The Washington Post

Reality Check: Does Pot Cause Teens to Lose 8 IQ Points?

“I think for states to go the full legalization route is a problem, precisely for my ‘mommy reason,’” confessed Ruth Marcus, a columnist for the Washington Post, on this week’s Meet the Press.

Ruth told the panel decriminalizing pot, or making it legal for public consumption--even with age restrictions--will make it easier for kids to get their hands on.

And that, Ruth concluded, would be extremely problematic.

“It is a vast social experiment, and we do not know the outcome except that the best evidence is that, if you use marijuana as a teenager regularly, you lose eight IQ points. And I don’t know about the rest of the table, but I don’t have eight to lose!”

Is this true, that smoking pot "regularly" as a teen means you’ll sacrifice eight IQ points?

The answer is yes, it’s true in the most extreme cases, but rather unlikely for most members of the general population.

Marcus was likely referencing a 2012 Duke University study that looked at a population of more than 1,000 people in New Zealand, over the course of several decades, tracking the participants from the age of 13 until 38.

The researchers recorded pot usage in five separate “waves,” at the ages of 18,21,26,32 and 38. Then, they compared brain testing from before the study began with testing after the individuals turned 38.

You can read the results for yourself here:

The major points to take away, however, include the following:

  • People who never smoked saw a slightly higher IQ at the end of the study
  • Individuals who reported smoking, but not ‘regularly,’ saw *on average* a 1-point IQ drop
  • Individuals who reported smoked regularly [as defined by 4 times a week or more], saw on average an IQ drop between three and six points, depending upon how long the smoking occurred
  • That eight-point IQ decline is only applicable in “the most persistent adolescent-onset cannabis users."

So, who exactly are the folks who qualify as the most persistent adolescent-onset cannabis users?

“The very earliest, heaviest users, starting at age 13 and using marijuana very regularly,” explained Dr. Timmen Cermak, an addiction psychiatrist and author of the book, "Marijuana, What’s a Parent to Believe?"

Cermak explained more sweepingly, “Clearly what this [study] shows is that the people we need to be worried about the most are the ones that start the youngest and smoke the heaviest, during their teenage years.”

Bottom line--if you smoke pot early and often as an adolescent, it can really affect your brain development.

But eight IQ points? That’s pretty unlikely.

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