Teenagers who smoke marijuana on a daily basis cause damage to their developing brains, making them more susceptible to depression and anxiety in adulthood, a new McGill University study suggests. The finding is particularly significant in the Canadian context, given previous research showing that more teens in this country consume cannabis than do adolescents in the United States or Europe.

"Just because marijuana is a plant doesn't mean it's harmless," said Gabriella Gobbi, one of the study's co-authors. "Our study demonstrated that the cannabinoid, when consumed daily during adolescence, can induce a permanent change in the brain."

Scientists at the Research Institute of the McGill University Health Centre experimented on adolescent and adult lab rats. They discovered that when they exposed the rats for 20 days to cannabinoids — the active ingredient in marijuana — only the adolescent rats were adversely affected. This suggests that the adolescent brain — since it's still developing — is particularly vulnerable to chronic drug use.

The researchers observed a decrease in serotonin levels in the brains of the adolescent rats. Serotonin is a neurotransmitter that plays a role in emotional perception. Conversely, the researchers noticed an increase in norepinephrine levels. Norepinephrine plays a key role in the so-called fight-or-flight response, and increased levels could make one more anxious.

The findings were published in the journal Neurobiology of Disease. Although the research was carried out on lab rats, Gobbi said one can assume the same effects on the human brain. Still, Gobbi plans to carry out a different type of study on humans to confirm the effects of the animal trials.

Gobbi cited research by the Public Health Agency of Canada in 2004 that showed that 42.7 per cent of Canadian adolescents had tried marijuana compared with 40 per cent of teens in the United States and 35 per cent of teens in Europe.

Earlier research by Gobbi has also challenged some of the myths surrounding marijuana. In 2007, she published a study that found that although marijuana does act as an anti-depressant in the short-term, it produces the opposite effect in the long run.