that 18 percent reported cannabis use in the past year. Use climbed with grade level, from three percent in grade 7 to 39 percent in grade 12.

The researchers adjusted for 32 early life factors, including family context and earlier medical use. They balanced the groups based on these confounders.

The early start signal remained after adjustments, which strengthens the case that timing and intensity matter.

Early and frequent cannabis use

In statistical terms, the team compared groups using an odds ratio, a measure of how much more likely an outcome is in one group.

Early starters who used frequently had higher odds of care for depression and anxiety, and higher odds for injuries and other physical illnesses.

The pattern did not mean that every early user will have these problems. It meant that, at the population level studied, early and frequent use tracked with more care for both mental and physical conditions by age 23.

How cannabis use affects teens

Cannabis can blunt attention, the mental process that selects what to focus on, during intoxication and shortly after. That shift may raise risk during driving, sports, and other tasks that demand quick decisions.

Repeated use during sensitive windows could intensify stress, sleep loss, and other common issues that teens face.

The result may be a mix of short-term safety issues and longer term mood and learning challenges.

In Quebec, the legal law sets the minimum age at 21 to buy or possess cannabis. Legal limits can reduce retail access, but teens still report getting cannabis through friends and family.

Helping teens safely

Public health messaging works best when it is concrete and age specific. Tell younger teens that postponing use protects mental health, and explain that frequent use raises risks more than occasional experimentation.

Parents can look for ongoing patterns rather than isolated incidents. A simple check-in that explores how often, where, and why a teen uses cannabis can uncover potential risks and open the door to support.

Focus on practical, non-judgmental steps – such as offering guidance on safer use, improving sleep and stress habits, and creating a gradual plan to cut back – to reduce harm and build trust.

The data show that the when and the how often of teen cannabis use matter for their health later in life. Delaying use, and reducing frequency if use has already started, are both realistic goals.

Health systems should be ready for preventable visits tied to injuries and anxiety in young adults. Schools and families can help by guiding choices earlier, when habits are forming and brains are still building critical skills.

The study is published in the journal JAMA Network Open.

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