Early, frequent cannabis use tied to higher mental and physical health risks, McGill study finds

Adolescents who begin using cannabis before age 15 and continue using it regularly are more likely to require health care for both mental and physical problems as young adults, according to new research led by McGill University.

The study, published in JAMA Network Open, found that early, frequent cannabis users were more likely to seek care for depression, anxiety, suicidal distress, and physical issues such as respiratory problems and injuries. Those who started using the drug after age 15, by contrast, were more likely to experience only physical health problems later on.

“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 Massimiliano Orri, Assistant Professor of Psychiatry at McGill and Canada Research Chair in Suicide Prevention. “Cannabis can also impair attention and cognition, which may increase the likelihood of accidents causing injuries.”

Researchers used data from the Quebec Longitudinal Study of Child Development to follow cannabis use patterns among participants ages 12 to 17 and linked those findings to health care records up to age 23. They accounted for early-life environmental and health factors to better isolate cannabis’s potential impact.

“Even when we considered several pre-existing risk factors for cannabis use, we still found increasing risks of using healthcare services for mental and physical health problems for youth with early-onset cannabis use,” said Pablo Martínez, postdoctoral fellow at McGill and the study’s lead author. “That suggests cannabis itself may play a role.”

The findings contribute to ongoing discussions about what age is “too young” to begin using cannabis, reinforcing public health advice to delay initiation. In Quebec, the legal age to buy cannabis is 21—higher than the federal minimum of 18 and the age of 19 in most other provinces.

Researchers caution that despite legal restrictions, changing social attitudes and easier access since legalization may still encourage underage use. “Efforts to steer youth away from consuming cannabis too young are important,” Orri said. “Public health initiatives should focus on identifying young people likely to start early and use frequently, as they may benefit from clinical interventions to reduce long-term risks.”