​State laws allowing for the legal use of medical marijuana by qualified patients do not increase teen marijuana use, and if anything decrease teen use or have no effect at all, according to data published online in the journal Annals of Epidemiology.

Investigators at McGill University in Montreal obtained state-level estimates of marijuana use from the US National Survey on Drug Use and Health for the years 2002 through 2009, reports Paul Armentano at the National Organization for the Reform of Marijuana Laws (NORML). Researchers used difference-in-differences regression models to estimate the causal effect of medical marijuana laws on cannabis use, and simulations to account for measurement error.

“Difference-in-differences estimates suggested that passing MMLs [medical marijuana laws]decreased past-month use among adolescents … and had no discernible effect on the perceived riskiness of monthly use,” McGill University researchers Sam Harper, Erin C. Strumpf and Jay S. Kaufman reported. “[These] estimates suggest that reported adolescent marijuana use may actually decrease following the passing of medical marijuana laws [emphasis added].”