Assessing the public health impact of cannabis legalization in Canada: core outcome indicators towards an ‘index’ for monitoring and evaluation

Assessing the public health impact of cannabis legalization in Canada: core outcome indicators... Abstract The legalization of non-medical cannabis use and supply is impending in Canada. This constitutes a major policy change with the declared objective of improving public health outcomes, which requires rigorous monitoring and evaluation. While numerous different aspects associated with legalization will be examined, a focused perspective is required for effective policy evaluation purposes. To these ends, we have identified a set of 10 core indicators associated with cannabis-related risk/harm outcomes—based on current best evidence—that are expected to measure the primary impacts of legalization on public health outcomes. We briefly review these indicators, and their respective data availability in Canada. As ideally an integrated outcome assessment of cannabis legalization’s impact on public health will be available, we further propose options to merge the individual indicators into an integrated, weighted ‘index’, considering their expected relative impact for public health. One possible approach to undertake this is ‘multi-criteria decision analysis’ as a method to weight the relative indicator impact on public health; alternative approaches are proposed. The integrated ‘public health index’ for cannabis legalization will allow for scientifically comprehensive, while focused, monitoring and evaluation of the effects of legalization in Canada for the benefits of science and evidence-based policy alike. Canada, cannabis, harm, indicators, legalization, policy, public health Introduction In 2018, Canada will move to legalize, and regulate, the non-medical use and supply of cannabis.1 This will constitute a fundamental policy change—or ‘social experiment’2—for Canada, following a century of control of cannabis by means of criminal law.3 It will also be a landmark reform internationally, as Canada will be the only G-20 nation to legalize cannabis (and the second nation to do so, following Uruguay).1,4,5 Major objectives of cannabis legalization in Canada include the protection of ‘public health and safety’.1 While, by standard (e.g. burden of disease) measurements, the public health burden of cannabis is smaller than that for licit drugs (e.g. alcohol or tobacco), and other illicit substances, it is substantial by international and Canadian assessments.6–8 Given the stated objectives, it is essential to systematically monitor and evaluate the impact of legalization on public health outcomes. In general, major substance—or health—policy reforms should be rigorously evaluated for evidence-based policy principles.2,9,10 This is even more important here, as the likely impacts of cannabis legalization are uncertain, at best; experiences in other jurisdictions are mixed.11 For example, increases in select acute harm outcomes—e.g. cannabis-impaired driving, hospitalizations, poisoning calls—have been found in the US legalization states (since 2014) of Colorado and Washington.12,13 But how to meaningfully evaluate the impact of cannabis policy reform on public health, especially in a focused fashion? Since announcing the prospects of legalization in Canada, an almost uncountable array of question and data inquiries and efforts into different aspects of cannabis use and harms have been discussed and initiated, e.g. by governments, funding agencies and researchers.14 Assessments from other legalization jurisdictions, to date, also mainly present individual data or outcome pieces.12,15 However, it is unclear whether these will generate both concise while comprehensive summary information, or allow for overall conclusions on the public health impact of legalization. On this basis, we outline a core set, and subsequent ‘index’ framework, of 10 essential indicators for monitoring health impacts of cannabis legalization within the specific case study of Canada. The indicators presented are derived from currently best conceptual and empirical evidence, and documented to be—directly or indirectly—linked to population-level health harms,16–18 thus comprising essential outcomes for public health and a focused ‘gauge’ on legalization impact.9,16 We believe that if this core set of indicators—combined with the weighted ‘index’ laid out—is monitored over a sufficient examination period, the empirical impact of cannabis legalization on public health in Canada can be meaningfully ascertained for both science and policy-making purposes. Cannabis use prevalence A common question concerning the consequences of legalization is whether cannabis use in the population will increase. While ‘use’ is not a tangible harm in itself,16 it constitutes the necessary exposure to any possible cannabis-related harm, and should be measured on this basis alone. In the USA, cannabis use rates (including among young people), have been found higher among jurisdictions with liberalized (legal) cannabis regimes (e.g. through ‘medical marijuana’) compared to non-legalization states; however, these appear to be mainly ecological rather than representing causal effects.19,20 Importantly—and specifically in Canada—current (e.g. past-year) cannabis use prevalence has fluctuated in the Canadian adult population (e.g. from 7.4% (1994) to 14.1% (2004))21,22 despite consistent policy parameters. Distinctly relevant for cannabis use levels in the population is use initiation among young people. Specifically, ‘age of initiation’ is a strong predictor of subsequent cannabis-related problems, as inversely related to risk for future acute and chronic (e.g. brain functioning, mental health, educational) problem outcomes.23–27 Hence, in addition to cannabis use prevalence in the population, age of use initiation should be a focus of monitoring. In Canada—national and provincial—general (e.g. adult, youth/student) population surveys can provide relevant data.22,28,29 Patterns of use Just like for other psychoactive substance use (e.g. alcohol), harm outcomes are unequally distributed across cannabis users; rather, specific patterns of use are recognized as a major predictor of acute and chronic adverse outcomes.30,31 Specifically, intensive or frequent (e.g. daily or near-daily) patterns of cannabis use are associated with increased risk for brain functioning and mental health problems, dependence, and other outcomes.30,32–36 Various survey data (e.g. among North American adults/adolescents) show that the proportion of frequent (i.e. daily/near-daily) cannabis users has almost doubled since the early 2000’s.21,22,37 Thus, while legalization