Marijuana use did not increase among teenagers in the states in which medical marijuana has become legal, researchers reported Monday.
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The new analysis is the most comprehensive effort to date to answer a much-debated question: Does decriminalization of marijuana lead more adolescents to begin using it?
The study found that states that had legalized medical use had higher prevailing rates of teenage marijuana use before enacting the laws, compared with states where the drug remains illegal. Those higher levels were unaffected by the changes in the law, the study found.
The report, published in The Lancet Psychiatry, covered a 24-year period and was based on surveys of more than one million adolescents in 48 states. The research says nothing about the effect of legalizing recreational use, however.
A primary concern on both sides of the debate over medical marijuana has been that loosening marijuana restrictions might send the wrong message to young people, and make the drug both more available and more appealing. Teenagers who develop and sustain a heavy, daily habit increase their risk of having cognitive difficulties later on, several studies now suggest.
Previous research on usage trends in the wake of the laws has been mixed, some reporting evidence of an increase among adolescents and others — including two recent, multistate studies — finding no difference. The new analysis should carry far more weight, experts said, not only because of its size and scope but also because the funders included the National Institute of Drug Abuse, whose director has been outspoken about the risks of increased use.
“We have a war going on over marijuana, and I think both sides have been guilty at times of spinning the data,” said Dr. Kevin Hill, an assistant professor of psychiatry at Harvard and director of the substance abuse consultation service at McLean Hospital. “It’s nice to have a scientifically rigorous study to guide policy.”
Dr. Hill, author of the book “Marijuana: The Unbiased Truth About the World’s Most Popular Weed,” said this study was about as definitive as could be expected.
Researchers opposed to legalizing marijuana, for medical or other purposes, disagreed, saying the study would have to go further to be convincing.
“Medical marijuana laws vary drastically across the U.S. and often take years to be implemented, so what we need to see is the longer-term effects of these laws and the accompanying commercialization efforts, which this study does not do,” said Kevin Sabet, a former Obama administration adviser and president of the group Smart Approaches to Marijuana, or SAM, which opposes legalization.
In the study, a research team led by Deborah Hasin of Columbia University analyzed data from a large, continuing University of Michigan survey of 8th, 10th and 12th graders, asking about their use of a variety of drugs, including alcohol, tobacco and marijuana.
The team focused on responses to several questions in particular, including those asking about use within the last 30 days and frequency of use. The researchers adjusted the data for factors known to correlate with marijuana use, like gender, education level of parents, and whether a school was urban or rural.
The overall rate of use among teenagers in states that passed laws was 16 percent, compared with 13 percent in those that had not, the analysis found. The researchers then compared samples of teenagers before and after laws passed in specific states: for example, before and after August 2013 in Illinois, and before and after April 2007 in New Mexico.
“We showed no hint of an increase at all after the laws were passed,” Dr. Hasin said.
The University of Michigan surveys have found that marijuana use among teenagers has been generally on the rise, in contrast to trends in alcohol, opioid and nicotine use — and perceptions of marijuana’s health risks are steadily shrinking.
Those risks are probabaly small for occasional users who are adolescents, most experts say. But heavy, daily users who start young are at risk of blunting their mental acuity over time, several studies have found, because of biological and social factors that are not yet understood.
The research group is planning to study the effect on usage trends when recreational marijuana is legalized, as in Colorado.
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