title: The effects of medical marijuana laws on potency.
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 Δ(9)-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.
  
  
  CONCLUSIONS
  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.
authors: Department of Criminology and Criminal Justice, University of South Carolina, 1305 Greene Street, Columbia, SC 29208, USA. Electronic address: sevigny@mailbox.sc.edu.
authors: Eric L
authors: EL
authors: Sevigny
authors: RAND Drug Policy Research Center, RAND Corporation, 1776 Main Street, P.O. Box 2138, Santa Monica, CA 90407-2138, USA.
authors: Rosalie Liccardo
authors: RL
authors: Pacula
authors: RAND Drug Policy Research Center, RAND Corporation, 1776 Main Street, P.O. Box 2138, Santa Monica, CA 90407-2138, USA.
authors: Paul
authors: P
authors: Heaton
mesh_terms: D013759:Dronabinol / Q000032:analysis*; D004335:Drug and Narcotic Control / Q000331:legislation & jurisprudence* / Q000639:trends; D006291:Health Policy / Q000639:trends*; D006801:Humans; D035846:Law Enforcement; D064086:Medical Marijuana / Q000737:chemistry*; D014481:United States
chemical_list: D064086:Medical Marijuana; D013759:Dronabinol
keywords: Differences-in-differences; Dispensaries; Mediation; Medical marijuana laws; Sinsemilla; Δ(9)-Tetrahydrocannabinol (THC)