Worldwide, cannabis is the most widely used and cultivated illicit drug. In 2017, approximately 15% of the Canadian population (15 years and older) used cannabis during the previous year, with higher rates of use observed among youth and young adults [1]. In fact, approximately one-fifth of youth (15 to 19 years) and one-third of young adults (20 to 24 years) used cannabis in the past year [2].

In 2015, the Government of Canada committed to legalizing, regulating, and restricting access to cannabis. In 2016, a nine-member Task Force produced ‘A Framework for the Legalization and Regulation of Cannabis in Canada’ [3]. The Task Force applied a public health approach to the regulation of cannabis and proposed a number of recommendations that have the potential to minimize the harms of use. In 2017, Bill C-45 was introduced—an Act to amend the Controlled Drugs and Substances Act, the Criminal Code and other Acts. The legalization of recreational marijuana took place in October 2018.

The liberalization of cannabis use is partly due to its prevalence of use, social acceptability surrounding use, and the view that cannabis has a host of medicinal properties. While there is evidence for the use of cannabis to relieve symptoms of nausea, vomiting, loss of appetite, and pain in cancer patients [4–6], as well pain reduction in patients with fibromyalgia, arthritis, and neuropathic pain [7–9], there is also evidence that indicates that cannabis is not an entirely benign substance. Some of the adverse health effects of cannabis are dependent on the method of consumption. Combustion, one of the most common ways to consume cannabis, is associated with inflammation of large airways, increased airway resistance, and lung hyperinflation [10], and contains much of the same carcinogenic agents as tobacco smoke [11]. Regardless of the method of consumption, the adverse health effects listed below are the result of the primary psychoactive agent found in cannabis: delta-9-terahydrocannabinol, or THC.

An acute adverse health effect of cannabis use is nonlethal overdose. This toxic reaction occurs after an individual session of consumption. While a cannabis overdose is not known to be fatal, side effects can include severe anxiety, nausea, vomiting, psychotic episodes, or hypotension and loss of consciousness [3]. At a population level, much of the mortality and morbidity associated with cannabis use is due to traffic-related accidents involving impaired driving [12]. In Canada, it is estimated that 4 to 12% of vehicle-related fatalities and injuries involve cannabis [13]. The evidence also indicates that a cannabis-impaired driver is approximately twice as likely to be in a motor vehicle accident than former cannabis users, or non-users [14]. Further, evidence from Colorado indicates that after the legalization of recreational cannabis, impaired driving accidents increased, while the rates of accidents stayed the same in states that had not legalized cannabis over the same period of time [15]. In terms of cognitive impairment, exposure to THC during young adulthood (before the age of 25) has been shown to impair neural connectivity in adulthood [10]; an examination of regular cannabis users found decreased activity in prefrontal regions of the brain and reduced volumes of the hippocampus [16]. Cannabis use has also been associated with a variety of mental health issues, including increased risk of depression [17], anxiety [18, 19] and suicidal thoughts [20, 21]. Additionally, cannabis use in adolescence has been associated with the development of schizophrenia in adulthood [14]. Lastly, over the past two decades, cannabis dependence has been found to be the most common drug dependency after alcohol and tobacco [14]. The evidence indicates that the earlier in life and the more regular the usage, the more likely dependency will develop [10].

Despite this evidence, perceptions of the risks and harms associated with cannabis use may not be widely recognized. Previous research has shown that the adverse health effects of cannabis are not as well understood as the adverse effects of cigarettes [22, 23]. According to a 2016 survey examining perceptions of cannabis risk and harm, less than half of Canadians considered cannabis use to be more harmful than helpful to mental and physical health (42 and 41%, respectively). In addition, just over half of Canadians (58%) perceived cannabis to be addictive [24]. Thus, there is a need to address these knowledge gaps, and communicate this health information to the public.

In the domain of tobacco control, health warnings on cigarette packaging are one example of a cost-effective medium to communicate health information, given their reach and frequency of exposure at the point of purchase and during use. Health warnings have been shown to increase health knowledge, perceptions of risk, and are associated with greater motivation to quit smoking [25]. Health warnings are thought to achieve their effectiveness by eliciting a fear response, which is in line with models of health behavior including the Health Belief Model, and Protection Motivation Theory [26, 27]. In general, these models posit that emotions, such as fear, guide our decision-making; that when faced with threatening information, individuals will be motivated to engage in behaviors that protect themselves from the perceived threat. In addition to fear, the evidence from tobacco control also suggests that the believability or credibility of the message is another mechanism that underlies a warnings’ effectiveness [28, 29], and is in line with dual-process theories of attitude change such as the Elaboration Likelihood Model [30], which underscores the importance of engaging not only affective pathways, but also cognitive pathways [31].

In addition to communicating the health effects of cannabis, it is also critical to reduce product appeal and delay initiation, given the fact that cannabis use is concentrated among youth and young adults; a developmental period during which exposure to THC could result in detrimental developmental and neurocognitive effects. Plain packaging (removal of all brand imagery and graphics, uniform color and standardized font) is one example of a policy measure meant to reduce brand appeal among young people. In 2012, Australia became the first country in the world to implement plain packaging for cigarettes. Studies evaluating this policy have shown that brand appeal was reduced among adults and youth [32, 33], graphic health warnings became more effective among adult smokers [34], and that calls to the Quitline increased with the introduction of plain packaging [35]. Since then, France, the United Kingdom, New Zealand, Norway, Ireland, and Hungary have implemented plain packaging. Canada also recently committed to implementing plain packaging for cigarette packaging.

In line with these tobacco control strategies, the Framework report on cannabis regulation requires plain packaging, appropriate labelling (i.e., levels of THC and CBD must be listed on the package) and health warning labels on cannabis products. Specifically, the proposed regulations would require producers to include one of six health warnings on each package. Producers are also required to rotate through the six health warning messages in each calendar year.

Although plain packaging and health warnings have proven to be effective population-level interventions in tobacco control, it remains to be seen whether these policy measures will produce similar results in the context of cannabis use. Thus, the current study seeks to examine: [1] perceptions of plain (versus branded) cannabis packaging on ratings of product appeal, [2] the perceived effectiveness, believability, and levels of evoked fear of different health warning messages, [3] whether viewing health warnings increased levels of health knowledge, and [4] a behavioral measure of purchasing a cannabis package.