The legalization of cannabis affects substance use, with the potential for both negative and positive impacts on public health, a new McGill-led study found.

The researchers conducted a systematic review of evidence from 176 studies spanning 30 years in Canada, the United States, Europe, Uruguay and Australia. Published in The Milbank Quarterly, the review is one of the most comprehensive assessments to date of how cannabis policy reforms affect substance use and public health.

“We found evidence that cannabis legalization can have ripple effects on the use of other substances, sometimes in ways that may reduce harm and sometimes in ways that raise new concerns,” said lead author Sarah Windle, a PhD candidate in McGill’s Department of Epidemiology, Biostatistics and Occupational Health. “Looking at those broader impacts helps provide a more complete picture of what these policies mean for public health.”

Both medical and recreational cannabis legalization increased the number of adults using cannabis, whereas effects were less apparent among youth. Cannabis-related emergency department visits and poison centre reports also increased.

Alcohol use showed different patterns depending on the policy and population studied, Windle noted. Medical cannabis legalization may have the potential to reduce alcohol use among youth and adults, while recreational legalization painted a more complex picture. Among young adults, it was linked to a rise in alcohol use and binge drinking.

Tobacco use, by contrast, generally stayed the same or dropped after cannabis legalization.

As for opioids, most studies found no increase, and in many cases found a decrease, in measures such as opioid prescribing, opioid sales and health-care visits related to opioid use.

Researchers say few studies have examined the effects of cannabis decriminalization or legalization on the use of other illicit drugs, highlighting priorities for future research.

“Impacts of Global Cannabis Policies on Substance Use: A Systematic Review of Quasi-Experimental Studies” by Sarah Windle and Arijit Nandi et al., was published in The Milbank Quarterly. This research was supported by the Canadian Institutes of Health Research.