Legalizing Marijuana Does Not Increase Substance Abuse (Study)

man holding cannabis in one hand and pills in another while smiling
"A recent study published by Cambridge University Press, involving researchers from the University of Minnesota, the University of Colorado Boulder, and the University of Colorado Anschutz Medical Campus, has found that legalizing marijuana does not lead to an increase in substance abuse. The study, which analyzed data from over 4,000 twins, found that while marijuana use was 20% higher in states where it was legal, there was no associated increase in substance abuse disorders. Furthermore, the study found no increase in illicit drug use following marijuana legalization, challenging the notion of marijuana as a gateway drug. The research also indicated that residents of states with legal recreational marijuana were less likely to engage in hazardous drinking.
Table of Contents

In 1920, the U.S. government banned the sale of alcohol. It didn’t work. Consumption declined at first, but within a few years, Americans were drinking almost as much as ever and were buying illegal and often dangerously poor-quality products from criminals.

Advocates for legal marijuana draw on the history of Prohibition to suggest that lawful access to cannabis results in a similar drop in organized crime, that regulated products are safer to use, and that legalization can provide economic benefits. The response from the anti-marijuana camp is to downplay these facts and instead stoke fears of the corruption of youth, a general social decline due to an increase in substance abuse, and the dangerous role of marijuana as a “gateway drug.”

There’s a wealth of both popular and scholarly literature debating the pros and cons of marijuana legalization, some of it dating back decades. But in many cases, conclusions in these older studies were based on limited information, as marijuana research has long been hampered by the illegality of use and the need to rely on self-reported data and broad-based trends that incorporate a number of uncounted variables.

Recent researchers have the advantage of a more cannabis-friendly environment made possible by ongoing legalization efforts, which in turn allows for more accurate and evidence-based research and reporting. A prime example of this phenomenon is a recent statement from the Journal of the American Medical Association based on results from a long-term analysis of more than 1.4 million adolescent subjects.

After reviewing the data, the authors reported:

There was no evidence that the legalization of medical marijuana encourages marijuana use among youth. Moreover, the estimates showed that marijuana use among youth may actually decline after legalization for recreational purposes (8% decrease in marijuana use and 9% decrease in frequent use).

That takes care of the “corruption of the youth” argument. But what about the fears that easy access to marijuana leads to social decline, an increase in psychotic behaviors, and the pathway toward hard drugs? For the latest on that, we turn to twins.

Why Two Are Better Than One

In January of 2023, Cambridge University Press released a new study with the headline, “Recreational cannabis legalization has had limited effects on a wide range of adult psychiatric and psychosocial outcomes.”

While this reporting is not the first of its kind, the methodology behind this latest study is what adds an extra degree of weight and credibility to its conclusions. This research paper is a collaborative effort between the University of Minnesota, the University of Colorado Boulder, and the University of Colorado Anschutz Medical Campus. It is based on cumulative data gathered on twins born in Colorado and Minnesota.

What’s so special about twins? Well, for starters: They tend to have a lot in common, and that can eliminate a lot of unknowns. Unlike tightly controlled laboratory trials where mostly all variables can be eliminated or documented, broad-based studies typically rely on a combination of record keeping and self-reported information. This can result in information gaps and inaccuracies that skew results, as everything from early home life, genetic dispositions, and education to diet, occupation, and peer group associations comes into play. Researchers employ a wide range of formulas to mitigate these variables, and one of the most powerful tools available is the twin study.

The rationale behind a twin study is that two individuals raised in the same family will be shaped by the same parental influences, home environment, community norms, and socioeconomic background. In addition, fraternal twins have similar genetic makeups, while identical twins share the exact same genetics. These commonalities allow researchers to discount many of the unknown physical, social, and economic variables that might otherwise cloud results.

What Twins Tell Us About Marijuana

For the new study, researchers examined more than 4,000 individuals enrolled in two of the nation’s most comprehensive databases on twins — the Minnesota Center for Twin Family Research and the Colorado Center for Antisocial Drug Dependence. Both of these databases provided detailed information on twins born between 1972-1994, tracked from adolescence on through their current ages of 24 to 49. Insights gathered for each participant included use of marijuana, alcohol, tobacco, and several illegal drugs, along with specific measures of psychological health, economic success, and social well being.

The study authors identified two goals for their research. First was making comparisons between the 40% of participants living in states where marijuana was legal and the remaining 60% living in states where it remained illegal. The authors postulated that the detailed and high-quality information gleaned from the databases would allow them to identify any broad-based impacts of marijuana legalization on things such as drug and alcohol use and abuse.

Second, the authors identified 240 twin pairs where one sibling was living in a marijuana-legal state and the other was not. These individuals became the subject of a very detailed examination into the effects of cannabis legalization on psychiatric and psychosocial outcomes that included everything from alcohol and illicit drugs to psychotic behavior, financial problems, cognitive problems, unemployment, and relationships at work and at home.

