Authors examined nearly 4,000 students in the Montreal region over four years, starting when the average participant was about 13 years old.
The students took yearly memory tests and self-reported their alcohol and marijuana use. Those reports were kept confidential "unless such information indicated imminent risk of harm," authors wrote.
By the fourth year, three-quarters of the students had consumed alcohol at least occasionally, while only about 30 percent of participants had used marijuana. But the study observed more daily marijuana users than alcohol users, Conrod said.
The study found some of marijuana's negative effects were short-term, while others were lasting.
A particularly troubling finding: Young cannabis users may cause long-term damage to a brain function associated with substance abuse.
When studying response inhibition — that's an individual's ability to change their actions to help meet a goal — researchers found that teens using marijuana caused long-term damage to their brains.
Conrod said that finding may help explain a previously "perplexing" phenomenon: Young cannabis users have been shown to be at a greater risk for addiction later in life.
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