marijuana shrinks brain

Neuroscientists found ‘no evidence’ that smoking pot damages an important brain region

A new study published today in the journal JAMA Psychiatry casts doubts on the idea that marijuana causes certain regions of the brain to shrink.

In order to study the brain on pot, neuroscientists studied siblings or twins, one of whom was a pot smoker (some regularly, some less so, though none at the “problematic” level) and the other who had never touched the stuff.

If the pot-using sibling had brain changes while the other didn’t, that would imply that marijuana use had caused those differences to appear.

The researchers were specifically interested in the amygdala, because of its association with emotion and motivation and the fact that previous research had shown this area was smaller in pot users.

But, surprisingly, they didn’t find any differences between pot smokers and their abstemious siblings.

“We found no evidence for the causal influence of cannabis exposure on amygdala volume,” the authors write in the paper.

In other words, that important region of the brain would have been the same size, whether those study participants had smoked pot or not. This blows a hole in the theory that casual marijuana use damages the brain — though there are still questions to answer about marijuana’s specific effects.

What they looked at exactly

Dr. David Pagliaccio of the National Institute of Mental Health and co-authors used publicly available data from the Human Connectome Project for the study.

They found 483 participants that had a sibling or twin who had also had their brain mapped as part of the project, all between 22 and 35 years old. Of that group, 262 had used cannabis.

They didn’t find a statistically significant difference in the size of the hippocampus, another area that’s frequently discussed when talking about the effects of marijuana.

And it’s important to note that we’re not talking impaired-brain levels of smaller here — everything was within the normal human spectrum, though the consistently smaller regions are noteworthy.

It didn’t matter whether users had smoked more than 100 times (30% of the group) or had starting using pot by the time they were 17 (49%) — none of that was associated with more changes than those experienced by occasional or casual users.

But what’s really fascinating is that the pot smokers’ sober siblings showed the same smaller brain regions, even though they’d never smoked pot (people who claimed they hadn’t smoked pot but had tested positive for THC were excluded from the research).

So what does that mean?

In the case of the amygdala, the researchers write that the smaller brain region may be explained by genetic factors — their analysis didn’t show a significant contribution from environment. But, as Francesca Filbey, a researcher and associate professor in the School of Behavioral and Brain Sciences at the University of Texas at Dallas, who was not involved in the research, tells us, it’s very hard to completely control for childhood environment, which likely has a strong influence on brain development.

What they can’t explain is the smaller ventral striatum region in marijuana users and their siblings. They haven’t ruled out any combination of factors for brain differences there, but some other research has shown that region to be larger in occasional marijuana users, which they say complicates the discussion. More research will be needed to figure that out.

But this does help start to answer the question of whether pot causes brain changes or whether people who are likely to smoke pot have different brains in the first place. In the case of the amygdala, this research implies that it’s the latter.

Reasons to be cautious

Even with this step forward, we are still far from explaining all the effects that marijuana has on the brain.

An additional study published in the same issue of JAMA Psychiatry shows increasing risk for schizophrenia among young men (though not women) who are already at high risk for that mental illness and who smoke pot. But that research has the same chicken-and-egg problem as previous studies: the work can’t say whether pot causes those risks to increase, or whether people are more likely to smoke pot as their risk increases.

As Dr. David Goldman of the National Institute of Alcohol Abuse and Alcoholism writes in an accompanying editorial also published in JAMA Psychiatry, this shouldn’t be taken as an endorsement that cannabis is safe.

Pagliaccio and co-authors note that even though this is the largest study to examine the causal effects of smoking marijuana, there are still groups that weren’t included in the study that would be important to analyze.

They didn’t look at any cannabis users who had been hospitalized for two or more days for substance abuse, nor did they include anyone who had been in substance abuse treatment for a year or longer — it’s possible that they would have seen different effects among marijuana users in that group.

The fact that these users were not necessarily chronic or heavy users may have had an impact on the results, according Filbey. “It is possible that any effects of marijuana may not have been detected because the sample may not be heavy users . 100 times in a lifetime is not much,” she tells Tech Insider.

Still, those caveats noted, Filbey does say that the “study shows that marijuana did not influence any changes” in the amygdala, at least for casual pot smokers.

Additionally, the researchers only looked at a few regions of the brain: The hippocampus, right ventral striatum, and amygdala. There’s a lot more going on in the brain outside of those areas, and it’s likely that everything we do, from the food we eat to the music we listen to, has some effect somewhere. We know, for example, that alcohol use changes the brain.

A better understanding of whether or not marijuana has a significant impact on the brain is incredibly important as states around the country continue to discuss legalization of the drug.

This is the biggest study to ever look at whether marijuana causes brain changes, and it has some startling findings.

Heavy marijuana use shrinks brain parts: study

WASHINGTON (Reuters) – Long-term heavy use of marijuana may cause two important brain structures to shrink, Australian researchers said on Monday.

young man smokes marijuana before the 10th annual “Marijuana March” in downtown Toronto May 3, 2008. REUTERS/Mark Blinch

young man smokes marijuana before the 10th annual “Marijuana March” in downtown Toronto May 3, 2008. REUTERS/Mark Blinch

young man smokes marijuana before the 10th annual “Marijuana March” in downtown Toronto May 3, 2008. REUTERS/Mark Blinch

Brain scans showed the hippocampus and amygdala were smaller in men who were heavy marijuana users compared to nonusers, the researchers said. The men had smoked at least five marijuana cigarettes daily for on average 20 years.

The hippocampus regulates memory and emotion, while the amygdala plays a critical role in fear and aggression.

The study, published in the American Medical Association’s journal Archives of General Psychiatry, also found the heavy cannabis users earned lower scores than the nonusers in a verbal learning task — trying to recall a list of 15 words.

The marijuana users were more likely to exhibit mild signs of psychotic disorders, but not enough to be formally diagnosed with any such disorder, the researchers said.

“These findings challenge the widespread perception of cannabis as having limited or no harmful effects on (the) brain and behavior,” said Murat Yucel of ORYGEN Research Centre and the University of Melbourne, who led the study.

“Like with most things, some people will experience greater problems associated with cannabis use than others,” Yucel said in an e-mail. “Our findings suggest that everyone is vulnerable to potential changes in the brain, some memory problems and psychiatric symptoms if they use heavily enough and for long enough.”

Among the 15 heavy marijuana users in the study, the hippocampus volume was 12 percent less and the amygdala volume was 7 percent less than in 16 men who were not marijuana users, the researchers said.

The researchers acknowledged that the study did not prove it was the marijuana and not some other factor that triggered these brain differences. But Yucel said the findings certainly suggested marijuana was the cause.


While about half of the marijuana users reported experiencing some form of paranoia and social withdrawal, only one of the nonusers reported such symptoms, Yucel said.

The heavy marijuana users, average age 40, said they had used other illicit drugs less than 10 times, the researchers said.

A U.S. group supporting legal sales and regulation of marijuana took issue with the findings, particularly because they were based on men who were such heavy, long-term users.

“These were people who were essentially stoned all day every day for 20 years,” Marijuana Policy Project spokesman Bruce Mirken said by e-mail. “This study says nothing about moderate or occasional users, who are the vast majority — and the (study) even acknowledges this.”

“The documented damage caused by comparably heavy use of alcohol or tobacco is just off-the-charts more serious, and you don’t need high-tech scans to find it,” Mirken added.

Yucel said the researchers have begun new research on the effects of both short-term and long-term and moderate and heavy use of marijuana.

Long-term heavy use of marijuana may cause two important brain structures to shrink, Australian researchers said on Monday.