The Dirty Dozen #2: How Does Marijuana Affect IQ?

By Laura Stack

Welcome to the Marijuana Dirty Dozen article #2, where we review the second of twelve common impacts of marijuana use on adolescents: it decreases IQ. To put it simply, it makes you dumber. Johnny was an ace in math (he scored a perfect 800/800 on the math portion of the SAT as a junior in high school). By the summer following graduation from high school, he could no longer do the higher math. He forgot how to do calculus. He told me he thought he had a brain tumor and needed an MRI. He described his brain as “green poo.” It was heartbreaking to see his IQ fall and that beautiful brain of his ruined from his marijuana use.

It’s no wonder. In “Adverse Effects of Cannabis on Adolescent Brain Development: A Longitudinal Study,” researchers found “high amounts of cannabis use during the 18-month interval predicted lower intelligence quotient and slower cognitive function measured at follow-up. These data provide compelling longitudinal evidence suggesting that repeated exposure to cannabis during adolescence may have detrimental effects on brain resting functional connectivity, intelligence, and cognitive function.”

Based on IQ tests performed by mental health professionals on large cohorts of individuals at different points in their lives, it’s a fact that heavy marijuana users permanently lose between 2-8 IQ points between childhood and adulthood when they start using marijuana as adolescents. Adult initiation of marijuana use doesn’t have the same serious effect on IQ. Think about that. It’s obvious that cannabis is very toxic to developing young brains.

Eight points may not seem like much, but it works out to 8-10% of the average American’s IQ. Losing eight IQ points puts you in a different intelligence bracket. If you got an A, now it’s a C. This fits with the whole slow, goofy image we all have of stoners, though users argue that’s a not fair characterization, since it’s based on how they act when stoned, not when they’re sober.

No one really questions the fact that heavy users who begin using as adolescents lose IQ points as they age. What they question is why. Does marijuana use cause the observed IQ-drop, or is the IQ-drop due to the adoption of a high-risk, brain-damaging lifestyle that also encourages marijuana use? Or could it be more of a chicken-and-egg situation, where both factors contribute and feedback on each other? In the research, there is no way to invalidate the results, because there is nothing else present that would cause the demonstrated IQ-drops except marijuana.

It’s because marijuana damages the developing brain. In “A Population-Based Analysis of the Relationship Between Substance Use and Adolescent Cognitive Development,” researchers concluded, “Common vulnerability effects were detected for cannabis and alcohol on all domains. Cannabis use, but not alcohol consumption, showed lagged (neurotoxic) effects on inhibitory control and working memory and concurrent effects on delayed memory recall and perceptual reasoning (with some evidence of developmental sensitivity). Cannabis effects were independent of any alcohol effects. Conclusions: Beyond the role of cognition in vulnerability to substance use, the concurrent and lasting effects of adolescent cannabis use can be observed on important cognitive functions and appear to be more pronounced than those observed for alcohol.”

Marijuana is a part of “everything that comes with it,” often the most significant part. Using marijuana, whether occasionally or daily, is a conscious choice. Whether it’s genetics, mental problems, false messaging that makes marijuana use seem harmless, or peer pressure to get high that triggers marijuana use and the attendant lifestyle, youth who choose to use are harming their brains.

Now, of course not all correlations are significant. Just because you always see firetrucks at building fires doesn’t mean firetrucks cause the fires. In this case, however, all the other data about the negative effects of marijuana on the human brain and body make this conclusion a slam dunk.

Science confirms this without question. In “Persistent cannabis users show neuropsychological decline from childhood to midlife,” study members with more persistent cannabis use showed greater IQ decline. In this study, each study member served as his or her own control; given that the groups were not equivalent on childhood IQ. They accounted for premorbid IQ differences by looking at IQ change from childhood to age 38 years. Study members who never used cannabis experienced a slight increase in IQ, whereas those who diagnosed with cannabis dependence at one, two, or three or more study waves experienced IQ declines of −0.11, −0.17, and −0.38 SD units, respectively. An IQ decline of −0.38 SD units corresponds to a loss of ∼6 IQ points, from 99.68 to 93.93. Results of analyses for persistent cannabis dependence and persistent regular cannabis use were similar.”

We have hit the tipping point for the marijuana/IQ decline evidence. It’s time for marijuana fans, both in the scientific community and in the general public, to admit long-term marijuana use starting in adolescence damages mental acuity.

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