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Joel Carillet

Chronic use of the chronic before age of 18 can cause "lasting harm to a person's intelligence, attention and memory" – and quitting pot later in life doesn't reverse the damage, says daunting new research out of New Zealand.

The study, which followed 1,037 Kiwis for nearly 40 years, found that adolescents who smoked marijuana persistently for years showed declines of eight IQ points when their scores were tabulated at age 13 and then at 38. Teens who got stoned regularly all scored significantly worse than their sober counterparts on tests measuring memory, reasoning and processing speed, with family and friends of users corroborating the findings anecdotally.

"Marijuana is not harmless, particularly for adolescents," lead researcher Madeline Meier, a post-doctoral researcher at Duke University, said in a release. "Somebody who loses eight IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come."

Adolescent tokers are particularly vulnerable to lasting mental deficits because their brains are still developing, the researchers explained. Subjects who didn't hit the bong until they were adults "with fully-formed brains" did not exhibit these drastic mental declines.

(Approximately five per cent of the respondents were deemed "marijuana-dependent," that is, lighting up more than once a week before turning 18. The researchers controlled for other drug and alcohol use and disparities in education.)

What isn't clear from this study is what quantity of weed causes damage, and what age (if any) might be safe for regular use.

Marijuana use is up among American teens, who are now more likely to smoke pot than tobacco, according to a 2011 University of Michigan study.

That study found one in every 15 high-school seniors getting high on a daily or near daily basis, the most substantive rates seen since 1981. One hypothesis for the resurgence is that teens perceive few risks associated with the drug, with many refusing to even call it a drug.

In Canada, the prevalence of pot use among Canadians aged 15 and over decreased to 9.1 per cent in 2011 from 10.7 per cent in 2010.

Still, the rates for youth aged 15 to 24 were three times higher than for their over-25 counterparts: 21.6 per cent versus 6.7 per cent.

Without naming names, do the findings ring true in your circle of friends?