Cannabis And IQ: What The Science Actually Shows

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Does consuming marijuana actually make you stupid? That’s the Reefer Madness argument made by a doctor in an attempt to prevent Oklahomans from voting to legalize cannabis.

Dr. Harold Urschel, chief medical strategist for EnterHealth, a Dallas-based addiction treatment program, said this on KOCO News 5:

The better high you get, the more addicting it is, but also the more you get into your brain, the more destructive it is. It changes how your mood states, messes with your sleep. For teenagers, it decreases your IQ by eight points, which is a significant drop. It causes significant risk of heart attacks, lung cancer. Doubles your risk of stroke.

We see a lot of the devastation caused by marijuana and other drugs as well. Probably 20 percent of our patients are addicted to marijuana. For the couple of hours you’re feeling the high, you do feel better. But then it wears off, and the injury it does to your brain makes the symptoms two or three times worse the more you need marijuana again.

That’s what’s called a “hot take,” in the world of “shout TV.” But the doctor’s statements, especially when it comes to IQ, are simply not rooted in available science.

Here are just a few studies that dispel Urschel’s warning:
In a study out of the United Kingdom, researchers concluded, “The notion that cannabis use itself is causally related to lower IQ and poorer educational performance was not supported in this large teenage sample.”

Data published in the Proceedings of the National Academy of Sciences led researchers to conclude: “In the largest longitudinal examination of marijuana use and IQ change, … we find little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline. … [T]he lack of a dose-response relationship, and an absence of meaningful differences between discordant siblings lead us to conclude that the deficits observed in marijuana users are attributable to confounding factors that influence both substance initiation and IQ rather than a neurotoxic effect of marijuana.”

Researchers published in the journal Addiction wrote: “[W]e found that youth who used cannabis … had lower IQ at age 18, but there was little evidence that cannabis use was associated with IQ decline from age 12 to 18. Moreover, although cannabis use was associated with lower IQ and poorer executive functions at age 18, these associations were generally not apparent within pairs of twins from the same family, suggesting that family background factors explain why adolescents who use cannabis perform worse on IQ and executive function tests.” Investigators concluded, “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.”

These findings are consistent with those of several other studies – including those here, finding that cannabis use alone during adolescence does not appear to have a significant, direct adverse effect on intelligence quotient.