Does Marijuana Cause Permanent Brain Damage in Teens?

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

“Since the commercialization of marijuana in Colorado, its use among adolescents and young adults has increased significantly.” Up to a 50% increase reported in a single year. And, according to the American Academy of Pediatrics: “Marijuana is not a benign drug for teens. The teen brain is still developing, and marijuana may cause abnormal brain development”—which is why they and the Academy of Child and Adolescent Psychiatry have “officially oppose[d]…legalization.” “Whereas adult users appear comparatively immune to cannabis-induced [long-term changes in brain function and structure], the same cannot be said of [those starting] during their early teens, when effects are both more severe and more long-lasting.” During puberty, parts of the brain are actually reorganizing themselves, making this a “vulnerable” period. Remarkably, “the brain does not complete development until approximately age 25.”

Okay, but is this just Reefer Madness revisited? Show me the data. Yeah, “[s]tudies of long-term, heavy…users [tend to] show [they] perform worse on [various] tests.” But, how do we know they weren’t that way before they even started using? Then, of course, here you are demonstrating they have memory impairments, and yet, you’re relying on their answers in terms of when they started and how much they’ve smoked over the years. So, what you need are “prospective, longitudinal investigation[s]”—meaning following kids over time, before and after, to see what’s really going on.

Even better, you might think, would be a randomized, controlled trial where you force half the kids to smoke. And, “even if [that] were ethical, it could merely show that cannabis has [the] potential…to impair cognition.” Only a prospective, longitudinal study can really get at whether it’s actually impairing brain function in the real world, and how much.

This was the first study ever published. About a hundred young adults, “assessed since infancy,” and after controlling for other factors like alcohol use, and their brain function before they started smoking, the bad news is that they did find that “regular heavy users did [do] significantly worse…in [terms of] overall IQ, processing speed,…and…memory.” But, the good news was that the effects seemed to be temporary. The brains of those who smoked heavily but then stopped appeared to start functioning normally again after like three months. So, yeah, if you’re in school, of course, you want to function at your best. But at least there’s no permanent brain damage—or so we thought. The average use of these former smokers was only about two years; they were testing them when they were about 18 years old.

In this study, they didn’t just follow a hundred kids, but a thousand, from birth all the way to age 38. What did they find? They found that same decline in brain function, confirmed by reports of trusted friends and family, especially among those who started younger. But, here’s the kicker: “…cessation of cannabis use did not fully restore [brain] function…among [those who started in their teens, even if they subsequently quit].” So, this suggests a true, long-standing “neurotoxic effect…on the adolescent brain,” which justifies why public health authorities are so concerned.

And, it was a “global” decline in mental function, “across [all five tested] domains:” executive function, memory, processing speed, perceptual reasoning, and verbal comprehension, consistent with the thought that there’s a critical brain development window that you just don’t want to mess with. The decline in IQ of about six points is the kind of brain damage you see with low-level lead exposure, both of which “are potentially preventable.” But how?

Do we need more DARE, Drug Abuse Resistance Education? Maybe. If it wasn’t a complete failure. No beneficial effects in terms of changing “drug use or [even] attitudes towards drug use,” which appear to be getting more permissive over time, combined with earlier ages of initiation. So: “Increasing efforts should be directed toward delaying the onset of cannabis use by young people,” at least until adulthood.

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Image credit: Rafael Castillo via flickr. Image has been modified.

Motion graphics by Avocado Video.

• brain health
• cannabis
• children
• cognition
• marijuana
• memory
• neurotoxins
• puberty
• smoking