A marijuana plant flourishes under grow lights at a warehouse in Denver in October 2010. (Ed Andrieski, Associated Press file)

Study: Marijuana use alters the young brain, gray matter negatively

Current research indicates the brain doesn't reach maturity until age 25 or 30, and people should hold off heavy pot use before then, said Francesca Filbey, who co-authored the study

Heavy marijuana users had different brain shapes and lower IQs than non-smokers in a newly published study, suggesting a potential danger to young people who abuse the drug.

The research in the Proceedings of the National Academy of Sciences study published Nov. 10 used magnetic resonance imaging to measure people who used marijuana three times a day on average. The users had smaller amounts of gray matter and increased connectivity in the orbitofrontal cortex — a section associated with decision-making and response to rewards — and the changes were more pronounced in people who had started using earlier.

The study adds to a growing collection of evidence that marijuana alters the young brain, just as more parts of the U.S. are decriminalizing the drug and more young people are using it. Current research indicates the brain doesn’t reach maturity until age 25 or 30, and people should hold off heavy pot use before then, said Francesca Filbey, who co-authored the study.

The data included 48 heavy marijuana users, 28 years old on average, plus 62 non-users of the same genders and ages. The research, which controlled for alcohol and tobacco use, suggested people who used frequently had increased connectivity in their brains — possibly compensating for the effects of drug use, Filbey said.

After about five years of drug use, the increased connectivity dropped off, which could mean that the brain was no longer able to make up for the negative effects of the marijuana. The earlier someone started, the worse the result.


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“I think that it’s possible the brain can’t sustain continued compensatory mechanisms,” said Filbey, an associate professor at the School of Behavioral and Brain Sciences at the University of Texas at Dallas. “What it might really boil down to is a difficulty in stopping marijuana use. It might be what drives people to continue use, despite the negative consequences.”

Study participants who consumed marijuana also scored lower on IQ tests than non-users, though the study didn’t draw a correlation between those results and brain differences. It also couldn’t show what the participants’ brains looked like before they started using marijuana, and more studies are needed that can capture that information, the authors said.

And the study’s focus on habitual users doesn’t address whether more limited use has any effect. Alcohol, which is legal across the U.S., can also harm young people during this time period, data suggest.

Still, similar findings linking pot use to poorer performance on memory and IQ tests have caused researchers to sound a note of caution even as the stigma of smoking pot diminishes.

“We just had our first baby this past summer,” said Matthew Smith, a research assistant professor at Northwestern University who has studied marijuana and the brain. “I would tell her if you’re curious about using it, delay until your early 20s.”

Marijuana use has grown in the past few years, including among young people. The trend may partly be fueled by the changes in enforcement against the drug’s use, said Susan Weiss, associate director for scientific affairs at the National Institute on Drug Abuse, which funded the UT Dallas study.


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New York City Mayor Bill de Blasio said this week he would direct police to reduce arrests for small amounts of pot by issuing tickets instead. Alaska, Oregon and District of Columbia voters opted to legalize it in the Nov. 4 election, joining Washington and Colorado, while 23 other states allow medical use of the drug.

About 6.5 percent of U.S. 12th graders say they use pot daily, according to NIDA, which is part of the National Institutes of Health. More than 36 percent of 12th graders say they’ve used marijuana in the last year.

The share of kids getting high has ticked up for several years, though it’s lower than it was in the 1980s, according to Weiss.

“When perceptions that it’s risky goes up, use goes down,” she said. “That’s the perspective we have which makes us concerned about all of these changes in policy about other messages going out.”

Scientists have known about evidence of marijuana’s effects on the brain for some time. One study, published in 2012 in Proceedings of the National Academy of Sciences, measured people at ages 13 and again at 38. It suggested that people who used marijuana regularly at a young age had a drop in IQ that could be seen later into life.

Researchers have debated whether that study controlled correctly for the socioeconomic backgrounds of its participants, and there is little other research measuring marijuana users both before and after they started on the drug.

NIDA is working on funding more examinations of young people and marijuana and carrying out a larger-scale study that looks at people before and after they start using marijuana, to determine whether pot is causing brain changes, Weiss said.


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Meanwhile, other research has suggested that young marijuana users’ brains look differently than non-smokers’. Smith, of Northwestern, co-authored a study where researchers gave basic memory tests and scanned the brains of people who used pot heavily for about 2 1/2 years starting around age 17 and then stopped, comparing them with people who never used marijuana.

The scans, which took place when the people were 25 on average, showed pot users had different brain shapes than non- users. The former users also performed worse on memory tests than those who remained clean.

Smith said the data suggested marijuana could hurt adolescents, though he acknowledged that the changes could have other sources. It’s also unclear whether the brain differences were caused by pot use or whether they indicated the person was genetically predisposed to abuse marijuana, he said.

“Most recent studies show that there’s a difference in the brain related to having used marijuana,” he said. “The caveat is no one’s showing change over time. If you do a study, you don’t know whether it’s cause or effect.”

Still, the effects seem to be more pronounced in people who consume more and earlier.

Marijuana advocates argue that the drug should be legalized because users don’t represent a dangers to others and shouldn’t clog the court system. And even in the four states where it’s legal to smoke, younger users aren’t supposed to, with a minimum age of 21 for recreational users to legally buy pot.


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Colorado Gov. John Hickenlooper, D, has routinely said he’s concerned about teen marijuana use and its impacts on brain development. In his budget request for the 2016 fiscal year, Hickenlooper asked the state legislature to dedicate about $6.7 million from taxes the state collects on marijuana sales for programs geared toward preventing youth marijuana use.

Complicating research is the fact that marijuana products are changing. Modern-day pot has a higher concentration of its active ingredient, THC, than its predecessors, according to researchers. THC is the component in cannabis thought to cause pot’s psychoactive effects.

On average, cannabis today is almost 13 percent THC, compared with almost 4 percent in 1995, according to data from Mahmoud ElSohly, a research professor at the University of Mississippi who examines samples confiscated by law enforcement.

Researchers don’t know what that means for young users or whether people are ingesting more marijuana or using less to compensate for the increased potency, Weiss said. The rising popularity of edible marijuana, such as pot brownies, also represents a risk, she said.

“It’s very hard to know how much you’re taking and it’s easy to overdose,” she said.

With assistance from Jennifer Oldham in Denver and Alison Vekshin in San Francisco