If you start smoking marijuana as an adult, how much cannabis is too much? 

“Can the recreational use of marijuana cause cognitive impairment? The most obvious answer is ‘yes’—after all, this is the basic reason for its recreational use.” As I discuss in my video Does Marijuana Cause Permanent Brain Damage in Adults?, “people clearly do not use cannabis only for its harms.” Boosting creativity, for example, is one reason people give for smoking it, but does cannabis have “the potential to enhance their creativity”? You don’t know, until you put it to the test. Researchers looked at divergent thinking, the ability to brainstorm solutions to problems. At a typical dose people might use to get high, their creativity took a hit. So, the relationship between cannabis and elevated creativity may just be an illusion. People may think they’re more creative when they’re high, but it “may not be the best choice when in need of breaking the ‘writer’s block,’ or overcoming other artistic inhibitions,” for example, and “might actually be counter-productive.”

As you can see at 0:55 in my video, one’s learning, memory, and attention may also be impaired for a few hours after smoking, but does it cause any lasting problems? In other words, “is cannabis neurotoxic for the healthy brain?” Researchers have found that cannabis users, compared to non-users, have a significantly smaller hippocampus, the memory center in the brain. Okay, but a snapshot-in-time study can never prove cause and effect. Only by following people over time can you see which came first. 

When researchers did just that, they found 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 to be “a consequence of chronic cannabis exposure.” Is it permanent? 

A famous study was 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. “Hippocampal volume correlated with 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 “reduced hippocampal volume,” that is, shrinkage, but what about years later? 

Researchers found that while “hippocampal volume is reduced in long-term cannabis users,…this atrophy can be restored following prolonged abstinence.” Even after 15 years of using cannabis, the size of their hippocampus appeared to bounce back just 29 months after quitting. And, it’s the same with cognitive impairments. They’re gone within a month or two after stopping, unless, however, regular cannabis use started as a teen.

As you can see at 2:57 in my video, those with the most persistent cannabis use starting as an adolescent may lose up to 8 IQ points, which is a significantly greater loss than if they had started as an adult. What’s more, even if they then quit, starting at such a young age appears to cause permanent brain damage. However, to get that lasting damage, “both adolescent onset and almost 2 decades of persistent cannabis use may be required.  

It sounds as if there may not be any irreversible neurological problems if cannabis use begins in adulthood—that is, unless you smoke perhaps 16 joints a day. In 2017, a study was published on extreme “chronic cannabis abuse in heavy doses.” Researchers found that the subjects’ brains really did seem to go to pot: “long-lasting brain dysfunction in all users and…long-lasting schizophrenia-like psychotic symptoms in more than half of all users,” such as hallucinations and delusions. They also found memory problems and even difficulty drawing basic figures, for instance. However, keep in mind this was at ten times the average daily dose in Colorado, for example, a total lifetime consumption of around 75,000 joints.

What if you start using cannabis before age 25? See my video Does Marijuana Cause Permanent Brain Damage in Teens?.


Recreational marijuana use is often to boost creativity, but researchers looked at divergent thinking, the ability to brainstorm solutions to problems, and found that cannabis negatively impacted creativity.
After smoking marijuana, one’s learning, memory, and attention may also be impaired for a few hours.
Researchers found that cannabis users have a significantly smaller hippocampus (the brain’s memory center) compared with non-users and that 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. As well, the shrunken hippocampus in marijuana users seems to be “a consequence of chronic cannabis exposure.”
Hippocampal volume is not static, suggesting the brain’s structure is in constant flux.
Researchers found that, although there was still reduced hippocampal volume (shrinkage) after quitting marijuana use for six months, the atrophy can be restored following prolonged abstinence.
Cognitive impairments also appear to be restored after stopping cannabis use unless regular usage began during adolescence.
Persistent cannabis use starting as a teen may cause permanent brain damage.

To see all of the videos I’ve produced on cannabis, check out the marijuana topic page.

Want an herb that can help your brain? See Flashback Friday: 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  Does Cell Phone Radiation Cause Cancer? and Cell Phone Brain Tumor Risk?. You may also be interested in another video in that series: Do Mobile Phones Affect Brain Function?

In health,

Michael Greger, M.D.

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