Despite warnings from opponents of medical marijuana, legalizing the drug for medical purposes does not encourage teens to smoke more pot, according to new research that compared rates of marijuana use in Massachusetts and Rhode Island after the latter state changed its laws.
Rhode Island legalized medical marijuana in 2006, but Massachusetts did not. “We wanted to pair these two states because they have so much in common culturally and geographically,” says Dr. Esther Choo, assistant professor of emergency medicine at Brown University’s Warren Alpert Medical School and emergency medicine physician at Rhode Island Hospital.
Choo’s analysis used data collected from 1997 to 2009 for the Centers for Disease Control and Prevention’s annual Youth Risk Behavior Survey. The analysis involved nearly 13,000 youth in Rhode Island and about 25,000 in Massachusetts. In each state in any given year, the study found, about 30% of youth reported using marijuana at least once in the previous month.
In other words, while marijuana use was common, there was no significant difference in rates of pot use between the years before and after legalization in Rhode Island. “We found no effect of the policy change,” says Choo.
PHOTOS: Inside Colorado’s Marijuana Industry
These results are consistent with a 2005 analysis conducted by Mitch Earleywine, associate professor of psychology at the State University of New York–Albany, for the Marijuana Policy Project. He found that between 1996 — when California passed its medical marijuana law — and 2004, previous-month pot use by ninth graders declined by 47%. That was a slightly steeper decline than seen nationally during the same period, and Earleywine found a similar effect in all of the medical marijuana states he studied.
Of the new study, Earleywine says, it is “very careful about potential covariates and looks closely at a couple of states across time in a way that my work didn’t.” He adds: “Of course, I’m delighted to see that their work confirms my previous report that medical marijuana laws do not increase teen use.”
Although legalizing medical marijuana may increase access for some, Choo notes that it is typically a very small population who uses marijuana for therapy — and these aren’t people whom teens are likely to emulate. “Whether they are taking it for pain or for vomiting control or appetite, this is not a group we think of as superinspiring for young people to take up their drug pattern. It’s an older population who is generally very ill,” says Choo.
PHOTOS: Stoner Cinema: Pot Movie Classics
It’s important to note that neither Choo’s nor Earleywine’s analyses have been subjected to peer review. Choo’s work was presented on Wednesday at the American Public Health Association’s annual meeting in Washington, D.C. She is planning to do further analysis, including data from additional states, for future publication.
But both studies are consistent with state and international data showing similar trends. Research has found that rates of youth drug use don’t correlate with marijuana arrests or, at the other end of the spectrum, total decriminalization of marijuana possession.
Could it be possible that associating pot with uncool old people and making it legitimate, rather than rebellious, actually deters use by youth?
MORE: Obama’s Misguided Crackdown on Medical Marijuana
Maia Szalavitz is a health writer at TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.