The legalization of cannabis has positive and negative effects on substance use and public health, according to a systematic review led by investigators at McGill University in Montreal.

Medical and recreational cannabis legalization was associated with an increased number of young adults using cannabis, whereas the effects were less apparent among youth. Cannabis-related emergency department visits and poison center reports also increased after legalization in some studies.

“We wanted to know if substance use — particularly the use of noncannabis substances such as alcohol, tobacco, or opioids — changed following cannabis policy reforms, such as cannabis legalization,” lead author Sarah Windle, a PhD candidate in epidemiology, biostatistics, and occupational health, told Medscape News Canada.“This is an extremely important issue for public health. Tobacco and alcohol together are estimated to account for about 95% of substance-related mortality worldwide. When you consider their contributions to cardiovascular disease and cancer, knowing if cannabis legalization increases or decreases other substance use could have substantial implications for public health,” she said.

The data were published on July 28 in The Milbank Quarterly.

Results Varied by Policy and Age Group

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

Most studies found no change or a decrease in opioid-related healthcare utilization outcomes following medical and recreational legalization, and most studies suggested no change or a decrease in opioid prescribing outcomes. The evidence was less consistent for opioid-related mortality, and these outcomes may be particularly vulnerable to time-varying confounding.

The findings from a small number of studies suggested a potential decrease in tobacco use across age groups and cannabis policy changes.

“Interestingly, we found that the effects on alcohol differed by policy,” said Windle. “A potential reduction in alcohol use following medical legalization suggested that some individuals may be using medical cannabis to replace the effects of alcohol, for example, for pain management. In contrast, there was a potential increase in alcohol use following recreational cannabis legalization, specifically among young adults. This [finding] suggests that some young adults are using cannabis and alcohol in combination.”

“Overall, I’m cautiously optimistic that the measures that have been taken to prevent increases in cannabis use among youth have largely been successful,” said Windle. “One of these key measures is the minimum legal age for cannabis consumption, which is between ages 18 and 21 years, depending on the province. Another issue is reducing the appeal of cannabis products to youth, including refraining from selling sweet flavors and dessert flavors in edibles, for example, that might be particularly appealing to children and youth.”

Physicians Need Training

“Only about 8 years have passed since the most recent major legislative changes, which might be insufficient for some long-term outcomes to occur. Given the lack of any worldwide effort to systematically define, identify, collect, and publish relevant outcome data, these results should be interpreted accordingly,” Irina Kudrina, MD, assistant professor of family medicine at McGill University and associate member of the Anesthesia Department at McGill University Health Center, told Medscape News Canada. Kudrina did not participate in the research.

An increase in diagnoses of cannabis use disorder may indicate greater awareness among patients and providers and better availability of training and information, said Kudrina. It may also reflect better awareness of chronic pain and addiction. “In addition, the novelty effect is possibly still in play, and recreational cannabis use prevalence may stabilize with time or change again as other factors like access to healthcare and funding for rehabilitation programs evolve,” she added.

“A good example from the review is the almost universal tobacco use decrease, for which one of the likely explanations is Canada’s ongoing antismoking campaign. Canada’s Tobacco Strategy, which was launched in 2018, set a national goal to reduce tobacco use and tackled everything from smoking-free public spaces to internet ads, plain packaging, and warning-label requirements,” said Kudrina.

Given the prevalence of substance use, physicians and other health providers must be trained to evaluate their patients’ substance use histories and provide counseling. They must consider not only cannabis but also other legal and illicit substances, as well as the use of multiple substances, according to Kudrina.

“Many cannabis, alcohol, smoking cessation, and opioid prescribing guidelines exist around the world. However, understanding the reason for the substance use…could direct providers’ efforts and establish trusting relationships to support the patient,” she added. For example, patients may use substances to improve mood, sleep, or pain. They may be coping with abusive relationships or socioeconomic challenges. Patients may not know about available therapies and rehabilitation options, Kudrina explained. Once they establish trust, clinicians could involve these patients in multidisciplinary care to address the relevant comorbidities.

The study was supported by the Canadian Institutes of Health Research. Windle and Kudrina reported having no relevant financial relationships.

Evra Taylor is a freelance medical writer and reporter with 20 years’ experience covering a broad range of therapeutic sectors, including family health, cardiology, psychiatry, ophthalmology, and dermatology.