A new study suggests that smoking marijuana may not be as safe as many teen users seem to think it is.

Montreal researchers say they’ve found evidence that pot-smoking interferes with the healthy development of teens’ brains and puts them at risk for developing a dependence to the drug, as well as for mental health problems.

Whether marijuana is addictive or a “gateway drug” to harder drug use has long been up for debate. Researchers at the University of Montreal decided to review more than 120 studies that looked at how pot affects the biology of the brain and the chemical reactions that occur when the drug is used.

They say it’s difficult to confirm that pot use helps contribute to later drug behaviour and mental health issues, such as schizophrenia. But they say there is good evidence that the brain changes seen in lab rats given marijuana also occur in humans.

Marijuana interacts with our brain through cannabinoid receptors, which are in brain areas that govern learning, motivation and reward, decision-making, and habit formation. Because the structure of the brain changes rapidly during adolescence, the researchers believe that pot use during this time can greatly influence the way those parts of the brain develop.

As for why only some teen pot users become dependent on the drug or develop other addictions or mental health problems, the researchers say a number of factors appear to play a role.

The age at which teens may be one key, according to researcher Didier Jutras-Aswad of the University of Montreal.

“When the first exposure occurs in younger versus older adolescents, the impact of cannabis seems to be worse in regard to many outcomes, such as mental health, education attainment, delinquency and ability to conform to adult role," Dr. Jutras-Aswad said in a statement.

Psychological factors also can influence heavy pot use, such as a tendency to impulsivity, aggression, and negativity, Jutras-Aswad said.

“Some of these traits are often exacerbated with years of cannabis use, which suggests that users become trapped in a vicious cycle of self-medication, which in turn becomes a dependence."

And there are studies that show that marijuana dependence can be inherited through the genes that produce the cannabinoid receptors.

The researchers say their research is strong evidence that pot smoking is not as harmless as many believe.

"It is now clear from the scientific data that cannabis is not harmless to the adolescent brain, specifically those who are most vulnerable from a genetic or psychological standpoint,” Jutras-Aswad said.

Co-author Dr. Yasmin Hurd, of New York's Icahn School of Medicine at Mount Sinai, says it’s important to continue research on marijuana dependency before governments begin relaxing rules on marijuana use.

“Without such systematic, evidenced-based research to understand the long-term effects of cannabis on the developing brain, not only the legal status of cannabis will be determined on uncertain ground, but we will not be able to innovate effective treatments such as the medicinal use of cannabis plant components that might be beneficial for treating specific disorders," Hurd said.

The review appears this month in the journal Neuropharmacology.