(CNN) Marijuana use among young people in the United States overall has climbed in recent years, but a new paper suggests that in states where recreational marijuana has been legalized, marijuana use among youth may actually be falling.

Laws that legalized recreational marijuana were associated with an 8% drop in the number of high schoolers who said they used marijuana in the last 30 days, and a 9% drop in the number who said they'd used at least 10 times in the last 30 days, according to the paper published in the medical journal JAMA Pediatrics on Monday.

"Just to be clear we found no effect on teen use following legalization for medical purposes, but evidence of a possible reduction in use following legalization for recreational purposes," said Mark Anderson, an associate professor at Montana State University in Bozeman, Montana, who was first author of the paper.

"Because our study is based on more policy variation than prior work, we view our estimates as the most credible to date in the literature," he said.

The paper involved analyzing data, from 1993 to 2017, on about 1.4 million high school students in the United States from the US Centers for Disease Control and Prevention's annual national Youth Risk Behavior Surveys

The researchers took a close look at self-reported marijuana use in the surveys among the students as well as survey responses in areas where medical or recreational marijuana was legalized.

The researchers examined the responses before and after the marijuana laws were implemented.

The data showed that marijuana use among high schoolers was not statistically associated with medical marijuana laws, but there was a link with recreational marijuana laws.

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The paper had some limitations, including that only an association was found in the study -- not a causal relationship -- and more research is needed to determine why this association exists.

"Because many recreational marijuana laws have been passed so recently, we do observe limited post-treatment data for some of these states," Anderson said. "In a few years, it would make sense to update our estimates as more data become available."

The new study appears to contradict some separate state-level studies that suggest marijuana use among teens remains unchanged -- instead of declining -- following legalization, said Bonnie Halpern-Felsher, professor of pediatrics in Stanford University's Division of Adolescent Medicine in Palo Alto, California, who was not involved in the new paper.

When it comes to the new paper, "I think the big question is why," Halpern-Felsher said. "Why are they seeing in this national dataset decreases -- pretty significant decreases -- when other studies are finding no difference?"

The researchers wrote in the paper that one possible explanation could be that in states where recreational marijuana is legal, "it is more difficult for teenagers to obtain marijuana as drug dealers are replaced by licensed dispensaries that require proof of age."

Halpern-Felsher called that "a good point."

"Maybe now because of having legalization, you don't have the street sales anymore," she said. "So dispensaries, we would hope, would be better at carding and checking for age verification."

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Meanwhile on the national front, the overall prevalence of marijuana-only use among youth in the United States since the early 1990s increased from 0.6% in 1991 to 6.3% in 2017, according to a study published in the American Journal of Public Health on Wednesday.

"The other question is, are youth getting the message about the fact that using marijuana during adolescence is more harmful because of their brain development?" Halpern-Felsher said. "Given the legalization, we need more education around marijuana or cannabis use for youth and we don't really have a lot of education."