Long-term study of twins finds no link between legalization and drug abuse

Once again, the Gateway theory – the belief that cannabis use leads to other drugs that became popular some 40 years ago – is shattered by new evidence, this time finding no evidence of aggravated drug abuse in legal states where twins are used as controls be used.

A new long-term study looked at twin pairs, over 4,000 people, and found that state legalization status was not associated with increases in substance use disorders and other psychological problems and vulnerabilities. Researchers also found that legalization led to increases in cannabis use but decreases in alcohol use disorder (AUD).

The study, “Legalization of recreational cannabis has a limited impact on a wide range of psychiatric and psychosocial outcomes in adults,” was published online by Cambridge University Press on January 5. In it, the researchers sought to “quantify possible causal effects of recreational cannabis legalization across substance use, substance use disorders, and psychosocial functioning, and whether vulnerable individuals are more vulnerable to the effects of cannabis legalization than others.”

Addiction usually goes well beyond the substances involved: The Colorado Sun reports that researchers have measured general mental dysfunction that extends beyond substance use disorders, but also measured financial problems, mental health, community withdrawal, and relationship problems, some of which are is assumed to use them related to grass.

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

The researchers collected data from longitudinal studies of twins in two contrasting states, one with legal pot and one without: Colorado or Minnesota. States provided near-perfect controls to examine the full impact of legalization compared to a state that bans most forms of cannabis. Researchers in both states followed the twins for long periods of time. By using twins there are more automatic controls over socioeconomic status or genetic differences.

The researchers collected data from 4,078 people who were first screened in adolescence and are now aged 24 to 49 and currently reside in states with different cannabis policies (Colorado or Minnesota). Study participants were recruited as teenagers using birth certificates from 1972-1994, beginning before 2014 when adult-use cannabis stores opened in Colorado. Parents gave informed consent if the study participants were minors.

Living in a legal state was “not associated with substance use disorders,” although they found it resulted in higher pot use but lower alcohol consumption. Indeed, living in a legal state was associated with lower AUD rates.

“In the co-twin control design, which accounts for prior symptoms of cannabis frequency or alcohol use disorder (AUD), the twin living in a recreational state used cannabis more frequently and had fewer AUD symptoms, on average, than their co-twin living in one lived non-recovery state. Cannabis legalization was not associated with any other adverse outcome in the co-twin design, including cannabis use disorder. No risk factor significantly interacted with legalization status to predict an outcome.”

This led the researchers to several conclusions.

“Recreational legalization was associated with increased cannabis use and decreased AUD symptoms, but was not associated with other maladjustments,” researchers wrote. “These effects persisted in pairs of twins who were incompatible for stay. Furthermore, vulnerabilities to cannabis use have not been exacerbated by the legal cannabis environment. Future research could explore causal relationships between cannabis use and outcomes.”

While living in a legal state was associated with higher cannabis use, this did not impact substance abuse and other mental health problems. “At least from a psychological point of view,” Stephanie Zellers, one of the researchers, told The Colorado Sun. “We really haven’t found that the policies (legalizing cannabis) are having a major negative impact, 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 funded much of the research based on grants from the National Institutes of Health.

Researchers say more data is needed to determine the impact of cannabis legalization on psychiatric disorders and addiction.