MONDAY, Jan. 30, 2023 (HealthDay News) -- Living in a U.S. state where recreational weed is legal does not appear to increase the average adult’s risk of succumbing to “reefer madness,” a new study of twins has determined.

An adult living in a “legal” state is not more likely to develop any sort of substance abuse disorder than their twin residing in a state where marijuana remains outlawed, researchers found.

They also aren’t more likely to break the law or have problems with their mental health, relationships, work, finances, friendships or standing in the community, according to the report published recently in the journal Psychological Medicine.

“We found mostly a lot of nothing, which I think is personally interesting,” said lead researcher Stephanie Zellers, a postdoctoral researcher with the University of Helsinki in Finland. “I think this is a case where we don't find much is actually more interesting maybe than finding a bunch of results.”

For the study, Zellers and her colleagues analyzed data on more than 4,000 twins who have been participating in long-term studies conducted by research teams at the University of Minnesota and the University of Colorado.

They found 240 twin pairs where one twin lives in a state with legal weed and the other lives in a state where it’s still banned. There are 21 U.S. states that have legalized recreational cannabis, the researchers said in background notes.

Twin studies are valuable because they share the same upbringing and -- in the case of identical twins -- the same genes, said Zellers, who began the research as a graduate student at the University of Colorado Boulder.

“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.”

Not surprisingly, researchers first found that an adult twin living in a legal state was more likely to partake in weed than their sibling in a state where toking can get them busted.

“That was kind of obvious,” Zellers said. “Yes, people can legally buy a drug, they're going to use it more.”

However, a twin in a legal state was slightly less likely to develop a drinking problem, Zellers said. That’s possibly due to the “substitution effect” – they use weed instead of alcohol to unwind.

A twin in a state with legal weed also was less likely to “drink in situations that could be physically hazardous,” such as drinking and driving, Zellers said.

“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.”

Marijuana has long been considered a “gateway” drug to more addictive substances, but the researchers found no evidence of that.

“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.”

Further, twins in states with legal weed aren’t more susceptible to mental or emotional problems, financial woes, unemployment or relationship problems, the study reports.

“I would like to see this be a reassuring result for public policy, at least with respect to psychological well-being,” Zellers said. “Legalization really isn’t causing great psychological harms.”

Linda Richter, vice president of prevention research and analysis for the Partnership to End Addiction, remains skeptical regarding the safety of recreational marijuana, even though “the analyses were rigorous and the descriptions of the results within the journal article were measured and appropriate," she said.

That’s because this study focused on adults rather than teens, Richter said.

“The concerns surrounding marijuana legalization from much of the public health community primarily center on young people -- adolescents and early adults -- who are more vulnerable to substance use and its consequences, since they are still undergoing significant brain development and are highly susceptible to increased normalization of and access to addictive substances that come with legalization and commercialization of cannabis,” Richter said.

“In youth samples, a growing body of research is pointing to a broad range of detrimental effects of legalization on youth, including higher rates of cannabis use, cannabis use disorder, driving under the influence, other substance use and mental health disorders,” Richter said.

Zellers agreed that “preventing adolescent use is something that is pretty important going forward, and can be addressed with policies around legal purchasing.”

Zellers also acknowledged that her study doesn’t consider how legal marijuana might impact higher-risk people who use weed more frequently. The adult twins in this report tended to use “maybe a few times a month at most,” she said.

“I think if you are approaching legalization from the question of substance abuse, for the average low-using person, we’re not seeing harms,” Zeller said. “I think that’s important to know.”

More information

The U.S. National Institute on Drug Abuse has more about marijuana.

SOURCES: Stephanie Zellers, PhD, postdoctoral researcher, University of Helsinki, Finland; Linda Richter, PhD, vice president, prevention research and analysis, Partnership to End Addiction; Psychological Medicine, Jan. 5, 2023