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Fewer Massachusetts high school students used marijuana in the years before the state’s first cannabis stores opened last year, according to a new report.

In 2017, about 24 percent of the state’s public high school students said they used cannabis in the previous month, down from 28 percent in 2011. Meanwhile, the share of the students reporting heavy cannabis use — at least 20 times per month — also dropped, from 9 percent to 5.6 percent.

It’s too early to know whether these trends have changed since legalization. But officials said the report, by the state Cannabis Control Commission, establishes a baseline for future study.

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“It’s going to take some time for us to realize, ‘What is the impact of legalization in Massachusetts, and how does it affect youth use and substance use disorder and the like?’ ” Commissioner Jen Flanagan said after the study was unveiled at the agency’s recent public meeting. “There’s still work to be done.”

In Washington state, cannabis use among teenagers fell after the drug was legalized in 2012, according to a December study published in the Journal of the American Medical Association. Colorado also legalized pot that year, and a study found that youth marijuana use stayed relatively flat — around 20 percent — from 2013 to 2017.

The Massachusetts Legislature required the study, among others, in its cannabis regulation law to monitor changes in how adolescents under 18 view and interact with marijuana. Studies suggest the drug, like alcohol, can harm the development of young people’s brains.

Research has shown that about 9 percent of people who use cannabis become addicted, compared to 15 percent who use alcohol.

For teenagers, especially, cannabis addiction can lead to long-lasting damage, such as poor school performance, dropping out, and other drug addictions, said Dr. Tim Wilens, chief of child psychiatry and codirector of the Center for Addiction Medicine at Massachusetts General Hospital.

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“Marijuana affects cognition — how you think, the structure of the brain, and the functioning of the brain,” Wilens said. “The more frequently you use marijuana and the earlier [in life] you use marijuana, the more likely you are to have an addiction.”

Cannabis can have some medicinal benefits for youths, but the potential harm, including a possible link to mental health problems, is serious and should be considered, Wilens said. He said research suggests cannabis can help treat epilepsy, muscle spasticity, and nausea and vomiting associated with chemotherapy.

Dr. Eric Ruby, a Taunton pediatrician who specializes in medical cannabis, said he has seen transformations in the roughly 300 young patients he has certified to use the drug.

“The benefits have to outweigh the risks,” Ruby said. “We’re talking about kids whose epilepsy could kill them, whose autism is family-dysfunctional, whose anxiety doesn’t let them go to school, whose Crohn’s disease doesn’t allow them to have an interactive social life, whose cancer and chemotherapy are not allowing them to participate in anything.”

Both Ruby and Wilens said more research is needed to truly know the harms and benefits of cannabis on young people.

Since licensing recreational cannabis businesses to open last fall, the state commission has overseen strict regulations to prevent young people from obtaining legal pot.

Among the rules: Retail customers must prove they are at least 21; companies must enter all their plants and products into a complex digital tracking system; and products must be packaged in a child-proof manner with warning labels.

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Marijuana businesses can’t be located within 500 feet of schools.

Even so, youth advocates have long worried that legalizing cannabis would send a message to teenagers that the drug is harmless, leading to more youth use.

Nationwide, teens’ perceptions of marijuana’s risk have declined since 1991, the cannabis commission report said; however, studies suggest those decreases aren’t linked to medical marijuana becoming legal.

Marijuana use among Massachusetts high school students exceeded the national average, according to the commission’s report. Nationwide, 22 percent of 12th-graders reported current cannabis use, the report said, compared to more than 30 percent in Massachusetts.

The report relied on data collected during the statewide survey of thousands of students every two years at randomly selected public schools.

The report, which also gauged alcohol use among high-schoolers, said the share of students who drank alcohol in the previous month also fell, from 40 percent to 32 percent, from 2011 to 2017.

Binge drinking was substantially more common among teens than heavy cannabis use, according to the report. While 5.6 percent of high school students in Massachusetts said they were heavy cannabis users in 2016, nearly 16 percent reported binge drinking in the prior month.

And it turns out, the report said, that youths do care what their parents think. Three other studies cited in the report found that teens who believed their parents disapproved of cannabis were less likely to use it.

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Dan Adams of the Globe staff contributed to this report. Naomi Martin can be reached at naomi.martin@globe.com. Follow her on Twitter at @NaomiMartin.