​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].”
“This could be plausibly explained by social desirability bias or greater concern about enforcement of recreational marijuana use among adolescents after the passage of laws,” the researchers speculated.
But the researchers were skeptical that the passage of medical marijuana laws had any causal effect at all on broader usage.
“We find limited evidence of causal effects of medical marijuana laws on measures of reported marijuana use,” the investigators concluded.
Previous investigations by teams at Brown University in 2011 and Texas A&M in 2007 had similar results, concluding, “[C]onsistent with other studies of the liberalization of cannabis laws, medical cannabis laws do not appear to increase use of the drug.”
The findings also agree with those of a study released by the Marijuana Policy Project (MPP) last year, which found that medical marijuana laws had no effect on teen cannabis use.
The findings are in direct conflict with public statements made by Drug Czar Gil Kerlikowske, who in recent years has often claimed that the passage of medical marijuana laws is directly responsible for higher levels of self-reported cannabis consumption among American teenagers.
Full text of the study [PDF]: “Do Medical Marijuana Laws Increase Marijuana Use? Replication Study and Extension”