may not influence the overall prevalence of use, it—for example, through changing cannabis availability and/or social norms—may lead to increases in use patterns, and subsequent population-level harm outcomes.9,38,39 Frequency patterns of cannabis use can be measured with relevant items in general population surveys; these items, however, have been defined inconsistently in surveys in the past. Modes of use Modes of cannabis use have greatly diversified in recent years.40 While, for long, the predominant mode of cannabis use occurred by smoking burnt cannabis products (e.g. a ‘joint’), various alternative use modes have become popular,41,42 including inhalation-based (e.g. vaporizer/e-cigarette utilizing electronic processes) as well as non-inhalational (e.g. edible/drinkable) modes.40 Smoking ‘burnt’ cannabis remains the most common use mode according to (US- and Canada-based) survey data, with however, up to one in three users reporting alternative (non-smoking) use modes.29,41,43 Cannabis use modes influence key public health-relevant outcomes. Specifically, cannabis ‘smoking’ is associated with a variety of acute and chronic bronchial/pulmonary problems, and may independently be associated with lung cancer.44–46 While available non-smoking use modes come with some of their own distinct risks (e.g. specific toxins, psychoactive delay and over-use effects), and more rigorous studies are needed to compare inter-mode outcomes, some of these alternatives (e.g. vaporized or edible cannabis use) can overall be assumed to be safer at least for pulmonary-bronchial health outcomes.40 Based on its mandate for direct education and regulation, legalization may influence and promote (safer) use modes among users.16,40 Monitoring of cannabis use modes can occur both based on user surveys, as well as retail distribution/sales data for different (e.g., mode-specific) cannabis products. Cannabis potency Potency of cannabis products—i.e. primarily its tetrahydrocannabinol (THC) content concentration—is positively associated with the risk for severe (acute and chronic) adverse outcomes.31,47,48 Concretely, high-potency cannabis is associated with a 3-fold risk for a psychotic disorder.49–53 Despite contexts of varying cannabis control regimes, cannabis potency levels have substantially increased in different jurisdictions (including North America but also Europe) in recent decades.54–58 For example, average THC values for cannabis flower products have increased from ~4 to ~12% or higher in the US 1994–2014; similar trends have been reported from Europe;55,57 newer THC extract (concentrate) products (e.g. ‘shatter’) reach THC concentrations of up to 80–90%.59,60 Similarly, in Canada, limited evidence indicates current average THC concentrations ranging between 15 and 20%.61–63 At the same time, the psychoactive potency of cannabis products may be moderated by other cannabinoid components (e.g. CBD).64–66 Furthermore, the possibility of users ‘titrating’ higher-potency cannabis, and thus reducing their dose-intake, exists, yet evidence is limited.67 Overall, the potency of cannabis products consumed is an important indicator to measure for public health outcomes; basic information can be monitored both from (legal) product sales information and/or population survey data (i.e., legal and illegal products used). Cannabis product sourcing A large extent of legalization’s expected public health benefits rests on the assumption that users will switch from illegal (e.g. ‘black market’) to legal (e.g. retail) sources to obtain regulated and quality-controlled cannabis products.1,16 For long before legalization, illegal cannabis markets have been well-established and active in Canada, and were recently (2015) estimated to be more than $6 billion in value;68–70 even for minors, accessing cannabis products is as easy as obtaining alcohol or tobacco.28,37 Moreover, illicit cannabis markets are not bound by regulatory restrictions, and are highly adaptable.10,71 Thus, it cannot be safely assumed to which extent users will switch from illegal to legal cannabis sources under legalization, for various factors that may include product availability, pricing, access and other restrictions or regulations (as well as potentially other subjective or ‘cultural’ factors).72 Recent survey data indicate that among a convenience sample of self-identified users from US legalization states, 45% were still purchasing cannabis from the black market.73 A failure to bring at least a substantial majority of cannabis users into legal cannabis distribution may crucially undermine the potential to realize the desired benefits of legalization.10,74 This, hence, is a key indicator for monitoring for which relevant data can be generated from both (user) surveys, as well as licit distribution (sales) and illicit market (e.g. enforcement) data. Cannabis-impaired driving and injuries Seminal studies indicate that acute cannabis-impairment—i.e. as occurring shortly following use—is associated with moderately elevated risk (factor 2–4) for motor-vehicle accident involvement, and related injuries and/or fatalities;75–77 these risks are further elevated in combination with alcohol.78,79 This makes cannabis-impaired driving a - while rare - direct cannabis-related cause of mortality.8,80,81 In Canada, approximately 3% of general population adults, but as many as 9% among young drivers (or about one in three cannabis users in the respective age groups), report (past-year) cannabis-impaired driving.29,82–85 Roadside driver and emergency room admission samples indicate somewhat higher rates of cannabis-exposure, respectively,86,87 and recent estimates found cannabis-impaired driving to be a lead contributor of cannabis-attributable disease burden for Canada (both morbidity/mortality counts as well as DALYs).8,80,81 With this major impact potential, it is uncertain how cannabis-impaired driving will evolve under legalization. While greater cannabis availability may lead to increases, intensified targeted interventions (e.g. prevention or enforcement) may lead to reductions in cannabis-impaired driving and subsequent harm.88,89 In US legalization states, increases in cannabis-impaired driving injuries have been observed.12 To monitor these key harm indicators, general population and driver (roadside) surveys, as well as emergency room and accident fatality data (e.g. from coroner’s reports) can be utilized. Hospitalizations (including poisonings) Cannabis use can lead to severe acute and/or chronic morbidity requiring hospitalization (including emergency departments), for example, psychotic symptoms, poisonings and accident-related injuries.17,18 These, due to severity and