Does Marijuana Cause Permanent Brain Damage in Adults?


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.

“Can the recreational use of marijuana cause cognitive impairment?” Uh, yeah—that’s kind of the whole point. “[P]eople clearly do not use cannabis only for its harms.” Like, what about boosting creativity? That’s one of the reasons people smoke it. But, you don’t know, until you put it to the test. They looked at “divergent thinking”—the ability to brainstorm creative solutions to problems—and, at a dose people might typically use to get high, their creativity took a hit, too. So, it may just “be an illusion.” People think they’re more creative when they’re high, but it may not be the best strategy, and even turn out to be “counter-productive.”

For a few hours after smoking, one’s learning, memory, and attention may also be impaired. But, the question is: Does it cause any lasting problems? In other words: “Is cannabis neurotoxic for [a] healthy brain?” Researchers have found that cannabis users have a significantly “smaller hippocampus [the memory center in the brain]…compared to non-users.” Yeah, but a snapshot-in-time study can never prove cause and effect. What you have to do is follow people over time. Only then can you see which came first.

And, what they found was, both—there are pre-existing “structural abnormalities” in the parts of the brain that control inhibitions and decision-making that may make someone more likely to take up the drug. But, the shrunken hippocampus does seem “a consequence of chronic cannabis exposure.” Okay, but is it permanent?

There was a famous study published about pre-GPS London taxi drivers who spent literally years learning and memorizing how to navigate around the city, and they had hefty hippocampuses to prove it—”correlat[ing to] the amount of time spent as a taxi driver,” suggesting the structure of the brain is in constant flux. So, if you stop using marijuana, does your hippocampus grow back to full size? Researchers tested users six months after quitting, and still found shrinkage. But, what about years later? We didn’t know, until now.

Yeah, “hippocampal volume is reduced in long-term cannabis users.” But, “this atrophy can be restored following prolonged abstinence.” Even after 15 years of use, 29 months after quitting, the size of their hippocampus appeared to bounce back. And, the same with cognitive impairments—gone within a month or two after stopping, unless they started regularly using as a teen.

Those with the most persistent cannabis use starting as an adolescent may end up losing up to eight IQ points—significantly more than if they started as an adult, and even if they then quit, starting that young appears to cause permanent brain damage. But, to get that lasting damage may require “[b]oth adolescent onset and almost 2 decades of persistent…use.”

Sounds like if you start using as an adult, though, there don’t seem to be any irreversible neurological problems—unless, perhaps, you smoke like 16 joints a day. In 2017, a study was published on extreme chronic and heavy cannabis use, and their poor brains really did seem to go to pot. “[L]ong-lasting brain dysfunction…in more than half,” and even long-lasting “psychotic symptoms:” hallucinations, delusions, and, not just memory problems, but like difficulty drawing basic figures. But again, this was at 10 times the average daily dose in Colorado—for example, a total lifetime consumption of around 75,000 joints.


What if you start before age 25? See my last video: Does Marijuana Cause Permanent Brain Damage in Teens?

All my upcoming cannabis videos can be streamed right now for a donation to the 501c3 nonprofit that runs

Want a herb that can help your brain? Check out Benefits of Rosemary for Brain Function and  Best Aromatherapy Herb for Alzheimer’s.

The cannabis issue reminds me of a similar clash of politics and commercial interests in the cell phone debate. If you’re interested, check out my videos Does Cell Phone Radiation Cause Cancer? and Cell Phone Brain Tumor Risk? I continue that video series with the next installment, coming up: Do Mobile Phones Affect Brain Function?


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