Some experts in the field of substance abuse say there are significant problems with a recent study that concluded medical marijuana laws are associated with a reduction in traffic deaths. The critics point out the study was published as a working paper, and was not a peer-reviewed study in a scientific journal.

The study, which appeared in a working paper for the Institute for the Study of Labor, concludes that the most likely reason for the decrease in traffic deaths in states with medical marijuana laws is that some people in states with the laws use marijuana instead of alcohol. They note that alcohol is more deadly than marijuana when combined with driving.

Kevin A. Sabet, PhD, Senior Policy Advisor to the Director of the White House Office of National Drug Control Policy, Gil Kerlikowske, from 2009-2011, writes in The Huffington Post that “the paper ignores the fact that alcohol crash fatalities were already coming down far before the introduction of medical marijuana in any state.”

He notes the study includes Rhode Island and Vermont, which both had fewer than 300 members registered in their medical marijuana programs and no medical marijuana dispensaries. Montana, also included in the study, had 6,000 members at the time of the study, he says. The study says the state had more than 27,000 members, but that was as of 2011, after the study had concluded. He also points to a study to be published in the peer-reviewed Epidemiologic Reviews that analyzed nine studies on marijuana and the risk of car crashes. The study found that “drivers who test positive for marijuana or self-report using marijuana are more than twice as likely as other drivers to be involved in motor vehicle crashes.”

Gen. Arthur Dean, Chairman and CEO of Community Anti-Drug Coalitions of America (CADCA), wrote in a blog post that the working paper’s study methodology is greatly flawed, the study’s authors ignore a large body of evidence that shows marijuana and alcohol are compliments, and the authors also ignore mounting evidence that marijuana use is linked with impaired driving.

“The authors contend that medical marijuana laws cause people to drink less and therefore not drive drunk,” he wrote. “This is most certainly a spurious and coincidental relationship, however, as a large body of data points to other reasons why we have witnessed historic reductions in road fatalities over the last 20 years. Just because A happened when B happened does not mean that A caused B.”