consequences, are important contributors to, and indicators of, the cannabis-related public health burden. A California-based study documented that all-cause hospitalization rates among cannabis users were ~50% elevated compared to non-users;90 European data indicate similarly elevated rates in cannabis-related hospitalizations, primarily for mental health outcomes.91,92 In Colorado, substantial (i.e. >doubling) increases in cannabis-related hospitalizations have occurred through different steps (i.e., medical and recreational) of cannabis legalization (2001–13).12,93 In addition, cannabis-related poison center calls have increased in states with legalized cannabis.93–95 In Canada, rates of cannabis-related hospital separations have increased pre-legalization, from 4.64 to 6.49/100 000 population (2011/12–2015/16);96 select provincial data also indicate increases in cannabis-related poison center calls.97 In Canada’s single-payer/public healthcare system, data on cannabis-related hospitalizations are available from respective (provincial) administrative healthcare utilization databases, based on respective ICD-codes and established data collection mechanisms (e.g. the National Ambulatory Care Reporting System [NACRS]); poison call data can be available from provincially mandated poison centers. Cannabis use disorders A serious adverse (chronic) cannabis use outcome is cannabis use disorder (CUD, including dependence), measured by standardized diagnostic tools.98–100 Typically, CUD is a consequence of intensive (e.g. chronic/frequent and/or high-potency) cannabis use and requires professional treatment.98 Older estimates suggest that about 1-in-10 cannabis users develop dependence; recent (e.g. US-based) estimates indicate that as many as one-in-three current users meet CUD criteria.101,102 Several (e.g. European) jurisdictions indicated recent increases in CUD treatment demand, especially among young people.18,103,104 While CUD has been estimated to be a main cannabis-related burden of disease contributor,8,80 cannabis-related treatment demand data in Canada are sporadic and inconsistent. Among (publicly funded) treatment agencies in Ontario, the number of cannabis-related treatment admissions has been stable at ~30 000 annual cases (2007/08–2015/16).105,106 In Nova Scotia, this rate however has increased by about 50% (2009/10–2011/12).107,108 Given the severity of CUD as both a health outcome and considering its intervention needs, it is a key public health indicator to monitor post-legalization, where population-level use may become more intensive and thus problematic. In Canada, only select provincial, but no national, databases limited to publicly funded treatment services exist,106,109,110 implying limitations on data scope and quality for monitoring. Other psychoactive substance use Beyond direct cannabis use-related health outcomes, there is good reason to assess possible indirect consequences on population-level health outcomes under legalization. A key issue, specifically, concerns how availability of legal cannabis may impact the risky use of, or morbidity/mortality harm associated with, other commonly used psychoactive substances (e.g. [prescription] opioids, alcohol or tobacco) through so-called ‘substitution’ effects; conversely (undesirable) ‘complementarity’ effects may occur.111,112 Concretely, US-based studies documented that increased (legal) cannabis availability through medical (13 states, 1999–2010) or recreational (Colorado, 2000–15) legalization has been associated with reductions in opioid-related mortality,113–115 although these studies are largely ecological .116 Furthermore, associations with reductions in opioid-related hospitalizations and driving fatalities have been observed.117,118 For alcohol, increased cannabis use may be associated with reductions in risky drinking as well as injuries (e.g. driving-fatalities), however, existing evidence is mixed and inconsistent, and rigorous studies are lacking.112,119–122 Furthermore, there is some—yet also inconsistent—evidence that increases in legal cannabis supply and use may be associated with increasing tobacco (mainly co-) use.123–125 In Canada, no systematic population-level data on either outcome exist. As the aforementioned substance groups (e.g. opioids, alcohol and tobacco) are exerting extensive public health tolls, respective substitution effects might be beneficial for public health; these may however be outweighed by adverse complementarity effects (e.g. from tobacco use) or by increases in cannabis-related disease burden.120,122,126 Over-time analyses in respective general population survey, or injury/fatality datasets, would allow for ecological assessments of possible inter-substance use and harm outcomes; more rigorous analyses could be conducted with longitudinal cohort/population samples. Cannabis-related ‘harm-to-others’ While the focus of most cannabis-related harms is on outcomes with users, an important complementary perspective is ‘harm-to-others’, i.e. ‘non-users’ or the social environment. The ‘harm-to-others’-concept has become well-established for other substance (e.g. alcohol and tobacco) use realms, and can include health harms, e.g. injuries or disease (e.g. from violence or smoke), or quality-of-life hazards, occurring with others.127–129 The concept’s relevance for cannabis use is underscored by the fact that greater than four-in-five Canadians are current non-users,22 yet potentially vulnerable to related harms. While cannabis’ main effects, based on its distinct pharmaco-behavioral properties (unlike other psychoactive substances) do not involve extensive aggression or violence, its impairment or other use consequences (e.g. cannabis smoking) may result in substantial inconvenience or harm-to-others.130–132 A specific ‘harm-to-others’ consequence may include development issues among un-/new-born infants of cannabis-using mothers, although evidence is limited.133,134 Potential ‘harm-to-others’ dynamics are illustrated in emerging Canadian (provincial) regulations for permitted cannabis use locations, with some restricted to ‘private homes’ only.74,135 Particularly with legalization policy aiming for ‘public’ health and safety, ‘harm-to-others’ constitutes an essential element for outcome monitoring.1,71 Its assessment can occur through relevant items in general population surveys (including both users and non-users), similar to what has been used for alcohol, yet also comprise other ‘social cost’ perspectives.136 Specific types of cannabis-related harm-to-others (e.g. injuries, harm to infants) would require data which is not easily or readily available with respective diagnostics. Discussion Main finding The legalization