The first US jurisdictions to legalize cannabis for medical use was California in 1996, and most Americans now reside in a state with legal access to medical cannabis. Despite this relatively long history, the demographic characteristics of medical cannabis users and the conditions that they seek to treat with cannabis have remained poorly understood. This paper sought to understand the epidemiology of self-reported cannabis use for medical purposes by utilizing the International Cannabis Policy Study, a population-based survey conducted with national samples of 16–65-year-olds in Canada and the USA.

To our knowledge, this is the first USA- and Canadian-wide study that estimated prevalence of self-reported cannabis use for medical purposes across jurisdictions of different recreational and medical cannabis use policies. Prevalence and forms of recreational cannabis use had been previously published (Goodman et. al. 2020a). Results indicated a higher prevalence of self-reported cannabis use for medical purposes in the USA than in Canada, noting that the survey was conducted immediately prior to legalization of cannabis for recreational use in Canada (legal for medical use only). Prevalence rates differed significantly as a function of the legal status of cannabis. One in three respondents in states that had legal recreational cannabis sales reported medical cannabis use. Conversely, states that did not allow legal recreational or medical cannabis access had the lowest rates of cannabis use with only just above one in five for respondents indicating that they used medical cannabis. Overall, the presence of a cannabis market, recreational or medical, was associated with a higher prevalence of self-reported cannabis use for medical purposes than the absence of a market. This is consistent with data from household surveys of US adults, which has found that all forms of cannabis use are highest in states with legal recreational markets, followed by those with legal medical markets, and lowest in states with no legal access to cannabis (Carliner et al. 2017; Obradovic, 2019). While this could be an effect of legalization, it also reflects higher prevalence of use prior to legalization in many cases.

The most common medical ailment for which participants reported using cannabis was the management of pain. This may reflect the analgesic properties of cannabinoids, which have been found to produce clinically significant reductions in pain in a minority of chronic pain patients, although the proportion is only marginally greater than that in patients who received a placebo (Stockings et al. 2018). A further factor influencing the use of cannabis for pain relief may be the growing interest in its use as a substitute for opioids, prompted by the epidemic of opioid overdose deaths in the USA (Guy et al. 2017). Individuals experiencing pain may seek out medical cannabis, and doctors may avoid prescribing opiates as a result of increased regulation and monitoring of prescriptions (Chang et al. 2018).

To manage or improve symptoms of anxiety was the most common reason for use among participants who reported using medical cannabis for mental health. Our finding was consistent with an online survey of medical cannabis users registered with a Canadian licensed producer, which found that anxiety was the most common mental health condition that cannabis was prescribed to treat (Turna et al. 2019). This could be a reflection of the anxiolytic properties of cannabinoids, especially the use of CBD, which is suggested as an adjunctive treatment for anxiety or stress-related disorders (Sharpe et al. 2020). The possible anxiolytic effects of CBD and other cannabinoids are demonstrated to mediate anxiety, stress, and restlessness in several animal and human studies (Crippa et al. 2009; Fraguas-Sánchez & Torres-Suárez, 2018). However, there is also a strong positive association between cannabis use disorder and anxiety disorder, because frequent cannabis users are more likely to have anxiety disorders, and anxiety disorder patients have a higher risk to develop cannabis dependence (Kedzior & Laeber, 2014). More evidence is needed on the effectiveness of cannabis in alleviating anxiety symptoms.

