In October 2018, Canada became the first G7 country to legalize recreational cannabis use for adults nationwide. Nearly seven years later, researchers like Dr. André McDonald from the Peter Boris Centre for Addictions Research (PBCAR) are investigating how legalization has changed cannabis use. Through his research, Dr. McDonald hopes to inform policymakers in Canada and other jurisdictions about how changing cannabis legislation may affect public health.
Why Did Canada Legalize Cannabis?
By regulating cannabis at a federal level, the Canadian government aimed to reduce youth access to cannabis, reduce illegal cannabis vendors’ profits, and protect overall public health and safety.
Since legalization of cannabis, the amount of illegal cannabis purchases has plummeted, resulting in criminal enterprises having smaller profits, and fewer people consuming cannabis products with unknown safety profiles and potential contaminants.
One of Canada’s goals in legalizing cannabis nationwide was to redirect cannabis profits away from illegal organizations; Dr. McDonald’s research shows that this goal was successful. However, there are still a lot of questions about cannabis that we don’t have the answers to yet.
Most cannabis research in Canada suggests that since legalization, rates of cannabis use have generally increased among adults. This raises some public health concerns, since cannabis use can still pose harms, even in people who aren’t misusing the substance.
Dr. McDonald’s Background in Public Health Research
Dr. McDonald's career in public health began in Nunavut, where he did policy work on mental health and addiction issues in the territory. He then completed a Master of Public Health in social and behavioural health sciences at the University of Toronto, followed by a PhD in epidemiology.
Cannabis was legalized in Canada around the time that Dr. McDonald began his PhD; this policy change raised questions about its potential public health effects for many researchers. Dr. McDonald was particularly interested in the relationship between youth cannabis use and mental health, which became the topic of his thesis.
Since completing his PhD, Dr. McDonald’s research has continued to address the relationship between cannabis use and mental health, and has expanded to evaluating the impacts of cannabis legalization.
Dr. McDonald, pictured left, joined the PBCAR as a postdoctoral fellow in March 2023, with Dr. James MacKillop as his supervisor.
“There’s a great culture at the PBCAR,” said Dr. McDonald. “From other postdoctoral fellows to master's and PhD students, everyone is doing such interesting work and comes from interdisciplinary backgrounds. It’s exposed me to different areas of mental health and addiction research, some of which I never thought I’d be involved in. It’s been an incredible learning experience.”
More Research is Needed to Understand Cannabis Misuse Post-Legalization
Dr. McDonald was the lead author of a study titled “Cannabis Use and Misuse Following Recreational Cannabis Legalization,” which was published in JAMA Network Open in April 2025. In this study, Dr. McDonald and other researchers found that since cannabis was legalized in Canada, cannabis use frequency in this study sample slightly increased while rates of misuse have slightly decreased.
Statistically, it makes sense to expect that in groups of people where cannabis use has increased over time, cannabis misuse will have also gone up. So why does Dr. McDonald’s study suggest the opposite?
This study followed the same group of people over a period of five years. This longitudinal design allowed Dr. McDonald and his colleagues to look at whether changes in cannabis use behaviours depended on pre-legalization cannabis use status, which has rarely been done before. However, the study included a large group of young adults, and as the study progressed, many of these young adults may have “aged out” of cannabis misuse.
Aging out is a common phenomenon that describes how people’s substance use tends to change as they get older and take on new responsibilities; this could help explain why cannabis misuse decreased at the same time as cannabis use frequency increased.
It is important to note that the study sample was not perfectly representative of the Canadian population. The aging out effect seen in this case is just one piece of a very large and complex puzzle, which researchers are working to complete over time.
“We need more research to understand how cannabis use and misuse have changed since legalization, and the reasons behind these changes,” said Dr. McDonald.
Cannabis Research in Canada can Inform Worldwide Legislation
As researching illegal substances is difficult to do, jurisdictions that are considering legalizing these substances historically have relied on research coming out of other places that have already enacted legalization.
Since Canada was one of the first countries with legalized cannabis nationwide, researchers here have the opportunity to inform other jurisdictions around the world about the potential effects of legalization. Other countries such as Germany and the US are raising the idea of cannabis legalization, and Canadian research in this field may help guide this legislation.
Dr. McDonald is particularly interested in demystifying the relationship between cannabis use and outcomes in both mental and physical health.
“I’m going to be conducting a series of longitudinal studies that look at cannabis use, particularly in young people, and how it’s related to the development of various mood and anxiety disorders,” he said. “I’m also involved in research looking at psychosis as an outcome, and possibly other adverse health effects like cancer, cardiovascular disease, respiratory disease, and dementia.”
Learn more about the Peter Boris Centre for Addictions Research here..