of non-medical cannabis use is imminent in Canada, following similar reforms elsewhere. While it continues to be controversially discussed, primary objectives of cannabis legalization as a major policy reform include the protection of public health and safety in Canada. Also in the context of (mixed) experiences with similar reforms elsewhere to date,137 and given the various—acute and chronic—well-documented health risks associated with cannabis use,17 the outcomes of the legalization policy ‘experiment’ on public health need to be rigorously monitored and evaluated. To that end, we have presented 10 groups of principal indicators which, based on current information, will express and allow the assessment of the main impacts of cannabis legalization on public health relevant risks and outcomes in the population. This, ideally, will occur based on data including baseline (i.e. pre-legalization) data and trends, yet still be valuable if mainly including longitudinal observations post-legalization. On this basis, implementing these indicators will allow for both a comprehensive but focused monitoring and evaluation effort for the impacts of cannabis legalization on public health. What is already known on this topic The risks for acute and chronic health harms associated with cannabis use are well documented.17 While, on population levels, many of these have substantially evolved under relatively consistent control conditions (e.g. prohibition), there is good reason to assume that they may further change with legal cannabis availability and use. To that extent, ongoing cannabis legalization experiments—e.g. in US states or Uruguay—have shown rather mixed outcomes, i.e. with certain health outcome indicators such as cannabis-related treatment admissions remaining stable, but others, such as cannabis-related accidents and hospitalizations increasing.12,137 In Canada, many such indicators will be examined individually and/or within select sub-groups. Neither the ongoing international legalization experiments, nor the—many—planned Canadian outcome evaluation efforts, however, have presented or developed monitoring approaches to assess or gauge the impact of legalization on public health overall. What this study adds On their own, the here proposed outcome indicators still consist of individual measures which may see differential evolvement—partly independent—of one another, and in themselves will not yet provide an integrated assessment of public health impacts of legalization (as policy-makers ideally utilize for evidence-based policy assessments).138–140 Furthermore, each of the indicators features (quantitatively) differential potential impact on the public health burden. Thus, towards further meaningful development and utilization of the public health indicators presented, we propose for these to be integrated into a weighted ‘index’ to measure the impact of cannabis legalization on public health. This should involve scaling (for inter-indicator comparability) and relative weighting of the individual indicators, considering the expected relative individual impact of each on overall public health burden.141,142 On this basis, monitoring data can then be combined into ongoing weighted index values (e.g. for regular, annual, measurements).141,143 For example, it can be expected that ‘cannabis use disorder’ or ‘cannabis-impaired driving’ outcomes will be associated with substantially higher public health burden than cannabis-related ‘harm-to-others’ or ‘use modes’; this relative ‘weighting’ ought to be defined for each measure towards, and reflected in, an integrated index. Various scientifically established methods for such weighted indexing exist. If the final outcome is unidimensional—such as burden of disease, which is usually measured as disability-adjusted life years (a summary indicator composed of years of life lost due to premature mortality and disability)144—an index can be construed via epidemiologic modeling. This index can subsequently be re-assessed and compared, if the associations between indicators and outcome, and the assumptions behind them, remain stable. If more than one dimension is involved, human judgment and valuations about relative impact explicitly come into play. Another option is to assemble an index with the help of ‘multi-criteria decision analysis’ (MCDA).145–147 MCDA involves (at times subjective) expert judgment and valuations, and is usually applied to consider and integrate different factors towards policy-relevant decisions or outcomes, e.g. in health or other policy settings.148,149 MCDA has also been applied in the ‘drug policy’ field, to compare and rank the harm potential of different psychoactive substances based on multiple criteria.142,150 Independent of the chosen approach to combine and integrate the above indicator dimensions into a composite index, actual values, and over-time changes for each of the indicators could be weight-scored over regular monitoring points, and so allow for longitudinal integrated assessments of public health outcomes following cannabis legalization. Limitations The present study presents a conceptual framework for empirical measurement. The proposed indicators are data-driven but selective, and may not capture the full public health impact associated with cannabis legalization. Indicator changes to be observed may include ecological effects. Proposed indicator integration and indexing may involve subjective judgments for weighting towards approximated composite outcome assessments. Conclusions Cannabis legalization towards public health ends—like other major policy reforms—requires rigorous monitoring and evaluation, as to whether the main policy objectives are met and/or adjustments are needed.10,71,151 Numerous individual—and likely inconsistent—outcomes associated with legalization will be observed, however, a scientifically solid ‘big picture’ assessment of legalization’s impact on public health will be required in Canada for evidence-based policy evaluation.138,139 The proposed 10 key outcome indicators, and options for integration into a cannabis public health ‘indicator index’, offer such a—timely—tool that is relatively easy and feasible to develop and implement towards these ends. Funding This work was supported by the Canadian Institutes of Health Research (CIHR) for the Canadian Research Initiative in Substance Misuse (CRISM) Ontario Node Team [Grant #SMN-139150], and by the Chair in Addiction, Department of Psychiatry, University of Toronto. 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Assessing the public health impact of cannabis legalization in Canada: core outcome indicators towards an ‘index’ for monitoring and evaluation