Among the expected conclusions was that twins living in marijuana-legal states would be more likely to consume marijuana dues to increased availability. This theory was borne out, as data showed that residents of marijuana-legal states consumed about 20% more often than those living in states where cannabis remained illegal. These findings were in line with previous research from the same group. The authors did note, however, that while this trend might be an actual increase in use resulting from legal access to marijuana, it might also be skewed by an increased willingness to report use.

What was less expected was the finding that legal access to marijuana did not appear to result in a corresponding increase in the risk of substance abuse.

Instead, the researchers report:

Surprisingly, cannabis use disorder was not associated with legalization, in both the individual-level and within-twin results.

Even more eye-opening and reassuring was the lack of a correlation between the legalization of cannabis products and an increase in illicit drug use. In other words, marijuana doesn’t appear to be the much-feared gateway drug that some would have you believe. This finding was the subject of a January 2023 press release from the University of Colorado Boulder, which quoted one of the study-co authors, Dr. Christian Hopfe as saying:

While many critics of legalization have expressed concern that cannabis could serve as a gateway drug to other more harmful substances, the researchers found no changes in illicit drug use after legalization.

Another noteworthy finding from twin comparisons was the effect of marijuana legalization on patterns of alcohol consumption. Scientists reported that they found:

…no significant effects of recreational legalization on average alcohol quantity [consumed by individuals].

While also observing that:

With respect to maximum drinks in a 24-hour period, residents of recreationally legal states drank less than their non-recreational counterparts at the individual level. This indicates that those twins living in a recreational-legal state were less likely to risk harm while under the influence of alcohol.

And that:

Overall, the results suggest that although the patterns of consumption may not be changed by recreational legalization, physically hazardous drinking is lower in recreationally legal states.

Marijuana Legalization, Mental Health & Well-Being

In addition to details on drug and alcohol use patterns, the database records contained 15 specific measures of mental health and social and economic well-being, each of which was measured over a period of years. A comparison of  twin pairs — one living in a state supporting legal cannabis and the other in a state where cannabis remained illegal — showed no differences in financial success, legal problems, community engagement, or work-related issues.

The study also looked for any legalization-related changes in mental health and signs of increased negative behaviors such as detachment, aggression, and risk for psychosis and psychopathic disorders. Particular interest was focused on whether the legalization of marijuana impacted younger males who were identified as being at greater risk of displaying destructive or anti-social behaviors.

The data failed to show any changes in these behaviors that could be linked to the legalization of marijuana. The authors reported:

Broadly speaking, our co-twin control and differential vulnerability results suggest that the impacts of recreational cannabis legalization on psychiatric and psychosocial outcomes are otherwise minimal.

These findings are of particular importance to the ongoing discussion on marijuana and mental health, as they provide assurances that the vast majority of cannabis users are not prone to developing “reefer madness” when marijuana is easily accessible.

The authors did include one cautionary observation with regards to marijuana-related psychosis disorders. They noted that elevated levels of marijuana consumption may incite or exacerbate psychotic tendencies in certain at-risk individuals, and that these risks may increase with high-potency cannabis products. But again, the data failed to show that cannabis legalization increased risk factors for these individuals.

The authors write:

Individuals at higher risk of substance use problems and other psychosocial dysfunction are not at higher risk in recreationally legal environments as compared to non-recreational environments. This suggests that prevention and intervention efforts may be best implemented by continuing to target established risk factors rather than focusing on availability.

Commenting on this and other insights from the study, lead author Dr. Stephanie Zellers summarized her group’s work by saying:

 We really didn’t find any support for a lot of the harms people worry about with legalization.

Zellers went on to add:

I would love for us to get past this question of ‘Is legalization good or bad?’ and move toward more specific questions like, ‘Who is most at risk? Who can benefit the most? And how?’ so that people can make informed choices.

Updated: May 29, 2024

Article Written By:

Pierce Hoover

Pierce Hoover is a career journalist with more than three decades of experience in print, broadcast and online writing, editing and reporting, with more than 5,000 articles published in national and international print media and online. His focus on medical marijuana therapies mirrors his broader interest in science-based alternative medical practices.


Questions about medical marijuana? Ready to get your card and purchase legal cannabis products? CannaMD‘s state-certified network of medical marijuana doctors are here to help! Contact CannaMD‘s experienced team at (855) 420-9170 today. You can also find out if you qualify for medical marijuana treatment with our quick online application!

Find Out If You Qualify

You may be eligible for medical marijuana!

To stay up-to-date with the latest studies and legal regulations surrounding medical marijuana treatment, be sure to follow CannaMD on Facebook and sign up for our newsletter, below!

Join 100k+ Subscribers!

Get updates on more posts like this!

Related PostS

Feedback from our readers is very important and helps us provide quality articles. Please don’t be shy, leave a comment.

If you have a topic you would like us to cover in our blog or you are interested in writing guest posts please contact us for more information.

Get a medical marijuana certification