Marijuana use among college students has been trending upward for years, but in states that have legalized recreational marijuana, use has jumped even higher.
An Oregon State University study published today in Addiction shows that in states where marijuana was legalized by 2018, both occasional and frequent use among college students has continued to rise beyond the first year of legalization, suggesting an ongoing trend rather than a brief period of experimentation.
Overall, students in states with legal marijuana were 18% more likely to have used marijuana in the past 30 days than students in states that had not legalized the drug. They were also 17% more likely to have engaged in frequent use, defined as using marijuana on at least 20 of the past 30 days.
The differences between states with and without legalization escalated over time: Six years after legalization in early-adopting states, students were 46% more likely to have used marijuana than their peers in non-legalized states.
Between 2012 and 2018, overall usage rates increased from 14% to 17% in non-legalized states, but shot up from 21% to 34% in the earliest states to legalize the drug. Similar trends appeared in states that legalized marijuana more recently.
Conducted by Harold Bae from OSU’s College of Public Health and Human Sciences and David Kerr from OSU’s College of Liberal Arts, this is the first study of college students to look broadly at multiple states that have legalized recreational marijuana and to go beyond the first year following legalization.
It includes data from seven states and 135 colleges where marijuana was legalized by 2018 and from 41 states and 454 colleges where recreational use was not legal.
That scope allowed Bae and Kerr to examine trends in the earliest adopting states as well as more recent adopters – though, the data for the study is stripped of state- and college-identifying information, so does not speak specifically to any one state or institution.
The data comes from the National College Health Assessment survey from 2008 to 2018, which asks about a wide range of health behaviors including drug and alcohol use and is administered anonymously to encourage students to respond more honestly. More than 850,000 students participated.
Looking at specific demographics, researchers found that the effect was stronger among older students ages 21-26 than minors ages 18-20; older students were 23% more likely to report having used marijuana than their peers in non-legalized states.
The effect was also stronger among female students and among students living in off-campus housing, possibly because universities adhere to federal drug laws that still classify marijuana as an illegal substance.
“It’s easy to look at the findings and think, ‘Yeah, of course rates would increase,’” Kerr said. “But we need to quantify the effects these policy changes are having.”
Furthermore, he said, researchers are not finding increases in adolescents’ marijuana use following legalization. “So it is surprising and important that these young adults are sensitive to this law. And it’s not explained by legal age, because minors changed too.”
A recent companion study published in Addictive Behaviors in November by OSU doctoral candidate Zoe Alley along with Kerr and Bae examined the relationship between recreational marijuana legalization and college students’ use of other substances.
Using the same dataset, they found that after legalization, students ages 21 and older showed a greater drop in binge drinking than their peers in states where marijuana was not legal. Binge drinking was defined as having five or more drinks in a single sitting within the previous two weeks.
Researchers have not yet tested any hypotheses as to why binge drinking fell, but they have some ideas.
An outside study previously found that illegal marijuana use decreases sharply when people hit 21 – where there is a sharp increase in alcohol use.
“When you’re under 21, all substances are equally illegal,” Alley said.
“In most states, once you reach 21, a barrier that was in the way of using alcohol is gone, while it’s intact for marijuana use. But when marijuana is legal, this dynamic is changed.”
Binge drinking has been on the decline among college students in recent years, but dropped more in states that legalized marijuana than in states that did not.
A study published today in Addiction shows that in states where marijuana was legalized by 2018, both occasional and frequent use among college students has continued to rise beyond the first year of legalization, suggesting an ongoing trend rather than a brief period of experimentation. Image is credited to Oregon State University.
“So in these two studies we saw changes after legalization that really differed by substance,” Kerr said.
“For marijuana we saw state-specific increases that went beyond the nationwide increases, whereas binge drinking was the opposite: a greater decrease in the context of nationwide decreases.”
The magnitude of effect was much larger with marijuana than with any of the other substances, Bae added. “So the changes following recreational marijuana legalization were quite specific to cannabis use.”
Future research is needed to see how those trends hold up over time, as additional states legalize marijuana and existing states continue to tweak their current policies, the researchers said.
