Twins Study Busts Cannabis Gateway Theory

Legal access to recreational cannabis has no effect on increasing the probability of disorders using alcohol or illicit drugs, according to a recent study of twins.

In a recent report published by the journal Psychological Medicine, researchers observed data gathered from observing twins living in Colorado and Minnesota. They found no link to legal access to marijuana with the likelihood of developing substance abuse problems. 

“Cannabis legalization was associated with no other adverse outcome in the co-twin design, including cannabis use disorder,” researchers found. “No risk factor significantly interacted with legalization status to predict any outcome.”

“We found mostly a lot of nothing, which I think is personally interesting,” lead researcher Stephanie Zellers added. “I think this is a case where we don’t find much is actually more interesting maybe than finding a bunch of results.”

The study also noted that residents living in legal cannabis states didn’t appear to show an increase in problems associated with mental health, relationships, work and finances.

“Recreational legalization was associated with increased cannabis use and decreased alcohol use disorder symptoms but wasn’t associated with other maladaptations,” researchers wrote. “These effects were maintained within twin pairs discordant for residence. “Moreover, vulnerabilities to cannabis use were not exacerbated by the legal cannabis environment.”

Access to Legal Market

Zellers and her research team observed 240 pairs of twins where one lives in the legal state of Colorado while the other lives in Minnesota, where cannabis remains prohibited. Now aged 24 to 49, the participants have provided data on their personal use of alcohol, tobacco, cannabis and several illicit drugs, as well as measures of “psychosocial health” since adolescence.

“This co-twin design automatically controls for a wide range of variables, including age, social background, early home life and even genetic inheritance” that can influence health outcomes, said co-researcher John Hewitt, professor of psychology and neuroscience at CU Boulder. “If the association holds up, it provides strong evidence that the environment, in this case legalization, is having an impact.”

“There’s lots of things that could explain why one person is behaving one way or why people of one state behave one way compared to another,” Zellers said. “But with twins, we were able to rule out so many of those alternatives—not everything, but a lot of them.”

The recent study acted as a follow-up to prior research that found an increase in adult cannabis use where states have allowed recreational use. Despite the rise in use, however, the team found no relationship to a spike in cannabis abuse or addiction.

“Obviously the cannabis use increases, but we didn’t see an increase in cannabis-use disorder, which is a little surprising,” Zellers said. “We didn’t really see changes in how much people were drinking or using tobacco. No large personality or workplace or IQ differences or anything like that.”

But while cannabis use increased in legal situations, twins living in such areas were also less likely to drive drunk or develop alcohol use disorders.

“You’re combining drinking with something that could be physically unsafe,” Zellers said. “The residents of legal states do that less, which is interesting and maybe something a little unexpected.”

Disproving the Cannabis Gateway Theory

The findings also reject the gateway drug theory that using marijuana only leads to using stronger substances.

“We asked in the last 12 months have you tried or used heroin, prescription opiates, cocaine, methamphetamine, hallucinogens—kind of the whole 11 or 12 categories of illicit drugs,” Zellers said. “And there’s no difference there. People living in a state with legal cannabis, they’re not necessarily transitioning on to more illicit drugs.”

The results are quite promising but are far from an absolute conclusion. The study does have several limitations, as it focuses on adults, and few of whom consider themselves as heavy users.

“Our sample is an adult community sample broadly characterized by low levels of substance use and psychosocial dysfunction,” the researchers wrote. “This limits our ability to generalize relationships between legalization, outcomes and risk factors for the individuals at greatest risk.”

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