No, Science Has Not Proven Marijuana Lowers Your IQ

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As with so many things between 2016 and 2020, the idea that marijuana damages IQ became popular in some circles because soon-to-be-former President Donald Trump said it.

In audio secretly recorded in 2018 and leaked in early 2020, President Trump told a group gathered at a White House dinner party that marijuana "does cause an IQ problem. It lowers your IQ." 

Interestingly, his son, Donald Trump Jr., disagreed with him, saying: "I will say this, between that and alcohol, as far as I'm concerned, alcohol does much more damage. You don't see people beating their wives on marijuana. It's just different."

Research shows that the belief marijuana lowers IQ is mostly a myth. But the IQ thing stuck. In December, North Carolina Rep. David Rouzer Tweeted that the National Institutes of Health (NIH) "states that regular marijuana use can reduce IQ by 8 points." 

That sounds very authoritative. But it's not true, according to the NIH itself.

What the NIH says about marijuana 

NIH did release a report in July 2020 about marijuana, and it touched on the subject of IQ. However, it did not state that cannabis lowers IQ.

Here's what it states: "Some studies have also linked marijuana use to declines in IQ, especially when use starts in adolescence and leads to persistent cannabis use disorder into adulthood. However, not all of the studies on the link between marijuana and IQ have reached the same conclusion, and it is difficult to prove that marijuana causes a decline in IQ when multiple factors can influence the results of such studies."

Those factors, according to the NIH, include:

  • Genetics
  • Family environment
  • Age of first use
  • Frequency of use
  • Having a cannabis use disorder
  • Duration of use
  • Duration of the study

That's a far cry from stating that cannabis "does cause an IQ problem" or that it can "reduce IQ by 8 points."

PolitiFact certainly thought so. They gave Rouzer a "mostly false" rating for his Tweet. 

Details are more complex.

While it's unknown what study Trump alluded to, if any, Rouzer seems to be referring to a New Zealand study that found an IQ drop of six to eight points for heavy marijuana users. But even if that is the cause, the senator left out some salient points.

  • The loss in IQ was among people who started regular marijuana use as teens. 
  • No one in the legal cannabis industry advocates teen use of marijuana. Additionally, legal cannabis money goes to programs that work to reduce teen use as well as build better public schools.
  • Other studies have shown teen use of cannabis does not increase in places that legalize cannabis. 
  • The New Zealand study did not find any impact on IQ for adults who regularly use cannabis.

Beyond this, other studies have found no connection between cannabis and IQ. For example, the NIH notes that "no predictable difference was found between twins when one used marijuana and one did not."

A lack of context and detail can make politicians' statements mislead the public, especially in something as nuanced as brain research. That's something to keep in mind as the debate over legalization continues to expand to states around the country—and at the national level.

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