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. Research also found that legalization resulted in an increase in marijuana use, but a decrease in alcohol misuse 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.”

Sometimes, addiction extends well beyond the drugs involved. The Colorado SunReports state that the researchers measured psychological dysfunction that went beyond addiction. They also assessed financial difficulties, disengagement from the community, and relationships that may be related to marijuana use.

A previous study had shown that cannabis was consumed by twins in legal states about 20% more often than those in non-green states. The same researchers decided to reexamine the findings and see how this affects addiction to other substances.

The data was gathered from two states with legal marijuana and another without. These states were near perfect controls for comparing the effects of legalization to a state that bans all forms of marijuana. The twins were observed by researchers in both states for long periods. There are greater controls on socioeconomic status and genetic differences by using twins.

The data was collected from 478 people, who were first evaluated in their adolescence, then aged 24-49 and are currently living in states that have different cannabis policies (Colorado and 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. Minors were required to give informed consent by their parents.

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 state that is legal was actually associated 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. There was no adverse outcome to the co-twin plan, which included cannabis use disorder. No risk factor significantly interacted with legalization status to predict any outcome.”

These findings led to many conclusions by researchers.

“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. The legalization of cannabis did not increase vulnerability to marijuana use. 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 was also the leader of the previous study on the effects of legalization. Many of the funding for the research came from grants provided by the National Institutes of Health. 

Research indicates that there is more information needed in order to assess the impact of legalizing cannabis on psychiatric disorders or addiction.