Legalising cannabis does not increase substance misuse, finds study

New research refutes the theory that cannabis is a ‘gateway drug’, finding no increase in substance use disorders or psychological problems in states which have legalised adult-use. 

The research from CU Boulder in the US, found that the legalisation of adult-use cannabis at the state level does not increase substance use disorders or use of other illicit drugs among adults. In fact, it may reduce alcohol-related problems.

The study, which used data from more than 4,000 twins from Colorado and Minnesota, also found no link between cannabis legalisation and increases in cognitive, psychological, social, relationship or financial problems.

For the study, published in the journal Psychological Medicine, researchers at the University of Minnesota, CU Boulder and the CU Anschutz Medical Campus tapped data from two of the nation’s largest and longest-running twin studies: one housed at IBG and the other at the Minnesota Center for Twin Family Research.

Researchers have followed participants, now aged 24-49, since adolescence, collecting data on use of alcohol, tobacco, cannabis and several illicit drugs, as well as measures of ‘psychosocial health’.

In 2014, Colorado became one of the first two states in the US to commence legal sales of recreational marijuana. In Minnesota, recreational use of cannabis remains illegal. 

The power of twin studies

By comparing the 40% of twins who live in recreationally legal states to those who live in states where it is still illegal, the researchers set out to get a general sense of the impact of legalisation.

In addition, by specifically comparing twin to twin in 240 pairs in which one lives in a state that has legalised and the other in a state that has not, researchers sought to determine what changes, if any, cannabis legalisation causes.

John Hewitt, professor of psychology and neuroscience at CU Boulder, explained: “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.

“If the association holds up, it provides strong evidence that the environment, in this case legalisation, is having an impact.”

In a previous study, the group found that identical twins living in states where cannabis is legal used it about 20% more frequently than their twin in states where it’s illegal.

The logical next question was whether more use meant more problems.

To find out, the team compared survey results looking at 23 measures of ‘psychosocial distress,’ including use of alcohol and illicit drugs such as cocaine and heroin, psychotic behaviour, financial distress, cognitive problems, unemployment and relationships at work and at home.

They found no relationship between legalising cannabis and heightened risk of cannabis use disorder, or cannabis addiction. 

While many critics of legalisation 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 legalisation.

Co-author Dr Christian Hopfer, a medical doctor and professor of psychiatry at IBG and CU Anschutz who studies substance abuse disorders, said:  “For low-level cannabis use, which was the majority of users, in adults, legalisation does not appear to increase the risk of substance use disorders.

Twins in states where cannabis is legal did show fewer symptoms of alcohol use disorder and were less likely to engage in risky behaviour after drinking, such as driving while intoxicated.

‘No drug is risk-free’

The authors caution that the study did not look at impacts on adolescents and did not look closely at the type and dosage of cannabis being consumed.

“Our study suggests that we should not be overly concerned about everyday adult use in a legalised environment, but no drug is risk-free,” said Professor Hewitt.

“It would be a mistake to dismiss the risks from higher doses of a drug that is relatively safe in small amounts.”

The study found no evidence that legalising cannabis benefits psychosocial health, either.

Today half the US population lives in a place where cannabis is legal, and several more are considering legalisation.

Lead author Stephanie Zellers, who began the research as a graduate student at CU Boulder’s Institute for Behavioral Genetics (IBG), commented: “We really didn’t find any support for a lot of the harms people worry about with legalisation. From a public health perspective, these results are reassuring.”

Dr Zellers, who recently earned a doctorate from the University of Minnesota, hopes the paper, and more to come, will help inform policy conversations about legalisation.

She added: “I would love for us to get past this question of, ‘Is legalisation 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.”

More research on cannabis policy and psychosis

Last week a study exploring the impact of cannabis policies on psychosis-related diagnoses, found ‘no statistically significant increase’ in rates of psychosis in states where medical or recreational cannabis is legal, compared to those where the plant is still prohibited.  

Scientists examined commercial and Medicare Advantage claims data from more than 63 million individuals between 2003 to 2017. 

The number of unique claims with psychosis-related diagnoses, prescribed antipsychotics and follow-up time was analysed, alongside measures of state cannabis policy and state-level demographic, social, and economic characteristics.

The paper concludes: “In this retrospective cohort study of commercial and Medicare Advantage claims data, state medical and recreational cannabis policies were not associated with a statistically significant increase in rates of psychosis-related health outcomes. 

“As states continue to introduce new cannabis policies, continued evaluation of psychosis as a potential consequence of state cannabis legalisation may be informative.”

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