On Monday, The Lancet Psychiatry journal published a study that analyzed 24 years' worth of data from the 48 states in the continental US in an effort to sort out whether making medical weed legal increases the likelihood of kids getting high. The conclusion? "Our findings, consistent with previous evidence, suggest that passage of state medical marijuana laws does not increase adolescent use of marijuana."
The study was conducted by Columbia University's Deborah Hasin, an epidemiology professor. She and her colleagues looked at statistics on eighth, tenth, and twelfth graders' use of marijuana from 1991 until 2014, as gathered by an organization called Monitoring the Future.
That period of time saw 21 US states legalize medical marijuana. The survey found that those states did generally have higher overall rates of teen stoner-ism. Specifically, teens in places like California, Oregon and Michigan were 27 percent likelier to be regular marijuana user than those in states with blanket prohibition.
But importantly, there was no evidence that students used more marijuana after the laws were changed. After the study's authors controlled for other factors, teens didn't have any greater desire to become stoners when there were legal medical users nearby. (In their summary of their findings, the authors note, "State-level risk factors other than medical marijuana laws could contribute to both marijuana use and the passage of medical marijuana laws"—in other words, maybe weed-friendly states both have more smokers of all ages and are more likely to make the drug legal.)
In a related commentary piece in the same journal, Kevin Hill wrote, "The growing body of research that includes this study suggests that medical marijuana laws do not increase adolescent use, and future decisions that states make about whether or not to enact medical marijuana laws should be at least partly guided by this evidence."
In 2012, a slightly less comprehensive study from the CDC came to the same basic conclusion as the new Lancet survey. Another CDC survey from that same year that teens in general were smoking more weed, but as economist Daniel I. Rees of the University of Colorado Denver said at the time, "There is anecdotal evidence that medical marijuana is finding its way into the hands of teenagers, but there's no statistical evidence that legalization increases the probability of use."
Yet another study, this one from the Journal of Adolescent Health in 2013, also failed to find "increases in adolescent marijuana use related to legalization of medical marijuana."
It's worth noting, however, there's evidence that blanket legalization, rather than just medical legalization, might lead to greater use among teenagers. Last year, a paper published in the International Journal of Drug Policy relying on data from a Monitoring the Future survey found that 10 percent of students who weren't likely to break the law by trying marijuana said they'd try it if it were legal.
Follow Mike Pearl on Twitter.