The past decade has seen unprecedented shifts in the cannabis policy environment. Canada and Uruguay became the first two countries to legalize the sale and use of cannabis for recreational purposes at the national level.
Despite retaining its status as a strictly prohibited Schedule I substance at the federal level in the US, cannabis is currently (as of December 2018) legal for medicinal use in 33 states and the District of Columbia (policies hereafter referred to as “medical cannabis laws”); 10 states and D.C. have expanded their policies to also legalize cannabis use for recreational purposes (hereafter referred to as “legalization”) (1).
If decriminalization (i.e., the removal of penalties associated with possession of cannabis, with no protection for supply) and high-cannabidiol (CBD) medical cannabis laws (MCLs) (i.e., “high-CBD-only laws”) are also considered, then all but three states have implemented some form of cannabis liberalization.
This movement toward more liberal cannabis policies is mirrored by growing public support for legalization. In 2018, over 60% of US adults said use of cannabis should be legalized for recreational purposes, a considerable increase from the 32% in favor in 2006 (2).
Despite decades of policy experimentation, the current patchwork of state cannabis policies in the US (Figure 1) reflects ongoing disagreement about the potential benefits and harms of policies that regulate the production and consumption of cannabis. While some disagreements stem from limited scientific understanding of the potential harms and benefits of cannabis itself (3), often the discussions by policymakers regarding the impacts of legalization reflect mixed or uncertain evidence for how legalization policies influence key public health outcomes, including prevalence of cannabis use, risky cannabis use (e.g., cannabis use disorder [CUD]), and use of other substances.
Figure 1. Cannabis policy in the United States, laws in effect as of January 1, 2018.
Decriminalization refers to policies that remove penalties associated with possession of small amounts of cannabis for personal use, with no protection for supply. Medical cannabis laws are laws that remove criminal penalties for medicinal cannabis use and some form of supply.
CBD (cannabidiol)-only laws are medical cannabis laws that only permit certain low-delta9-tetrahydrocannabinol (THC) strains of cannabis to be used for medicinal purposes. Recreational legalization refers to laws that remove criminal and monetary penalties for the possession, use, and supply of cannabis for recreational purposes.
Recognizing there are a myriad of factors that characterize the interests of proponents and opponents of cannabis legalization (4), the present study focuses on one aspect of public health interest by reviewing the evidence for how medical and recreational cannabis laws (RCLs) impact cannabis use, as well as use of alcohol, opioids, and tobacco, three substances that generate substantial societal costs (5–8).
We focus on evidence from scientifically rigorous policy evaluations that use methods for causal inference, i.e., those that
(1) use time-series data,
(2) verify that policies preceded their effects on outcomes, and
(3) include a control or comparison group.
By focusing on these empirical designs, we draw attention to the limited takeaways from these policy experiments so far and emphasize how challenging it is, even when using sophisticated econometric techniques, to draw firm conclusions from the current evidence.
Considering evidence for the effects of cannabis policy
Understanding how cannabis policies impact cannabis use is key to making subsequent causal claims about their effects on the use of other substances, but it is also an important question in and of itself. If liberalization does not impact cannabis use, but instead shifts some or all existing use (or potential use) from the illegal to legal market, then arguably such policies are welfare enhancing from a governmental perspective (e.g., increased tax revenues, reduced law enforcement expenditures) and from a consumer perspective (e.g., a safer and more consistent product). Even if liberalization increases cannabis use, the impact on risks or harms will depend on whether increased consumption occurs among populations whose use more strongly predicts subsequent harms (e.g., adolescents) or by leading to more problematic use patterns, such as persistent daily use (3).
Conceptually, cannabis liberalization, whether MCLs and RCLs, could influence consumption through several mechanisms, including changes in perceived harmfulness, social norms, prices, potential legal consequences, search costs of locating a supplier, and potential social stigma associated with participating in illegal activity (9–11).
The extent to which particular mechanisms change in response to a policy and the timing of such changes depend on the specific provisions that comprise the law and how long it takes for particular provisions to influence cannabis markets.