Limitations

This study is subject to limitations common to survey research. Respondents were recruited using non-probability-based sampling, so the raw data do not provide nationally representative estimates. Therefore, the data were weighted by age group, sex, and region in both countries, and region-by-race in the USA. As explained in our methods, this adjusts our sample to have a similar distribution to the known population parameters. However, the study sample was somewhat more highly educated than the national population in the USA. In both countries, the ICPS sample had poorer self-reported general health compared to the national population, which is a feature of many non-probability samples (Fahimi et al. 2018). This may be partly due to the use of web surveys, which provide greater perceived anonymity than in-person or telephone-assisted interviews often used in national surveys (Hays et al. 2015). The rates of cannabis use were also somewhat higher than some national estimates, but this was likely because the ICPS sampled individuals aged 16–65 whereas the national surveys included older adults, who have lower rates of cannabis use than younger adults. The ICPS is also conducted online, whereas most national surveys are conducted in person. Compared to interviewer-assisted survey modes, self-administered surveys can reduce social desirability bias by providing greater anonymity for sensitive topics, including substance use (Dodou & de Winter, 2014; Kohut et al. 2012). The Canadian surveys were conducted in Canadian national languages of French and English. However, the US surveys were only conducted in English; therefore, the US sample may have under-represented people who did not speak English (e.g. Spanish speaking only). We excluded responses from smartphone use to avoid biases in data quality, as explained in the “Methods” section. However, our sample may have under-represented people without internet access outside of mobile devices. We did not have an adequate sample size of people who identified with non-binary gender, so our results may not be applicable in this minority population. Future research on the impacts of cannabis policy among ethnic minorities and disadvantaged groups are warranted.

Our study is a self-report survey of cannabis use for medical reasons. Individuals may self-rationalize or self-decided to use cannabis for medical reasons. This may explain why some of our participants reported using cannabis to help with some conditions that medical cannabis is not approved for, e.g. psychosis and schizophrenia. There is a distinction between authorized medical cannabis use under approval and supervisor by a health professional, compared to self-medication of cannabis for self-decided medical reasons. Consumers may define “medical use” broadly and the distinction between recreational and medical uses may be arbitrary in many cases, particularly with respect to relieving stress or enjoyment. We reported on self-reported common physical or mental health reasons for cannabis use for medical purposes. These reasons are not exhaustive. There were few rare participants who reported less common reasons; < 0.5% reported that they used medical cannabis for other reasons such as digestion, eating disorder, and attention problems. Future studies may review the list of conditions presented in the response options to ensure that common self-reported reasons for use are up-to-date. For future research, it would be important to examine whether cannabis users have received medical authorization for their self-reported cannabis use for medical purposes. This will help to illustrate the distinction between prescribed medical use, compared to a potentially large proportion of consumers who may be self-medicating for medical purposes without advice by a health professional. It would also be crucial to examine how people obtained medical cannabis in jurisdictions which did not allow legal recreational or medical cannabis access. This may allow us to differentiate risks and effects of medical cannabis obtained through legal medical prescription or other sources, so as to better inform policymakers and practitioners.

People who live in jurisdictions that have legalized cannabis use have higher exposure to cannabis marketing (Rup et al. 2020), and trends in reasons for cannabis use may be affected by promotional activities.

For example, cannabis is being promoted to reduce morning sickness and nausea to pregnant women, but cannabis use during pregnancy is associated with poorer birth outcomes (Hall et al. 2019). Goodman et al.’s (2020a, b) study on the use and perception of cannabidiol (CBD) products as part of the International Cannabis Policy Study shed light on CBD use in the USA and Canada. The marketing of CBD products and belief that CBD oil is beneficial for health are widespread, including for conditions for which there is little or no evidence of efficacy. Given the increasing ease of access to information, and misinformation, from online sources, it is important to generate evidence-based information on the benefits and risks of cannabis that are accessible to the general population. It is also important to conduct research into the uses of cannabis being promoted for by the industry and on social media to inform the communication of science-based information to the public.

Conclusion

A substantial proportion of the North American population report ever using cannabis to improve or manage symptoms of medical or mental health conditions. This includes those who live in jurisdictions where there were no legal medical cannabis markets. Use is most common among young adults who would be expected to have lower rates of the chronic medical conditions for which medical cannabis is used for, such as chronic pain, than older adults. While there is emerging evidence of therapeutic effects of cannabis for certain conditions, there are some conditions that have no empirical support for, and others that cannabis can have adverse effects (e.g. psychosis). Our findings have implications for how cannabis use are being used by the population, which may included authorized use with prescription by a health professional, and also self-defined medical use without professional guidance. Further research on the epidemiology of medical cannabis use is needed to understand populations who may experience unintended negative health outcomes from their medical cannabis use.