Dr. Madeline Meier

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Dr. Meier: I studied the effects of persistent cannabis use on IQ. So, we asked whether persistent cannabis users lost IQ points from childhood to adulthood.

We found that people who started using cannabis as teenagers, by the time they were 38, they lost about 8 IQ points.

We studied about 1,000 people who were born in Dunedin, New Zealand, in 1972 and '73.

And we followed them up approximately every other year all the way until they were 38 years old.

When they were 7 to 13 years old, we gave them IQ tests, then we assessed their cannabis use from the time they were 18 to 38, and then we gave them IQ tests again at 38 so we could look at their intellectual functioning in childhood before they began using cannabis and then again in adulthood, after some of them had already been using cannabis.

We found declines across measures of mental functions, so not just IQ. We looked at memory; executive functioning, which is the ability to multitask, plan ahead; processing speed; reaction time.

We also looked at whether informants noticed cognitive problems among persistent cannabis users—we didn't just look at standardized tests, we looked at how people were functioning in everyday lives, and we found that informants noticed more cognitive problems, attention and memory problems, among persistent cannabis users.

In this study, we defined persistent cannabis use as dependence on cannabis over multiple years.

Dependence generally refers to using cannabis despite experiencing problems associated with cannabis use. Those could be physical health problems, legal problems, or relationship problems.

So we looked at it in two different ways. We looked at persistent cannabis dependence, but we also looked at persistent regular cannabis use. And we defined regular cannabis use as using cannabis 4 or more days per week.

Regular teen cannabis users who stopped using later in life, by adulthood, they still showed IQ decline, so quitting use as an adult didn't result in recovery of IQ function.

We don't know when the damage is done, but we do think that the damage is lasting.

Our findings that adolescent or teenage cannabis users show IQ decline are consistent with other studies, both animal and human studies, suggesting that adolescents may be particularly vulnerable to the effects of cannabis on mental function.

We wanted to know if there were perhaps alternative explanations that could account for this finding of the effect of cannabis on IQ decline, and so we looked at some of the most obvious potential alternative explanations, such as recent cannabis use the night before neuropsychological testing at age 38; alcohol use or other drug use; tobacco use.

What we did is we took all of these factors into account, and after we took them into account, we still found that persistent cannabis users showed IQ decline, suggesting that none of these alternative explanations could account for the findings.

There was a hypothesis put forward that low social class could possibly explain our findings that persistent cannabis users experience IQ decline. And in fact, we went back and we controlled for social class, and we found that social class couldn't account for the effects. Even after taking into account low social class, persistent cannabis users still showed IQ decline.

It would be really nice to combine this kind of before-and-after study—so, "before used cannabis" and "after used cannabis"—with neuroimaging to look at how cannabis affects the brain, both the function and the structure.

I think it's also important, you know, people want to know, how much cannabis needs to be used and over what ages, and I think those are future areas for research.

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