Laws that allow the proliferation of dispensaries or that grant legal cannabis access to a broader segment of the population, for example, are likely to have greater impacts on perceptions and norms (as well as access) than laws that are more restrictive. Similarly, while perceptions, social norms, and legal risks may change immediately upon passage of a law, changes in price and access depend more directly on the size and structure of the supply-side of the legal market.
The implication of this is that studies that examine the impact of liberalization policies by focusing on when a law became effective or when the first store opens likely do not capture the full influence of when the market became “present” within the state and hence likely miss some of the more relevant impacts associated with norms, availability, and cost.
As cannabis policies have evolved, so too has the literature examining the impact of these policies on substance use. While recent evaluations of the effects of MCLs have begun to pay better attention to variability in specific policy provisions, issues remain due to lack of consideration of the length of time it takes for mature markets to emerge and fully influence perceptions, norms, prices, and product choice.
The outcomes evaluated thus far have also been relatively limited; while we have a relatively large number of studies examining cannabis use prevalence or days of use on average, we know far less about how liberalization policies may impact specific patterns of cannabis use or co-use of various substances with cannabis.
Some of these limitations may be challenging to address. We have few large-scale representative systematic data collection efforts that capture information on cannabis use in its various forms, and those we do have often only provide crude measures of use.
These systems, established prior to the rise of commercialized cannabis regimes, were not equipped to provide detail on the variety of consumption patterns that exist today. They also provide only limited information on polysubstance use, particularly with respect to simultaneous substance use. Similarly, we have limited data to assess the implementation and evolution of policies “on the ground.”
For instance, the conclusions drawn from most previous studies—particularly those that claim operating dispensaries provide evidence on substitution or complementarity—rely on the assumption that the opening of the first cannabis dispensary serves as a sufficient indicator for cannabis access. In the absence of data to inform the time course of MCL implementation, such indicators have often been the best that researchers had to work with, yet we now know from the experience of both MCLs and RCLs that cannabis prices and availability evolve dramatically as more suppliers and retailers enter these legal markets over time (50,56,80–82).
The rise of legal cannabis markets under RCLs may help reduce some of these challenges. Legalization has brought with it large-scale administrative datasets with more detailed information on retail outlets, product purchases, potency, and price.
These data bring their own set of challenges, but they may provide greater insights into how markets for cannabis evolve and how consumer behavior in the legal market changes alongside policy. Furthermore, with more detail on monetary prices of cannabis, future research may be able to more adequately assess how changes in the price for cannabis relate to changes in other substance use, offering greater insight into economic substitution or complementarity of cannabis with other substances.
However, evaluations of RCLs face some additional methodological complexity. Currently, all states with RCLs had preexisting MCLs, and many already had fairly robust cannabis distribution through medical dispensaries. Both the preexisting and co-occurring policy environments in RCL states are important to consider, as estimating RCL effects relative to the existing MCL environment may conflate heterogeneity in the “control” group of non-RCL states.
Serious consideration needs to be given to what makes a state a reasonable control group, given that no state has moved from strict prohibition to RCL. Additionally, since the literature suggests MCLs (and some provisions of them) increase adult cannabis use, models of RCL effects need to account for this potential differential trend when constructing an appropriate comparison group; including dummy variables for MCL or MCL provisions may not be an adequate enough adjustment if MCLs lead to a shift in cannabis use trends and not just levels.
Finally, while this review was restricted to studies that use methods most appropriate to identifying causal effects of MCLs and RCLs, a fundamental limitation of the state policy evaluations meeting this criterion is that they are largely estimating population-level associations using information from multiple years of cross-sectional data.
Thus, it is unknown whether observed population-level changes in alcohol, opioid, or tobacco use were driven by individuals whose cannabis use actually changed. The mechanisms underlying some of these associations remain unclear, and the models may be highly susceptible to confounding. In the case of MCL evaluations in particular, nationally representative survey data suggest that less than 10% of past-year cannabis users report use for medicinal purposes (118).
As such, MCLs target a group that is far too small to drive the large effects we are seeing in many of these population studies. In order to advance our understanding of how the use of cannabis and other substances interact, evidence from clinical and prospective cohort studies would greatly bolster any findings from evaluations of state policy effects.
Oregon State University