Globally, cannabis is the most widely used illicit drug [1], and cannabis use and dependence are estimated to have increased over the past two decades [2]. In 2016, the United Nations Office on Drugs and Crime estimated 192 million people aged 15–64 years used cannabis [3]. The Global Burden of Disease study estimated the age-standardized rate of cannabis use disorder in 2017 was 289.7 per 100,000 population (95% Uncertainty Interval (UI) 248.9–339.1), affecting 22.1 million people (95% UI 19.0–25.9 million) [2]. The United States and Canada are estimated to have among the highest age-standardized rates of cannabis use disorders (CUDs) in the world [2]. In Canada, the prevalence of cannabis use and CUDs have increased over the past decade, paralleling changes in the legal and political climate favoring legalization [4,5]. In 2017, 4.5 million Canadians aged 15 years or older reported past-year cannabis use, 1.5 million reported daily or near-daily use, and 6.8% met criteria for CUD [6,7].

People who use cannabinoids may seek varied effects that include relaxation, euphoria, relief from stress, increased appetite, improved sleep, and self-confidence [8]. However, a range of adverse physical and psychological consequences may also be experienced. The effects of short-term use include impaired short-term memory, impaired motor coordination, altered judgment, and—at higher doses—paranoia and psychoses [9,10]. The effects of long-term or heavy cannabinoid use include addiction, altered brain development, poor educational outcome, cognitive impairment, diminished life satisfaction and achievement, symptoms of chronic bronchitis, and increased risk of chronic psychotic disorders [9]. There are also potentially fatal harms, particularly among people who use cannabinoids regularly or who are dependent. These include risks of injuries (both unintentional and intentional, including exposure to violence), motor vehicle collisions, and suicide [9,11].

In recent years, recreational cannabis use has become increasingly decriminalized and legalized in many jurisdictions, including Canada in October 2018 [12]. Cannabis has also been legalized for recreational (and often medicinal use) in many U.S. states—including Colorado and California—while remaining a Schedule-I drug federally [13]. Despite this, illicit sales and use of cannabis continue to be prevalent, particularly among marginalized people who use illicit drugs [14].

Proponents of legalization have emphasized the potential to eradicate the marijuana black market [15], improve quality and safety control [15], increase tax revenues [16], improve the availability of medical cannabis [17] and lower gang-related drug violence [18]. As legalization regimes are established in multiple countries, public health professionals are increasingly synthesizing decades of knowledge from other policy areas to inform effective cannabis policy [13]. Conversely, opponents of legalization have stressed concerns about cannabis’ addictive potential [1], second-hand cannabis exposure [19], potential exacerbation of underlying and established mental illnesses [20], as well as alterations in perception and attitudes towards cannabis, particularly those that affect safety and driving.

Given these seemingly diametrically opposed views, the impact of legalization remains unclear. This is compounded by the fact that cannabis use is only legal in a handful of jurisdictions—including the Netherlands, Uruguay, Canada, and specific U.S. states. However, recent research involving cannabis has been increasing, and, studies measuring changes in cannabis-related outcomes before and after legalization are also on the rise. To date, there have been no reviews exploring the implications of cannabis legalization in jurisdictions where medical or recreational marijuana use is permitted.

The purpose of this review was to synthesize recent literature on this theme.