As a family physician, I often talk with patients and their families about cannabis and how it may affect growing brains. Many teenagers see the prevalence of use and may hear that marijuana is “natural.” They may assume that this means it is safe. But newer research suggests cannabis use during the teen years may negatively affect how the brain grows and works.

The teenage brain is forming. It keeps growing and changing well into the mid-20s. During this time, the brain is building important skills like memory, attention, learning, planning, and self-control. These are the same skills teens need to do well in school, make safe choices, and manage emotions.

A large recent study followed more than 11,000 children from ages 9 to 10 into their later teen years. Researchers tracked both brain development and substance use over time. They found that teens who used cannabis showed less growth in important thinking skills, including memory, attention, language, and processing speed. Some of these teens started out doing just as well as other children when they were younger. But over time, their progress slowed, while other teens kept improving.

Researchers also found that THC, the main chemical in cannabis that causes a “high,” was linked to worse memory over time. CBD did not show the same effect. This matters because many newer cannabis products contain much higher levels of THC than in the past. Do you think most users know what components are in their cannabis?

Brain scans in another long-term study showed that teens who used cannabis had faster thinning in parts of the brain that help with decision-making, attention, and impulse control. These changes were seen most in the prefrontal cortex, an area that is still developing during adolescence. The more cannabis a teen used, the stronger these brain changes appeared to be.

Even teens who did not have a cannabis addiction were more likely to struggle with depression, trouble focusing, school problems, and thoughts of self-harm compared with teens who did not use cannabis.

This does not mean every teen who uses cannabis will have lasting harm. Some may be more sensitive than others. But the evidence is clear enough to guide caution. Teens who start younger, use more often, or use high-THC products appear to be at greater risk.

The bottom line is simple: adolescence is a critical time for brain growth, and cannabis use during these years has risk. Patients and families should know that today’s cannabis products may carry real risks for learning, mental health, and healthy brain development. User beware.