Policy analysis
The effects of medical marijuana laws on potency

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Abstract

Background

Marijuana potency has risen dramatically over the past two decades. In the United States, it is unclear whether state medical marijuana policies have contributed to this increase.

Methods

Employing a differences-in-differences model within a mediation framework, we analyzed data on n = 39,157 marijuana samples seized by law enforcement in 51 U.S. jurisdictions between 1990 and 2010, producing estimates of the direct and indirect effects of state medical marijuana laws on potency, as measured by Δ9-tetrahydrocannabinol content.

Results

We found evidence that potency increased by a half percentage point on average after legalization of medical marijuana, although this result was not significant. When we examined specific medical marijuana supply provisions, results suggest that legal allowances for retail dispensaries had the strongest influence, significantly increasing potency by about one percentage point on average. Our mediation analyses examining the mechanisms through which medical marijuana laws influence potency found no evidence of direct regulatory impact. Rather, the results suggest that the impact of these laws occurs predominantly through a compositional shift in the share of the market captured by high-potency sinsemilla.

Conclusion

Our findings have important implications for policymakers and those in the scientific community trying to understand the extent to which greater availability of higher potency marijuana increases the risk of negative public health outcomes, such as drugged driving and drug-induced psychoses. Future work should reconsider the impact of medical marijuana laws on health outcomes in light of dramatic and ongoing shifts in both marijuana potency and the medical marijuana policy environment.

Introduction

Marijuana (cannabis) is the most widely used illicit substance in the United States, with about 17.4 million past-month users in 2010. Recent trends reveal an increase in marijuana prevalence, especially among younger populations. Between 1990 and 2010, rates of past-month marijuana use increased about 68% for youth aged 12–17, 46% for young adults aged 18–25, and 12% for adults aged 26–34 (Substance Abuse and Mental Health Services Administration, 2011). Over the same time period, average concentrations of Δ9-tetrahydrocannabinol (THC)—the main psychoactive component of marijuana—nearly tripled from 3.4% to 9.6% (ElSohly, 2008, ElSohly, 2012). This epidemiology has important public health implications, as mounting evidence links higher potency marijuana to an array of adverse outcomes, especially among novice users (Hall and Degenhardt, 2006, Hall and Degenhardt, 2009, McLaren et al., 2008). In particular, research supports claims of dose-dependency between THC levels and risk of acute anxiety (Crippa et al., 2009), psychosis (Di Forti et al., 2009), cognitive impairment (Ramaekers et al., 2006), and vehicular accidents (Li et al., 2012, Ramaekers et al., 2004).

Although there has been some recent attention in the academic literature to the question of whether permissive state medical marijuana laws (MMLs) have contributed to the recent rise in recreational use of marijuana, with results from published studies appearing quite mixed (e.g., Friese and Grube, 2013, Harper et al., 2012), virtually no attention has been given to the possible impact these state laws might have on consumption through their effects on the average potency of the marijuana consumed. Indeed, it is entirely possible that a rise in the average potency of marijuana could be associated with a decline in total quantity of marijuana consumed, as users consuming higher potency marijuana require less marijuana to reach the same level of intoxication (van Laar et al., 2013, Reinarman, 2009).

In light of the public health concerns associated with rising rates of high-potency marijuana use, particularly among youth, and the possible mediating effect this rise would have on total marijuana consumed, an obvious first question to ask is whether medical marijuana laws have contributed to rising potency trends over the past two decades. Although no state law directly regulates the THC content of medical marijuana, there is some evidence to suggest that the typical potency of medical marijuana is higher than that of recreational marijuana sold in black markets (Burgdorf, Kilmer, & Pacula, 2011). It may be the case that the general allowance for growing high-grade marijuana for medical purposes—including specific rules governing retail outlets or dispensaries, home cultivation, and patient caregivers—has contributed to the upward trend in potency observed in recreational markets.

The focal relationship we examine in this study, therefore, concerns the effect of state medical marijuana laws on cannabis potency. Specifically, we investigate state-level variations in potency for the years 1990–2010 using data from the University of Mississippi's Potency Monitoring Program (PMP), a federally-funded surveillance program that forensically analyzes marijuana samples seized by federal, state, and local law enforcement agencies (see Mehmedic et al., 2010). Recognizing that alternative state policies and programs may also affect potency, we explore the competing effects of rival explanatory factors, including marijuana decriminalization and law enforcement efforts. In the next section, we further explicate these policies and possible mechanisms of action.

Section snippets

State marijuana policies, markets, and potency

Marijuana is not a uniform product, varying considerably by strain (indica, sativa, hybrid), cultivation technique (hemp, sinsemilla, hydroponic), and manner of processing (herb, resin, oil). The resulting cannabis phenotypes contribute to wide variations in potency across both time and place (Burgdorf et al., 2011, Slade et al., 2012). Although direct empirical evidence is limited, insider and journalistic accounts suggest that MMLs—and the medical marijuana industry built up around them—have

Data

The measures for this study come from several data sources. Marijuana potency and state-level marijuana market indicators were derived from the University of Mississippi's Potency Monitoring Program (PMP), a federally funded forensic surveillance program that analyzes seized marijuana samples (see Mehmedic et al., 2010). The micro-level PMP data used for the current study comprise n = 39,157 observations of dried herbal marijuana seized by law enforcement in the 50 U.S. states and the District of

Results

Table 2 presents descriptive statistics for the sample, stratified by MML status at the state-year level. Notably, mean potency is nearly 3.5 percentage points higher in states with a medical marijuana law. Overall, about 21% of the observations were seized in medical marijuana states. For jurisdictions with an active MML, 40% of seizures were from states with legally operating dispensaries, 74% from states with de facto operating dispensaries, and 97% from states that allow home cultivation.

Discussion

A fundamental question that has of yet remained unanswered in the academic literature is whether state medical marijuana laws lead to a rise in the average potency of marijuana available on the market. Indeed, prior research by Pacula et al. (2010), which examined the impact of medical marijuana laws on self-reported price paid per gram among the arrestee population in the 2000–2003 Arrestee Drug Abuse Monitoring (ADAM) data, showed that self-reported marijuana prices were higher in states that

Acknowledgements

This research was supported by a grant from the National Institute on Drug Abuse to the RAND Corporation (R01 DA032693-01). We appreciate helpful comments on earlier versions of this work from David Powell, as well as participants of the 2012 American Society on Criminology annual conference and the 2013 International Society for the Study of Drug Policy annual conference. All errors are the authors. The opinions expressed herein represent only those of the authors and not the funding agency or

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