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© The Author(s) 2018. Published by Oxford University Press on behalf of Faculty of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com
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Abstract

Abstract The legalization of non-medical cannabis use and supply is impending in Canada. This constitutes a major policy change with the declared objective of improving public health outcomes, which requires rigorous monitoring and evaluation. While numerous different aspects associated with legalization will be examined, a focused perspective is required for effective policy evaluation purposes. To these ends, we have identified a set of 10 core indicators associated with cannabis-related risk/harm outcomes—based on current best evidence—that are expected to measure the primary impacts of legalization on public health outcomes. We briefly review these indicators, and their respective data availability in Canada. As ideally an integrated outcome assessment of cannabis legalization’s impact on public health will be available, we further propose options to merge the individual indicators into an integrated, weighted ‘index’, considering their expected relative impact for public health. One possible approach to undertake this is ‘multi-criteria decision analysis’ as a method to weight the relative indicator impact on public health; alternative approaches are proposed. The integrated ‘public health index’ for cannabis legalization will allow for scientifically comprehensive, while focused, monitoring and evaluation of the effects of legalization in Canada for the benefits of science and evidence-based policy alike. Canada, cannabis, harm, indicators, legalization, policy, public health Introduction In 2018, Canada will move to legalize, and regulate, the non-medical use and supply of cannabis.1 This will constitute a fundamental policy change—or ‘social experiment’2—for Canada, following a century of control of cannabis by means of criminal law.3 It will also be a landmark reform internationally, as Canada will be the only G-20 nation to legalize cannabis (and the second nation to do so, following Uruguay).1,4,5 Major objectives of cannabis legalization in Canada include the protection of ‘public health and safety’.1 While, by standard (e.g. burden of disease) measurements, the public health burden of cannabis is smaller than that for licit drugs (e.g. alcohol or tobacco), and other illicit substances, it is substantial by international and Canadian assessments.6–8 Given the stated objectives, it is essential to systematically monitor and evaluate the impact of legalization on public health outcomes. In general, major substance—or health—policy reforms should be rigorously evaluated for evidence-based policy principles.2,9,10 This is even more important here, as the likely impacts of cannabis legalization are uncertain, at best; experiences in other jurisdictions are mixed.11 For example, increases in select acute harm outcomes—e.g. cannabis-impaired driving, hospitalizations, poisoning calls—have been found in the US legalization states (since 2014) of Colorado and Washington.12,13 But how to meaningfully evaluate the impact of cannabis policy reform on public health, especially in a focused fashion? Since announcing the prospects of legalization in Canada, an almost uncountable array of question and data inquiries and efforts into different aspects of cannabis use and harms have been discussed and initiated, e.g. by governments, funding agencies and researchers.14 Assessments from other legalization jurisdictions, to date, also mainly present individual data or outcome pieces.12,15 However, it is unclear whether these will generate both concise while comprehensive summary information, or allow for overall conclusions on the public health impact of legalization. On this basis, we outline a core set, and subsequent ‘index’ framework, of 10 essential indicators for monitoring health impacts of cannabis legalization within the specific case study of Canada. The indicators presented are derived from currently best conceptual and empirical evidence, and documented to be—directly or indirectly—linked to population-level health harms,16–18 thus comprising essential outcomes for public health and a focused ‘gauge’ on legalization impact.9,16 We believe that if this core set of indicators—combined with the weighted ‘index’ laid out—is monitored over a sufficient examination period, the empirical impact of cannabis legalization on public health in Canada can be meaningfully ascertained for both science and policy-making purposes. Cannabis use prevalence A common question concerning the consequences of legalization is whether cannabis use in the population will increase. While ‘use’ is not a tangible harm in itself,16 it constitutes the necessary exposure to any possible cannabis-related harm, and should be measured on this basis alone. In the USA, cannabis use rates (including among young people), have been found higher among jurisdictions with liberalized (legal) cannabis regimes (e.g. through ‘medical marijuana’) compared to non-legalization states; however, these appear to be mainly ecological rather than representing causal effects.19,20 Importantly—and specifically in Canada—current (e.g. past-year) cannabis use prevalence has fluctuated in the Canadian adult population (e.g. from 7.4% (1994) to 14.1% (2004))21,22 despite consistent policy parameters. Distinctly relevant for cannabis use levels in the population is use initiation among young people. Specifically, ‘age of initiation’ is a strong predictor of subsequent cannabis-related problems, as inversely related to risk for future acute and chronic (e.g. brain functioning, mental health, educational) problem outcomes.23–27 Hence, in addition to cannabis use prevalence in the population, age of use initiation should be a focus of monitoring. In Canada—national and provincial—general (e.g. adult, youth/student) population surveys can provide relevant data.22,28,29 Patterns of use Just like for other psychoactive substance use (e.g. alcohol), harm outcomes are unequally distributed across cannabis users; rather, specific patterns of use are recognized as a major predictor of acute and chronic adverse outcomes.30,31 Specifically, intensive or frequent (e.g. daily or near-daily) patterns of cannabis use are associated with increased risk for brain functioning and mental health problems, dependence, and other outcomes.30,32–36 Various survey data (e.g. among North American adults/adolescents) show that the proportion of frequent (i.e. daily/near-daily) cannabis users has almost doubled since the early 2000’s.21,22,37 Thus, while