Marijuana and the Brain:
What Do We Know?

Massachusetts' first recreational marijuana shops still up in the air, but Worcester will play a central role. Photo courtesy of Creative Commons.

By Sarah Toy
Boston University Statehouse Program

Versions of this article were published in the Worcester Telegram & GazetteLowell Sun, and Sentinel & Enterprise.

BOSTON — Amy Turncliff, a mother of three, is concerned.

Her children are 10, 8 and 4 years old. In November, voters approved legislation that made recreational marijuana legal in Massachusetts. However, there is still a lot of legislation coming down the pipeline.

Turncliff, a neuroscientist who lives in Ashland, thinks a lot is at stake. She and her husband are worried their children will grow up in a culture where “marijuana use is treated like alcohol use and is believed to be a rite of passage.”

“I’m not even sure 21 is the right cut-off age,” she said, referring to the current legal age for owning, using, and home-growing marijuana. She thinks it should be higher.

As Massachusetts lawmakers prepare to debate possible changes to the existing recreational marijuana law and related public health measures, Turncliff and others are considering the question: What exactly are the effects of marijuana on the developing brain?

The answer, according to Dr. Marie McCormick of the Harvard School of Public Health, is not so simple. She and a committee of researchers and physician-scientists at the National Academies of Sciences, Engineering and Medicine analyzed over 10,000 studies to determine marijuana’s effects on health. The committee issued a report on their findings in January.

It was difficult to come to any hard conclusions, said McCormick, who served as the committee’s chair and is a professor of pediatrics at Harvard Medical School.

“One of the issues is that there is virtually no data on any forms of cannabis other than smoked cannabis,” she said. “Smoking is no good for you, no matter what you’re smoking.”

She also noted the problem of confounding factors. “It was extremely difficult to tease out the effects of cannabis alone versus everything else people were doing.”

The report did find evidence of a link between cannabis use and the development of substance dependence or abuse problems with alcohol, tobacco and illicit drugs. It also found that beginning cannabis use at a younger age was a risk factor for developing dependency later on in life.

Dr. Staci Gruber, the director of the Marijuana Investigations for Neuroscientific Discovery (MIND) program at McLean Hospital and an associate professor of psychiatry at Harvard Medical School, explained that the brain is still developing during adolescence.

“The brain doesn’t reach maturity until well into the second decade or even the third decade of life, depending on whose research you’re reading,” she said. “When the brain is at a point of neurodevelopmental vulnerability, there are a lot of things that can affect the way development continues.”

Some studies, including one of Gruber’s, have shown that adolescent marijuana users exhibit changes in the white matter of the brain, which is made up of millions of nerve fibers, or axons, that allow for communication between brain regions. She also found that those who started using earlier showed less organized white matter than those who started later.

“That’s really important because in the younger-onset users, lower white matter organization was also related to higher behavioral impulsivity,” she said.

She has also done research with heavy cannabis users — adults who use cannabis at least four or five times a week. She found that study participants who began using at an earlier age tended to do more poorly on tasks involving the brain’s executive functions — things like planning, reasoning, behavioral control and problem solving — than those who began using later in life, after age 16, and those who didn’t smoke marijuana at all.

To her, it makes sense. “The brain develops from back to front, bottom to top,” said Gruber. “These are tasks that are mediated by the frontal part of the brain, which is the part that comes online last.”

Gruber thinks these findings have public health implications, but doesn’t believe in messages of abstinence when it comes to marijuana.

“Parents and teachers want there to be a message that this is an inevitability — if you begin to use, you will become addicted,” she said. “Kids are smart. They know that’s not true.”

Rather, she is an advocate for education and open discussion.

“It’s important to be honest and open,” she said. “You can have honest dialogue with emerging adults and say here’s the deal — we see some changes in the brain early on, so it’s better to wait until you’re about this age.”

She likes to tell people: “We aren’t saying never. We’re just saying not yet.”

Sen. Jason Lewis, D-Winchester, and Rep. Hannah Kane, R-Shrewsbury, have both filed bills this session they say will address concerns surrounding marijuana’s effect on the developing brain, along with other public health concerns.

Lewis and Kane’s bills propose several changes to the current marijuana law: Dropping the home possession and home grow limits on marijuana; delaying the manufacture of edibles and concentrates another two years; allowing city council members to approve a ban on local marijuana sales without holding a special election; and creating a five-member independent commission modeled after the gaming commission to regulate and supervise the industry.

The bills have come under fire from legalization proponents. Jim Borghesani, the spokesman for the Marijuana Policy Project, has said that any changes to the law as it was passed in November would “violate the will of the voters.”

Lewis, however, thinks public health and safety is the bigger issue at hand.

“What I don’t want to see happen is us trading a criminal justice problem . . .  for a public health problem,” said Lewis. “The legislation I’ve filed is intended to essentially strengthen public health safeguards as we go about creating this new pot industry in Massachusetts.”

Lewis and Kane have also filed bills looking to restrict the potency of THC in cannabis products, create a research program looking at the social, economic and public health impacts of marijuana legalization in Massachusetts and fund public education campaigns directed at youth.

Fortunately, public education is something everyone can get behind, including long-established foes.

“We are all for public education regarding marijuana,” said Borghesani. “The Legislature should have done this a long time ago.”


Material from the State House News Service was used in this report.

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