Concerns associated with legalizing marijuana include the idea that it could lead to more teens smoking pot, but findings of a new study involving more than 1 million teenagers revealed that the number of teens who smoke weed has not increased in states where the drug is already permitted.

For the new study published in The Lancet Psychiatry on June 15, researchers wanted to know if pot use has increased among teenagers from the time states approved medical marijuana laws until 2014 and if the passage of the laws is tied with changes to the risks of youngsters starting to use marijuana.

The researchers looked at the data of more than 1 million teenagers, who were surveyed whether or not they have used marijuana in the last 30 days in a series of school surveys from 1991 to 2014 administered to student in grades 8, 10 and 12. They found that teen marijuana use was more prevalent in states that had approved medical marijuana laws as of 2014, with almost 16 percent of the teens claiming they have used marijuana in the last 30 days compared with only 13 of their counterparts in states where medical marijuana is not permitted.

The researchers, however, found that over time there was no change in marijuana use among teenagers in 21 states that legalized medical marijuana by 2014 after the law was green-lighted, compared with before.

Around 16 percent of the teenagers said that they did not use marijuana in the last 30 days before the law was passed while 15 percent of the teens said the same after the marijuana law was passed.

"Our findings provide the strongest evidence to date that marijuana use by teenagers does not increase after a state legalizes medical marijuana," said study researcher Deborah Hasin, from Columbia University Medical Center, adding that the states with approved medical marijuana laws already have higher marijuana use among teenagers compared with other states.

The study is not the first one that shows marijuana use among teenagers did not increase after medical marijuana legalization. Two earlier studies also had the same results, albeit they were conducted in only four to five states.

The researchers noted there could be other factors that contribute to marijuana use among teenagers. 

"Adolescent use is higher in states that ever passed such a law than in other states. State-level risk factors other than medical marijuana laws could contribute to both marijuana use and the passage of medical marijuana laws, and such factors warrant investigation," Hasin and colleagues wrote in their study.

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