legalization may not influence the overall prevalence of use, it—for example, through changing cannabis availability and/or social norms—may lead to increases in use patterns, and subsequent population-level harm outcomes.9,38,39 Frequency patterns of cannabis use can be measured with relevant items in general population surveys; these items, however, have been defined inconsistently in surveys in the past. Modes of use Modes of cannabis use have greatly diversified in recent years.40 While, for long, the predominant mode of cannabis use occurred by smoking burnt cannabis products (e.g. a ‘joint’), various alternative use modes have become popular,41,42 including inhalation-based (e.g. vaporizer/e-cigarette utilizing electronic processes) as well as non-inhalational (e.g. edible/drinkable) modes.40 Smoking ‘burnt’ cannabis remains the most common use mode according to (US- and Canada-based) survey data, with however, up to one in three users reporting alternative (non-smoking) use modes.29,41,43 Cannabis use modes influence key public health-relevant outcomes. Specifically, cannabis ‘smoking’ is associated with a variety of acute and chronic bronchial/pulmonary problems, and may independently be associated with lung cancer.44–46 While available non-smoking use modes come with some of their own distinct risks (e.g. specific toxins, psychoactive delay and over-use effects), and more rigorous studies are needed to compare inter-mode outcomes, some of these alternatives (e.g. vaporized or edible cannabis use) can overall be assumed to be safer at least for pulmonary-bronchial health outcomes.40 Based on its mandate for direct education and regulation, legalization may influence and promote (safer) use modes among users.16,40 Monitoring of cannabis use modes can occur both based on user surveys, as well as retail distribution/sales data for different (e.g., mode-specific) cannabis products. Cannabis potency Potency of cannabis products—i.e. primarily its tetrahydrocannabinol (THC) content concentration—is positively associated with the risk for severe (acute and chronic) adverse outcomes.31,47,48 Concretely, high-potency cannabis is associated with a 3-fold risk for a psychotic disorder.49–53 Despite contexts of varying cannabis control regimes, cannabis potency levels have substantially increased in different jurisdictions (including North America but also Europe) in recent decades.54–58 For example, average THC values for cannabis flower products have increased from ~4 to ~12% or higher in the US 1994–2014; similar trends have been reported from Europe;55,57 newer THC extract (concentrate) products (e.g. ‘shatter’) reach THC concentrations of up to 80–90%.59,60 Similarly, in Canada, limited evidence indicates current average THC concentrations ranging between 15 and 20%.61–63 At the same time, the psychoactive potency of cannabis products may be moderated by other cannabinoid components (e.g. CBD).64–66 Furthermore, the possibility of users ‘titrating’ higher-potency cannabis, and thus reducing their dose-intake, exists, yet evidence is limited.67 Overall, the potency of cannabis products consumed is an important indicator to measure for public health outcomes; basic information can be monitored both from (legal) product sales information and/or population survey data (i.e., legal and illegal products used). Cannabis product sourcing A large extent of legalization’s expected public health benefits rests on the assumption that users will switch from illegal (e.g. ‘black market’) to legal (e.g. retail) sources to obtain regulated and quality-controlled cannabis products.1,16 For long before legalization, illegal cannabis markets have been well-established and active in Canada, and were recently (2015) estimated to be more than $6 billion in value;68–70 even for minors, accessing cannabis products is as easy as obtaining alcohol or tobacco.28,37 Moreover, illicit cannabis markets are not bound by regulatory restrictions, and are highly adaptable.10,71 Thus, it cannot be safely assumed to which extent users will switch from illegal to legal cannabis sources under legalization, for various factors that may include product availability, pricing, access and other restrictions or regulations (as well as potentially other subjective or ‘cultural’ factors).72 Recent survey data indicate that among a convenience sample of self-identified users from US legalization states, 45% were still purchasing cannabis from the black market.73 A failure to bring at least a substantial majority of cannabis users into legal cannabis distribution may crucially undermine the potential to realize the desired benefits of legalization.10,74 This, hence, is a key indicator for monitoring for which relevant data can be generated from both (user) surveys, as well as licit distribution (sales) and illicit market (e.g. enforcement) data. Cannabis-impaired driving and injuries Seminal studies indicate that acute cannabis-impairment—i.e. as occurring shortly following use—is associated with moderately elevated risk (factor 2–4) for motor-vehicle accident involvement, and related injuries and/or fatalities;75–77 these risks are further elevated in combination with alcohol.78,79 This makes cannabis-impaired driving a - while rare - direct cannabis-related cause of mortality.8,80,81 In Canada, approximately 3% of general population adults, but as many as 9% among young drivers (or about one in three cannabis users in the respective age groups), report (past-year) cannabis-impaired driving.29,82–85 Roadside driver and emergency room admission samples indicate somewhat higher rates of cannabis-exposure, respectively,86,87 and recent estimates found cannabis-impaired driving to be a lead contributor of cannabis-attributable disease burden for Canada (both morbidity/mortality counts as well as DALYs).8,80,81 With this major impact potential, it is uncertain how cannabis-impaired driving will evolve under legalization. While greater cannabis availability may lead to increases, intensified targeted interventions (e.g. prevention or enforcement) may lead to reductions in cannabis-impaired driving and subsequent harm.88,89 In US legalization states, increases in cannabis-impaired driving injuries have been observed.12 To monitor these key harm indicators, general population and driver (roadside) surveys, as well as emergency room and accident fatality data (e.g. from coroner’s reports) can be utilized. Hospitalizations (including poisonings) Cannabis use can lead to severe acute and/or chronic morbidity requiring hospitalization (including emergency departments), for example, psychotic symptoms, poisonings and accident-related injuries.17,18 These, due to severity