One of the biggest cannabis-harm claims of the decade just got crushed. New research refuted the six-year-old claim that cannabis can lower a person’s IQ, but what makes this takedown so epic is that it happened at the hands of the very same researcher, Madeline H. Meier. 

The drama started with Meier’s 2012 study in Proceedings of the National Academy of Sciences that claimed cannabis use in youth can permanently lower the person’s intelligence quotient, or IQ. The study came out right before Colorado and Washington voted on cannabis legalization, so every media outlet imaginable reported on the harm finding, and it ignited a firestorm.

Time magazine summarized the findings as, “Heavy marijuana use is associated with cognitive decline in about 5% of teens, according to a new study, which suggests that the heaviest users could lose 8 IQ points.” As expected, Fox News seemed giddy when it proclaimed, “Pot does lower IQ, study finds.” 

And it only got uglier from there. 

Mere months after the findings came out, the same journal published two different rebuttals that said socioeconomic status and/or personality traits better explain the correlation between childhood cannabis use and lower IQs. This prompted the original researchers to respond in what became a public spat in clinical circles. It was the Sheldon Cooper equivalent of Wrestlemania. 

Since then, Meier moved from Duke University to Arizona State University, and she led a follow-up study that included three of the original co-authors (all still with Duke). This time, the team reached a very different conclusion. 

“Short-term cannabis use in adolescence does not appear to cause IQ decline or impair executive functions, even when cannabis use reaches the level of dependence,” the researchers reported in the February 2018Addiction journal. “Family background factors explain why adolescent cannabis users perform worse on IQ and executive function tests.”

Why the reversal? The 2012 research used data from the Dunedin Study, a longitudinal study that has been following 1,037 New Zealanders since their births in 1972 and 1973. For the follow-up, the research team utilized a longitudinal study tracking twins born in England and Wales. To this point, the twins have been tested at ages five, 12 and 18, and their data allowed the researchers to compare cannabis use and IQs at ages 12 and 18 in teens with very similar genetics. This made a big difference in the findings. 

The best summary comes from London-based researchers who wrote a response to the study for Addiction: “Assertions that adolescence represents a period of vulnerability to neurocognitive harms of cannabis use are based largely on a previous, highly influential study by Meier and colleagues using the Dunedin cohort…. [In the new study], Meier et al. found no evidence that adolescent cannabis use or dependence was associated with IQ decline…. Moreover, they found no difference in IQ at age 18 and minimal differences in executive functioning between co-twins discordant for cannabis use.”

If not cannabis, what associations possibly influenced the neurocognitive changes? The findings suggest that biology, not blunts, best explains the declines and impairments. To its credit, the Meier-led team also acknowledged that the new finding is consistent with what most researchers concluded in recent studies on cannabis use and neurocognitive performance. Worth noting, Meier also led Dunedin Study-based research in 2016 that claimed cannabis consumers had equal or better physical health than non-users on all metrics except for gum health. 

Along these same lines, the Addiction journal recently published a study that compared the effects of alcohol and cannabis on the brain, and it found that alcohol, but not cannabis, negatively impacted the volume of grey and white matter. After analyzing 853 adults and 439 teens, the researcher concluded, “Alcohol use severity is associated with widespread lower gray matter volume and white matter integrity in adults, and with lower gray matter volume in adolescents…. No associations were observed between structural measures and past 30-day cannabis use in adults or adolescents.”

What do all these studies tell us? In a nutshell, cannabis consumers might not be getting dumber, but researchers appear to be getting smarter. 

Photo credit: Flickr/Marcos de Madariaga.

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