The research was applauded by other social scientists for its size and strength. And the authors themselves suggested it is time for the debate over how to keep adolescents from using marijuana at an early age to move on to target on other factors.
"Concerns that increased adolescent marijuana use is an unintended effect of state medical marijuana laws seem unfounded. In view of the potential for harm from early use, other factors influencing wide segments of the population need to be investigated," the authors wrote.
The study is published in the journal Lancet Psychiatry. The work was led by Prof. Deborah Hasin of the Mailman School of Public Health, at New York City's Columbia University.
The researchers analyzed nearly a quarter-century's worth of data from a nationally administered survey American teenagers complete in Grades 8, 10 and 12. Their dataset included information provided by 1,098,270 adolescents.
During the time studied, 21 of the 48 states that administer the survey passed medical marijuana laws.
The researchers compared teen marijuana use in states with and without legislation, but also looked closely at usage rates in states with medical marijuana laws in the periods before and after the legislation went into effect.
The researchers found teen use was higher in the states that adopted medical marijuana laws, but that pattern of higher use existed before the laws came into effect.
Asked if parents or policy makers should feel reassured by the findings, Hasin said she didn't think so.
"Early adolescent marijuana use has many long-term adverse consequences, and the fact remains that teen use of marijuana is up considerably compared to the beginning of the 21st century," she said in an email.
"So, I don't think people should be reassured by our findings particularly because we still need to find out what causes such increases and what can be done to prevent them."
A Canadian addictions expert who was not involved in the study called it the best available on the topic.
"This is probably the ... highest-quality study on a question that has been lingering for some time, which is whether the liberalization of marijuana control and more specifically medical marijuana regimes in the U.S. lead to higher levels of cannabis use, especially among young people," said Benedikt Fischer, a senior scientist at the University of Toronto's Centre for Addiction and Mental Health.
"What this study importantly finds is that while states with medical marijuana regimes have somewhat higher levels of marijuana use among young people, the explanation is not that it is the medical marijuana regimes that make marijuana use go up."
"These states were different from the outset — probably had a more accepting or softer disposition towards marijuana use."
A chief concern of opponents of medical marijuana laws is that adopting them will lead to a general increase in use of the drug, especially among young people. Previous studies of the question have produced mixed results.
These researchers set out to resolve the issue. They found there was no statistically significant increase in marijuana usage rates among Grade 10 and Grade 12 students after medical marijuana laws came into effect in the states that adopted them. And use of the drug actually dropped among Grade 8 students after legislation was put in place.
"In conclusion, the 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," the authors wrote.
A commentary published with the study noted the work underscores how important it is to actually rigorously test assumptions before making public policy.
"Policies might sometimes be shaped by preconceived notions that do not end up being true, and Hasin and colleagues' study is an example of such an occurrence," wrote Dr. Kevin Hill from the Division of Alcohol and Drug Abuse, McLean Hospital, Belmont, Mass.Suggest a correction