A report published in the British medical journal “Lancet Psychiatry” suggests that marijuana use among adolescents did not increase in U.S. states where medical marijuana laws were enacted. The researchers did report, however, that overall teen use is higher in states that passed such laws than in other states.

The report is based on annual surveys conducted over a 24-year period from 1991 to 2014 of 8th, 10th and 12th grade students in about 400 schools. The study was funded by the U.S. National Institute on Drug Abuse, the Columbia University Mailman School of Public Health and the New York State Psychiatric Institute.

Because adolescent use of marijuana, and especially regular use, increases the likelihood of harmful effects, according to the study, the factors behind marijuana use bears “substantial public health importance.” The impact of medical marijuana laws is one factor that has been debated as encouraging use of the drug among teens. Some argue that medical marijuana laws send a message that using the drug is “acceptable,” and in 2013, 19% of high school seniors said they would try marijuana or use it more often if it were legal for general use. In states where medical marijuana use has been adopted, 55% of adolescents thought that such laws make it easier for teens to use the drug “for fun.”

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In the “Lancet” study, the researchers addressed two questions. First, were adolescents in states with medical marijuana laws at greater risk for marijuana use than those in other states? And second, were adolescents in states with medical marijuana laws at higher risk of use immediately after such laws were passed than they were before the laws were enacted?

Here are some of the study’s findings:

• Marijuana use in the previous 30 days was more prevalent in states that passed a medical marijuana law between 1991 and 2014 than in those that had not.
• [T]he risk of marijuana use did not significantly change after passage of a medical marijuana law.
• Among 8th graders, marijuana use decreased significantly after passage of medical marijuana laws … but no significant change was found before versus after passage in 10th or 12th graders.
• Substantial state-to-state variability was found for pre-passage versus post-passage risk of adolescent marijuana use.
• States also varied in whether the effects of medical marijuana laws differed significantly by grade.

The study concluded:

[T]he results of this study showed no evidence for an increase in adolescent marijuana use after passage of state laws permitting use of marijuana for medical purposes. Whether access to a substance for medical purposes should be determined by legislation rather than biomedical research and regulatory review is debatable.30 However, concerns that increased adolescent marijuana use is an unintended effect of state medical marijuana laws seem unfounded.

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The following chart from the study shows that use of marijuana has dropped significantly since reaching a peak in the late 1990s, and it has declined further in the past five years.

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