and consequences, are important contributors to, and indicators of, the cannabis-related public health burden. A California-based study documented that all-cause hospitalization rates among cannabis users were ~50% elevated compared to non-users;90 European data indicate similarly elevated rates in cannabis-related hospitalizations, primarily for mental health outcomes.91,92 In Colorado, substantial (i.e. >doubling) increases in cannabis-related hospitalizations have occurred through different steps (i.e., medical and recreational) of cannabis legalization (2001–13).12,93 In addition, cannabis-related poison center calls have increased in states with legalized cannabis.93–95 In Canada, rates of cannabis-related hospital separations have increased pre-legalization, from 4.64 to 6.49/100 000 population (2011/12–2015/16);96 select provincial data also indicate increases in cannabis-related poison center calls.97 In Canada’s single-payer/public healthcare system, data on cannabis-related hospitalizations are available from respective (provincial) administrative healthcare utilization databases, based on respective ICD-codes and established data collection mechanisms (e.g. the National Ambulatory Care Reporting System [NACRS]); poison call data can be available from provincially mandated poison centers. Cannabis use disorders A serious adverse (chronic) cannabis use outcome is cannabis use disorder (CUD, including dependence), measured by standardized diagnostic tools.98–100 Typically, CUD is a consequence of intensive (e.g. chronic/frequent and/or high-potency) cannabis use and requires professional treatment.98 Older estimates suggest that about 1-in-10 cannabis users develop dependence; recent (e.g. US-based) estimates indicate that as many as one-in-three current users meet CUD criteria.101,102 Several (e.g. European) jurisdictions indicated recent increases in CUD treatment demand, especially among young people.18,103,104 While CUD has been estimated to be a main cannabis-related burden of disease contributor,8,80 cannabis-related treatment demand data in Canada are sporadic and inconsistent. Among (publicly funded) treatment agencies in Ontario, the number of cannabis-related treatment admissions has been stable at ~30 000 annual cases (2007/08–2015/16).105,106 In Nova Scotia, this rate however has increased by about 50% (2009/10–2011/12).107,108 Given the severity of CUD as both a health outcome and considering its intervention needs, it is a key public health indicator to monitor post-legalization, where population-level use may become more intensive and thus problematic. In Canada, only select provincial, but no national, databases limited to publicly funded treatment services exist,106,109,110 implying limitations on data scope and quality for monitoring. Other psychoactive substance use Beyond direct cannabis use-related health outcomes, there is good reason to assess possible indirect consequences on population-level health outcomes under legalization. A key issue, specifically, concerns how availability of legal cannabis may impact the risky use of, or morbidity/mortality harm associated with, other commonly used psychoactive substances (e.g. [prescription] opioids, alcohol or tobacco) through so-called ‘substitution’ effects; conversely (undesirable) ‘complementarity’ effects may occur.111,112 Concretely, US-based studies documented that increased (legal) cannabis availability through medical (13 states, 1999–2010) or recreational (Colorado, 2000–15) legalization has been associated with reductions in opioid-related mortality,113–115 although these studies are largely ecological .116 Furthermore, associations with reductions in opioid-related hospitalizations and driving fatalities have been observed.117,118 For alcohol, increased cannabis use may be associated with reductions in risky drinking as well as injuries (e.g. driving-fatalities), however, existing evidence is mixed and inconsistent, and rigorous studies are lacking.112,119–122 Furthermore, there is some—yet also inconsistent—evidence that increases in legal cannabis supply and use may be associated with increasing tobacco (mainly co-) use.123–125 In Canada, no systematic population-level data on either outcome exist. As the aforementioned substance groups (e.g. opioids, alcohol and tobacco) are exerting extensive public health tolls, respective substitution effects might be beneficial for public health; these may however be outweighed by adverse complementarity effects (e.g. from tobacco use) or by increases in cannabis-related disease burden.120,122,126 Over-time analyses in respective general population survey, or injury/fatality datasets, would allow for ecological assessments of possible inter-substance use and harm outcomes; more rigorous analyses could be conducted with longitudinal cohort/population samples. Cannabis-related ‘harm-to-others’ While the focus of most cannabis-related harms is on outcomes with users, an important complementary perspective is ‘harm-to-others’, i.e. ‘non-users’ or the social environment. The ‘harm-to-others’-concept has become well-established for other substance (e.g. alcohol and tobacco) use realms, and can include health harms, e.g. injuries or disease (e.g. from violence or smoke), or quality-of-life hazards, occurring with others.127–129 The concept’s relevance for cannabis use is underscored by the fact that greater than four-in-five Canadians are current non-users,22 yet potentially vulnerable to related harms. While cannabis’ main effects, based on its distinct pharmaco-behavioral properties (unlike other psychoactive substances) do not involve extensive aggression or violence, its impairment or other use consequences (e.g. cannabis smoking) may result in substantial inconvenience or harm-to-others.130–132 A specific ‘harm-to-others’ consequence may include development issues among un-/new-born infants of cannabis-using mothers, although evidence is limited.133,134 Potential ‘harm-to-others’ dynamics are illustrated in emerging Canadian (provincial) regulations for permitted cannabis use locations, with some restricted to ‘private homes’ only.74,135 Particularly with legalization policy aiming for ‘public’ health and safety, ‘harm-to-others’ constitutes an essential element for outcome monitoring.1,71 Its assessment can occur through relevant items in general population surveys (including both users and non-users), similar to what has been used for alcohol, yet also comprise other ‘social cost’ perspectives.136 Specific types of cannabis-related harm-to-others (e.g. injuries, harm to infants) would require data which is not easily or readily available with respective diagnostics. Discussion Main finding