Once again the gateway theory—the belief that cannabis use leads to other drugs, popularized about 40 years ago—is being crushed by new evidence, this time finding no evidence of worsened drug abuse in legal states, using twins as controls. 

A new long-term study examined sets of twins, over 4,000 individuals, and found that state legalization status wasn’t associated with a rise in substance-use disorders of other drugs, and other psychological problems and vulnerabilities. Researchers also noted that legalization led to an increase in cannabis use but decrease in alcohol use disorder (AUD). 

The study, “Recreational cannabis legalization has had limited effects on a wide range of adult psychiatric and psychosocial outcomes,” was published online by Cambridge University Press on Jan. 5. In it, researchers sought to “quantify possible causal effects of recreational cannabis legalization on substance use, substance use disorder, and psychosocial functioning, and whether vulnerable individuals are more susceptible to the effects of cannabis legalization than others.”

Addiction usually goes well beyond the substances involved: The Colorado Sun reports that researchers measured general psychological dysfunction, going beyond substance-use disorders but also measuring financial problems, mental health, community disengagement, and relationship issues that are sometimes believed to be linked to pot use.

After noting that twins consumed cannabis about 20% more in legal states than non-green states in a previous study, the same team of researchers set out again to see if this impacts addiction of other substances, and other psychiatric disorders.

Researchers gathered data from longitudinal studies of twins in two opposing states, one with legal pot and one without: Colorado or Minnesota. The states provided near-perfect controls to examine the full effects of legalization versus a state that prohibits most forms of cannabis. Researchers in both states observed the twins over long periods of time. By using twins there are more automatic controls over socioeconomic status or genetic differences.

Researchers gathered data from 4,078 individuals, first assessed in adolescence and now ages 24-49, and currently residing in states with different cannabis policies (Colorado or Minnesota). Study participants were recruited as teens via birth records from the years 1972–1994, beginning before 2014, when adult-use cannabis stores opened in Colorado. Parents provided informed consent when the study participants were minors.

Living in a legal state was “not associated” with substance abuse disorders, although they found it led to higher pot use but lower alcohol use. Living in a legal state was associated, in fact, with lower AUD rates.

“In the co-twin control design accounting for earlier cannabis frequency and alcohol use disorder (AUD) symptoms respectively, the twin living in a recreational state used cannabis on average more often, and had fewer AUD symptoms than their co-twin living in an non-recreational state. Cannabis legalization was associated with no other adverse outcome in the co-twin design, including cannabis use disorder. No risk factor significantly interacted with legalization status to predict any outcome.”

This led researchers to come to several conclusions.

“Recreational legalization was associated with increased cannabis use and decreased AUD symptoms but was not associated with other maladaptations,” wrote researchers. “These effects were maintained within twin pairs discordant for residence. Moreover, vulnerabilities to cannabis use were not exacerbated by the legal cannabis environment. Future research may investigate causal links between cannabis consumption and outcomes.”

While living in a legal state was associated with higher pot use, it didn’t impact drug abuse and other psychological problems. “At least from the psychological point of view,” Stephanie Zellers, one of the researchers, told The Colorado Sun. “We really didn’t find that the policies (on cannabis legalization) have a lot of negative influence, which I think is important.”

“That twin component really allows us to rule out a lot of possible alternatives—maybe there were just cultural differences, family differences, things like that,” Zellers said, explaining the need to observe twins.

Zellers also led the earlier study looking at the impact of legalization. The team has funded much of the research based on grants from the National Institutes of Health. 

Researchers stipulate that more data is needed to determine the effects of cannabis legalization regarding psychiatric disorders and addiction. 

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