It is difficult to assess whether the legalization of adult cannabis use in some US states has increased cannabis use and cannabis-related harm among youths1,2—a major concern about the effects of the policy change.3 First, adult legalization has been implemented by citizen-initiated referenda and, with the exception of Washington State, these states have not funded evaluations of the impact of the policy on cannabis use by youths.1 Second, most evaluations of the policy have compared survey data on self-reported cannabis use among youths between states that have and have not legalized cannabis for adult use and have examined changes in cannabis use before and after the legislation of cannabis use.2,4 This method assumes that any short-term changes in use (or, more often, the absence of a change) provide a reliable guide to the longer-term effects of legalization1 and that the policy was implemented on the date that legislation passed.2
In this issue of AJPH, Kan et al. (p. 1386) have used survey data from two states from a longitudinal study of justice system–involved (JSI) adolescents and young adults (the Crossroads Study) to assess the impact of cannabis legalization on youths’ cannabis use across three periods: (1) before legalization, (2) immediately after legalization but before implementation, and (3) after implementation of legalization. Their data were collected from October 2015 to June 2018 in a population of youths involved with the juvenile justice system. This included 504 youths in California, where recreational use was legalized in 2016, and 478 youths in Pennsylvania, where recreational cannabis use was prohibited throughout their study. The authors measured participants’ self-reported cannabis use in the 24 hours before they were interviewed, and they retained most of the members of both samples over the three study periods.
Kan et al. found that the prevalence of cannabis use in the past 24 hours was higher in California than in Pennsylvania before legalization and remained unchanged in California across the three time periods. In Pennsylvania, by contrast, cannabis use increased in the past 24 hours after the recreational cannabis law passed in California, and it remained higher for the rest of the period. The authors suggest that the increase in cannabis use in Pennsylvania may reflect other policy changes within Pennsylvania, or possibly national reductions in the perceived harms of cannabis use as more states have legalized recreational cannabis use.
STUDY LIMITATIONS
The authors acknowledged several limitations of the study. First, it could compare only two states—one that had legalized adult cannabis use and one that had not—because these were the only states with the required data.
Second, the sample sizes were relatively small, and cannabis use was only measured as use in the past 24 hours. As Kan et al. argue, use in the past 24 hours is probably highly correlated with cannabis use over longer time periods, largely because daily users are overrepresented in justice-involved youths. Nonetheless, this measure may have limited the study’s capacity to detect any increases in the frequency of cannabis use, an important outcome because cannabis-related harms increase with the frequency of use.3
Third, a JSI population may not be as sensitive to the effects of legalization as youths in the general population who were not involved with the criminal justice system. This is because JSI youths were not discouraged from using cannabis by its illegal status, and they probably already had ready access to the illicit cannabis market before legalization.
IS IT TOO EARLY TO ASSESS EFFECTS OF LEGALIZATION?
The present study also shared a common limitation of studies of the impacts of cannabis legalization on cannabis use among youths: it was only able to assess the effects of legalization in the first year after implementation.4 Changes over this short period of time are unlikely to reflect the longer-term effects of legalization, for several reasons. First, the number of retail cannabis outlets has increased slowly in most US states after legalization. Their numbers and locations have often been limited by state restrictions on the number of licensed retailers and by local governments refusing to allow retail outlets to open. The California cannabis industry, for example, complains that these restrictions limit its capacity to compete with the illicit cannabis market.5
Second, federal cannabis prohibition has limited the commercialization of cannabis sales in US states that have legalized cannabis.1 Federal law restricts advertising and limits access to banking, and the Internal Revenue Service does not allow tax deductions for business costs. We should expect much larger changes in cannabis use if legalization becomes US national policy because the end of federal prohibition would allow online cannabis markets and sales, mergers between small cannabis companies, cross-state cannabis commerce, and investment by the tobacco and alcohol industries. It would also provide constitutional protection for the promotion of cannabis use under commercial freedom of speech.1
Third, many other studies have assumed that the effects of a state’s cannabis legalization are confined to that state.2 This is unlikely. Media stories and public debates about cannabis legalization spill easily across state boarders; so has cannabis. The patchwork of US state policies has increased cannabis trafficking between states that have legalized and those that have not. This was evident in the outbreak of vaping-related lung injuries caused by illicit cannabis vapes contaminated with vitamin E in the United States.6 The authors of the current study noted suggestive evidence for spillover effects in that rates of cannabis use increased in Pennsylvania after legalization had occurred in California.
Most evaluations of the effects of the legalization of recreational cannabis use assume that they can be distinguished from the effects of legalizing medical cannabis use. It is not a coincidence that the first US states to legalize cannabis had earlier legalized medical cannabis sales under liberal regulations.1 These liberal medical cannabis policies amounted to the de facto legalization of adult cannabis use by making it legal for any adult with a medical recommendation to purchase cannabis products from commercial cannabis dispensaries.1
Notwithstanding these limitations, Kan and colleagues’ findings are broadly consistent with population surveys in suggesting that, in the short term, the legalization of adult cannabis use has had a limited impact on cannabis use among youths in the states that legalized use.2,4 It would be unwise, however, to assume that the lack of any short-term impact of legalization indicates that there will be no increase in the longer term. We may yet see increased rates and frequencies of youth use as retail markets mature and expand, increasing cannabis access and potency, reducing price, and normalizing recreational use among adults. The limited impacts so far of state cannabis legalization under federal prohibition may poorly predict what will happen to youth cannabis use if federal cannabis prohibition ends in the United States.1
CONFLICTS OF INTEREST
The authors have no conflicts of interest to report.
Footnotes
See also Kan et al., p. 1386.
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