title: Do medical marijuana laws increase marijuana use? Replication study and extension.
abstract: OBJECTIVE
  To replicate a prior study that found greater adolescent marijuana use in states that have passed medical marijuana laws (MMLs), and extend this analysis by accounting for confounding by unmeasured state characteristics and measurement error.
  
  
  METHODS
  We obtained state-level estimates of marijuana use from the 2002 through 2009 National Survey on Drug Use and Health. We used 2-sample t-tests and random-effects regression to replicate previous results. We used difference-in-differences regression models to estimate the causal effect of MMLs on marijuana use, and simulations to account for measurement error.
  
  
  RESULTS
  We replicated previously published results showing higher marijuana use in states with MMLs. Difference-in-differences estimates suggested that passing MMLs decreased past-month use among adolescents by 0.53 percentage points (95% confidence interval [CI], 0.03-1.02) and had no discernible effect on the perceived riskiness of monthly use. Models incorporating measurement error in the state estimates of marijuana use yielded little evidence that passing MMLs affects marijuana use.
  
  
  CONCLUSIONS
  Accounting for confounding by unmeasured state characteristics and measurement error had an important effect on estimates of the impact of MMLs on marijuana use. We find limited evidence of causal effects of MMLs on measures of reported marijuana use.
authors.affiliation: Department of Epidemiology, Biostatistics and Occupational Health, McGill University, 1020 Pine Avenue West, Montreal, Canada. sam.harper@mcgill.ca
authors.forename: Sam
authors.initials: S
authors.lastname: Harper
authors.forename: Erin C
authors.initials: EC
authors.lastname: Strumpf
authors.forename: Jay S
authors.initials: JS
authors.lastname: Kaufman
mesh_terms: D000293:Adolescent; D000294:Adolescent Behavior; D000328:Adult; D002186:Cannabinoids / Q000627:therapeutic use; D002648:Child; D006291:Health Policy / Q000331:legislation & jurisprudence*; D006801:Humans; D016014:Linear Models; D008385:Marijuana Smoking / Q000453:epidemiology* / Q000331:legislation & jurisprudence*; D015995:Prevalence; D013219:State Government; D014481:United States / Q000453:epidemiology; D055815:Young Adult
chemical_list: D002186:Cannabinoids