The legalization of non-medical cannabis use is imminent in Canada, following similar reforms elsewhere. While it continues to be controversially discussed, primary objectives of cannabis legalization as a major policy reform include the protection of public health and safety in Canada. Also in the context of (mixed) experiences with similar reforms elsewhere to date,137 and given the various—acute and chronic—well-documented health risks associated with cannabis use,17 the outcomes of the legalization policy ‘experiment’ on public health need to be rigorously monitored and evaluated. To that end, we have presented 10 groups of principal indicators which, based on current information, will express and allow the assessment of the main impacts of cannabis legalization on public health relevant risks and outcomes in the population. This, ideally, will occur based on data including baseline (i.e. pre-legalization) data and trends, yet still be valuable if mainly including longitudinal observations post-legalization. On this basis, implementing these indicators will allow for both a comprehensive but focused monitoring and evaluation effort for the impacts of cannabis legalization on public health. What is already known on this topic The risks for acute and chronic health harms associated with cannabis use are well documented.17 While, on population levels, many of these have substantially evolved under relatively consistent control conditions (e.g. prohibition), there is good reason to assume that they may further change with legal cannabis availability and use. To that extent, ongoing cannabis legalization experiments—e.g. in US states or Uruguay—have shown rather mixed outcomes, i.e. with certain health outcome indicators such as cannabis-related treatment admissions remaining stable, but others, such as cannabis-related accidents and hospitalizations increasing.12,137 In Canada, many such indicators will be examined individually and/or within select sub-groups. Neither the ongoing international legalization experiments, nor the—many—planned Canadian outcome evaluation efforts, however, have presented or developed monitoring approaches to assess or gauge the impact of legalization on public health overall. What this study adds On their own, the here proposed outcome indicators still consist of individual measures which may see differential evolvement—partly independent—of one another, and in themselves will not yet provide an integrated assessment of public health impacts of legalization (as policy-makers ideally utilize for evidence-based policy assessments).138–140 Furthermore, each of the indicators features (quantitatively) differential potential impact on the public health burden. Thus, towards further meaningful development and utilization of the public health indicators presented, we propose for these to be integrated into a weighted ‘index’ to measure the impact of cannabis legalization on public health. This should involve scaling (for inter-indicator comparability) and relative weighting of the individual indicators, considering the expected relative individual impact of each on overall public health burden.141,142 On this basis, monitoring data can then be combined into ongoing weighted index values (e.g. for regular, annual, measurements).141,143 For example, it can be expected that ‘cannabis use disorder’ or ‘cannabis-impaired driving’ outcomes will be associated with substantially higher public health burden than cannabis-related ‘harm-to-others’ or ‘use modes’; this relative ‘weighting’ ought to be defined for each measure towards, and reflected in, an integrated index. Various scientifically established methods for such weighted indexing exist. If the final outcome is unidimensional—such as burden of disease, which is usually measured as disability-adjusted life years (a summary indicator composed of years of life lost due to premature mortality and disability)144—an index can be construed via epidemiologic modeling. This index can subsequently be re-assessed and compared, if the associations between indicators and outcome, and the assumptions behind them, remain stable. If more than one dimension is involved, human judgment and valuations about relative impact explicitly come into play. Another option is to assemble an index with the help of ‘multi-criteria decision analysis’ (MCDA).145–147 MCDA involves (at times subjective) expert judgment and valuations, and is usually applied to consider and integrate different factors towards policy-relevant decisions or outcomes, e.g. in health or other policy settings.148,149 MCDA has also been applied in the ‘drug policy’ field, to compare and rank the harm potential of different psychoactive substances based on multiple criteria.142,150 Independent of the chosen approach to combine and integrate the above indicator dimensions into a composite index, actual values, and over-time changes for each of the indicators could be weight-scored over regular monitoring points, and so allow for longitudinal integrated assessments of public health outcomes following cannabis legalization. Limitations The present study presents a conceptual framework for empirical measurement. The proposed indicators are data-driven but selective, and may not capture the full public health impact associated with cannabis legalization. Indicator changes to be observed may include ecological effects. Proposed indicator integration and indexing may involve subjective judgments for weighting towards approximated composite outcome assessments. Conclusions Cannabis legalization towards public health ends—like other major policy reforms—requires rigorous monitoring and evaluation, as to whether the main policy objectives are met and/or adjustments are needed.10,71,151 Numerous individual—and likely inconsistent—outcomes associated with legalization will be observed, however, a scientifically solid ‘big picture’ assessment of legalization’s impact on public health will be required in Canada for evidence-based policy evaluation.138,139 The proposed 10 key outcome indicators, and options for integration into a cannabis public health ‘indicator index’, offer such a—timely—tool that is relatively easy and feasible to develop and implement towards these ends. Funding This work was supported by the Canadian Institutes of Health Research (CIHR) for the Canadian Research Initiative in Substance Misuse (CRISM) Ontario Node Team [Grant #SMN-139150], and by the Chair in Addiction, Department of Psychiatry, University of Toronto. 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Journal of Public HealthOxford University Press

Published: May 30, 2018

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