Researchers at multiple institutions analyzed data from the Monitoring the Future study, which conducts annual national surveys of students in 8th, 10th, and 12th grade (modal ages 13–14, 15–16, and 17–18 years, respectively), in around 400 schools per year. Students complete self-administered questionnaires that include questions on marijuana use.
The researchers analyzed data from 1,098,270 adolescents surveyed between 1991 and 2014. The primary outcome of this analysis was any marijuana use in the previous 30 days, and they used statistical modelling to examine two questions: whether marijuana use was higher overall in states that ever passed a medical marijuana law up to 2014 and whether the risk of marijuana use changed after passage of medical marijuana laws.
They found that marijuana use was more prevalent in states that passed a medical marijuana law any time up to 2014 than in other states, but the risk of marijuana use in states before passing medical marijuana laws did not differ significantly from the risk after medical marijuana laws were passed.
“Our findings, consistent with previous evidence, suggest that passage of state medical marijuana laws does not increase adolescent use of marijuana,” the researchers state. “However, overall, 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.”
The effect of such laws on teenage marijuana use has been hotly debated, and the study received national media attention, including coverage by the New York Times. But at least one expert on substance use disorders who reviewed the report for Psychiatric News said the results should be viewed cautiously.
“Federal policy regarding medicinal marijuana did not relax until mid- 1999, and data from before that period only informs us about laws that were not particularly active, due to federal prohibitions,” said Christian Hopfer, M.D., a professor of psychiatry in the division of substance dependence at the University of Colorado, Denver. “The commercialization of marijuana and subsequent industrial growth is a recent phenomenon, and experiences from Holland’s depenalization demonstrated that it took a number of years for the full effect of depenalization and subsequent commercialization on adolescent marijuana use to be seen. Nationally, adolescent marijuana use has been on the increase over the past decade, and it is unclear to what extent news regarding marijuana’s medicinal effects and legalization has played a role in the national rise—that is, the effects of marijuana medicinal legalization may not be felt state-by-state, but on a national level through decreasing the perceived
For more information, see the Psychiatric News article "Expert Emphasizes Importance of Science in Debate Over Legalization of Marijuana."