Frequent marijuana use before age 18 tied to drop in IQ

August 27, 2012–A new study reinforces the need to protect adolescents from a commercial marijuana industry that will come after them, like the tobacco and alcohol industries do, if marijuana is legalized.

The study, reported today by the Associated Press, involved more than 1,000 people in New Zealand who were interviewed and tested several times, beginning at age 13 and ending at age 38.  Those who used marijuana persistently by age 18 experienced an average IQ drop of 8 points at age 38.  Reducing use or quitting did not fully restore the drop in IQ.  Those using marijuana frequently after age 18 did not experience a drop in IQ.

The study was published online today by the Proceedings of the National Academy of Sciences.



  • By mladyd, September 2, 2012 @ 6:49 am

    Surely the creators of this site are familiar with the “think of the children” meme and how those words are a joke to anyone who has been paying attention. If this campaign is to be taken seriously you are going to need a serious name. Or maybe you are trying to be charmingly ironic or sarcastic? Hard to tell. Surely the creators also know that children are able to get pot now very easily since nobody checks their ID.

  • By Joshua Schreiber, October 5, 2012 @ 2:21 am

    I was tested for IQ as a child, and received a 133 for my IQ. I started using marijuana at 15 or 16 years of age. Not only did my IQ not go down, it increased by 16 points elevating me to the documented level of genius (upon my most recent IQ test results). Individuals who are exposed to Mexican schwag will most probably experience a dip in IQ, due to the low THC and medium levels of CBD. Once individuals experience high THC cannabis, there is a positive impact on IQ, not a negative one. This is my personal field study now 19 years in the making. Once individuals discover 20% THC cannabis, intellect goes up, as does creativity and free thought. Your statements of the negative effects of cannabis are entirely unfounded. Yes, I have two children, 7 and 10. My eldest is in the top 1% of the entire nation in intellect, based upon her standardized test scores. And she understand my plight.

  • By Kevin, November 21, 2012 @ 6:23 pm

    You need to seriously re-think your concern for children if you think marijuana letalization would lead to it being targeted at children and make it easy for them to obtain. It’s already way more readily available to teens because the drug dealers don’t ask for ID. At least if it’s being sold in stores and not on the street you can enforce age restrictions and gain taxation to fund education about drugs for children. Teens will ALWAYS get ahold of drugs, they’ve never had a problem getting alcohol, tabacco, marijuana, heroine etc etc and they will after it’s legalized. The difference is the number of children that get exposed due to the ability of accessing it from a dealer, who doesn’t ask for ID.

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