Marijuana and IQ: The Latest Reefer Madness


What Is Weed Blog

Study Shows Adult Marijuana Use Has No Effect On IQ

If you want to believe that marijuana damages your brain, be my guest. There are plenty of folks who’ll be more than happy to support you in that belief, and now, weed-hater, there’s a new scientific study which you can brandish at your pothead friends, as well. Ah, the delicious superiority you’ll feel, as you make it clear to these weed-addled burnouts that they are just plain unacceptable in polite society.
But once you get over that last little orgasmic shudder of righteousness — at least, if you’re interested in maintaining some sort of tenuous contact with the non-Reefer Madness reality under which most of us operate — you might want to consider that maybe this study, trumpeted loudly by all the usual mass media suspects, might just reflect the fact that serious, practicing cannabis users aren’t that into taking IQ tests. You might want to also remember that — even according to this study — marijuana use by adults has no effect on IQ scores.

“Adolescent-onset cannabis users showed significant I.Q. declines, and more persistent use was associated with greater declines,” said lead author Madeline H. Meier, a rsearcher at Duke University. “Collectively, these findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects.”
When Meier makes that suggestion, however (which is, as she herself admitted, pure speculation), she is in conflict with scientific studies which show the neuroprotective and even neurogenerative effects of cannabis. That is to say, solid scientific evidence exists that marijuana not only helps protect the brain cells you have — it also helps to grow new ones.
The study, which its own authors admit is “not definitive,” suggested a drop (on the average) of eight points in IQ scores for those who, as teenagers, frequently smoked marijuana, was headlined by Fox News Latino as “Marijuana Linked to Low IQ In Teenagers, Study Says.” Of course, the study says no such thing — after all, an IQ of, say, 130, still isn’t “low” even at 122. But then again, two words: Fox News.
The study also showed that, for those who become frequent users of cannabis after age 18, there is no IQ drop. Researchers said that their primary concern is that people stay away from marijuana until adulthood.
The study has serious limitations, including the fact that researchers relied on subjective reports of marijuana use rather than actual chemical testing, reports Tanya Lewis at Wired. While they controlled for some factors which could skew the results, such as other drug addictions and education, other factors could have contributed to the IQ decline.
Still up for debate is whether the brains of adolescent-onset users “recover” after quitting marijuana. The study’s finding — that quitting did not restore mental function to pre-cannabis levels — was based on only a small sub-sample of subjects who quit pot.
Other studies have suggested there is some “recovery.” Harrison Pope and a team at Harvard Medical School found that users who showed cognitive deficits a week after quitting cannabis had “recovered fully” after 28 days of not using.
Canadian Study Found IQ Gains For “Light Users” of Cannabis
A 2002 longitudinal study published in the Canadian Medical Association Journal concluded that “marijuana does not have a long-term negative impact on global intelligence,” and that “current marijuana use had a negative effect on global IQ score only in subjects who smoked five or more joints per week.” 
In fact, that same study — which monitored subjects since birth, and examined IQ scores before, during and after cessation of regular cannabis use — found current “light users” and former users showed average IQ gains of 5.8 and 3.5 points, respectively, compared to an IQ gain of only 2.6 among those who had never used cannabis. (The study did, however, show an average IQ decrease of 4.1 points for heavy users.)
Marijuana is the most popular illegal drug in the world, with the United Nations estimating between 119 million and 224 million users planet-wide as of 2010. In the U.S., roughly one in four high school seniors admits recently smoking weed, making it more popular than cigarettes, the federal government reported in June.
The study drew on survey data from 1,037 people in New Zealand — everybody born in the town of Dunedin during 1973. In addition to IQ tests, they were interviewed five times between the ages of 18 and 38, including questions about their cannabis use.
The study relied on those who self-reported becoming “dependent on marijuana” at age 18; there were 52 such participants in the study. Ninety-two others reported “dependence” starting after 18.
Researchers compared their IQ scores at age 13 to the score at age 38 and found a drop only in those who had self-reported “marijuana dependency” by age 18.
“I’m not going to say that adult-onset cannabis use is safe,” demurred study author Meier. “But I would tell parents to talk to their kids and tell them to delay their onset until adulthood.”