Teens in the first two states to end pot prohibition do not appear any more likely to fall into harder substances, according to a new study, rebuking legalization opponents who have long contended that marijuana is a gateway drug.

The research, published in the March issue of the Journal of Substance Abuse Treatment, examined data on “adolescents” (ages 12-17), “early emerging adults” (18-20), and “late emerging adults” (21-24) living in Colorado and Washington, where voters in 2012 passed measures legalizing recreational marijuana use for adults. 

“A public health concern stemming from recreational marijuana legalization (RML) is the idea that marijuana may act as a ‘gateway’ drug among youth and young adults, where growing marijuana use will lead to increasing substance use disorder (SUD) for ‘harder’ illicit drugs,” the authors of the study wrote. “This study investigates whether SUD treatment admissions for cocaine, opioids, and methamphetamines increased following RML enactment in Colorado and Washington for adolescents and emerging adults.” 

The researchers concluded that recreational marijuana legalization in Colorado and Washington “was not associated with an increase in adolescent or emerging adult SUD treatment admissions for opioids, cocaine, or methamphetamines” compared with rates of such substance use in other states where marijuana legalization has not occurred.

“Future studies should extend this research to other states, other substances, for older adults, and over longer time periods; and consider how the effects of drug policies may differ across different jurisdictions,” the researchers, who are based at Temple University and the University of Tennessee, wrote in their conclusion.

A Flimsy Idea Leading Governmental Policies

The notion that marijuana is a “gateway drug” has long been disputed, and has often unraveled when subject to scrutiny. But that belief has long shaped government policy. While the Center for Disease Control acknowledges that the “majority of people who use marijuana do not go on to use other, ‘harder’ substances,” it also insists that “[m]ore research is needed to understand if marijuana is a ‘gateway drug.’”

The authors of the new study sought to fill that gap. 

“To the best of our knowledge, this is the first study to investigate whether RML [recreational marijuana legalization] in the U.S. led to an increase in SUD [substance use disorder] treatment admissions [for] illicit drugs other than marijuana,” they wrote, as quoted by NORML. “We found that the legalization of recreational marijuana use in Colorado and Washington did not result in an increase in SUD treatment admissions for cocaine, opioids, or methamphetamines among adolescents or emerging adults.”

NORML’s Deputy Director Paul Armentano hailed the study for striking at an oft-invoked argument levied against legalization advocates.

“This data further undermines long standing claims that marijuana acts as any sort of a ‘gateway’ to the abuse of other controlled substances – an allegation that has, historically, largely guided prohibitionist-based marijuana policies in the United States despite a lack of hard evidence,” Armentano said in a statement on Friday.

Different research published late last year may also help assuage concerns of parents concerned about what legalization means for their teenaged children. The study, published in December by the Journal of Adolescent Health, that legalizing marijuana use among adults does not result in a spike in use among teenagers.