Public health impacts of legalizing recreational cannabis use in Canada and the US

In 2012, the citizens of the US states of Colorado and Washington passed popular ballots to legalize adult non‐medical cannabis use, and this policy was implemented in 2014 ¹ . Uruguay legalized adult cannabis use and production in 2013; Canada did so in 2018; and approximately half of US states have since done so ¹ , ² . Other countries in Europe, Asia, and Central and South America have implemented, or are considering, the legalization of adult cannabis use ¹ .

How has cannabis legalization fared in Canada and the US, where it has been in place the longest and where data have been collected on its impacts?

Most Canadian provinces and US states have adopted a modified form of alcohol regulations for legal cannabis frameworks ¹ , ² , ³ . These set an age threshold for legal use (21 years in US states and 19 years in most Canadian provinces), restrict conditions of use and possession, and licence for‐profit companies to produce and sell legal cannabis products ¹ , ² , ³ . They place more restrictions on advertising cannabis than alcohol ¹ , ³ , ⁴ , but compliance with these restrictions has been variable ⁵ .

Legalization has produced large reductions in arrests for cannabis use offences, although this has not been accompanied by a reliable reduction of relevant racial disparities in the US¹. Legalization has increased adults’ access to cannabis products that are quality‐regulated and labelled for potency ¹ . It has also increased the diversity of cannabis products; greatly reduced their prices ¹ , ² ; increased the ∆9‐tetrahydrocannabinol (THC) content of herbal cannabis and edibles, extracts and vape products ¹ , ² , ³ ; and appears to have reduced the size of the illicit cannabis retail market in Canada ¹ , ² , ³ . Its impact on illicit markets in US states has varied, depending on the size of the illicit market before legalization, the barriers faced by new entrants to the legal market, and the level of enforcement against the illicit market ¹ .

The prevalence of daily cannabis use has increased markedly among adults in the US ¹ , ⁶ and more modestly in Canada ¹ , ³ , ⁴ . There are now more daily cannabis users than daily alcohol users in the US⁶. There has been a small increase in the proportion of adults who have used cannabis, including older adults, in the US ¹ , ² . There is limited evidence that adult legalization has increased cannabis use among adolescents ¹ , ² , ³ , possibly reflecting a general reduction in the use of tobacco and alcohol by young adults in many high‐income countries, and perhaps variations in age‐of‐purchase laws in Canada².

There has been an increase in emergency department attendances for the acute adverse effects of cannabis use, that probably reflects increased regular use of more potent cannabis ¹ , ² , ⁷ , ⁸ , ⁹ . These presentations include psychiatric symptoms, psychoses, anxiety and depression, and the poisoning of children who ingest edible cannabis products that look like confectionary ¹ , ² .

It is unclear how adult legalization has affected the population prevalence of cannabis use disorders ² . Some national US and provincial Canadian surveys suggest that the prevalence of symptoms of these disorders has increased, but other surveys do not ² . Fewer people have sought treatment for cannabis use problems in addiction services in the US, probably because fewer persons are referred to treatment by courts as an alternative to criminal charges ¹ , ² , ³ . Data in Ontario suggest that more people are presenting to emergency departments for problems related to their cannabis use after the number of cannabis retail outlets has increased ⁷ , ⁸ .

The evidence on the impact of legalization on car crashes in impaired drivers is mixed ¹ , ² , ³ . Some Canadian studies have found higher blood THC levels in persons injured in road accidents ⁴ , but the overall trends in accident fatalities in both the US and Canada have not been clearly related to the implementation of cannabis legalization ¹ , ² , ³ .

In the US and Canada, there have been increased rates of cannabis use reported among women during pregnancy, probably to manage nausea ¹ . Cannabis hyperemesis syndrome has increased in very heavy cannabis smokers ² .

There are increasing concerns that adults who smoke cannabis frequently may have an increased risk of cardiovascular disease. The risk may be higher in older adults who use cannabis to treat chronic pain or sleep problems ¹ , ² .

These adverse impacts of cannabis legalization may be greater in socially disadvantaged communities ¹ . In the US, there are more retail outlets in low socioeconomic status neighborhoods, and daily cannabis use has increased more among people in these areas ¹ . These trends could amplify social disadvantage, especially if they reduce educational attainment and impair work performance ¹ .

A major public health concern is about future reductions in the regulation of cannabis sales ² , ⁵ . The legal cannabis industry has been campaigning in Canada for the right to use cannabis brand names to promote their products ² . Along with the US industry, they are campaigning for lower taxes and less regulation, allegedly to compete more effectively with the illicit market ² . In many US states, the cannabis industry has been involved in designing regulations ¹ , and has resisted proposals to regulate or differentially tax higher potency cannabis products ² . A major concern in the US is the public health impact of an end to federal cannabis prohibition, because this would enable the creation of a large national cannabis market with greatly reduced prices and a capacity to invest in other countries planning to legalize cannabis ² .

We need more rigorous studies of the effects of legalization of cannabis use in higher‐risk populations ¹ , ² , ³ , ⁴ . These include adolescents, young adults with mental health disorders, pregnant women, and older adults who use cannabis for medical reasons, such as to treat chronic pain ¹ . Unfortunately, most US states that have legalized adult cannabis have not invested in the public health research infrastructure needed to evaluate the impacts of the policy changes ¹ .

We also need better studies of the impacts of legalization on the prevalence of cannabis use disorders, and greater research investment in developing more effective interventions to assist people who want to stop using cannabis ¹ , ² . The impacts of legalization on illegal cannabis production and supply are also under‐investigated.

The challenge for policy makers will be to design regulations that balance the competing goals of cannabis legalization, i.e., reducing illicit cannabis markets, minimizing the adverse effects of criminalization, and protecting public health ¹ , ⁵ . A reasonable concern, given historical experiences with the tobacco, alcohol and gambling industries, is that governments will give a higher priority to maximizing tax revenue from cannabis sales than they do to protecting public health, and so they will be receptive to cannabis industry lobbying to reduce public health‐oriented regulation ¹ , ⁵ .