Simultaneous drinking, smoking marijuana increases odds of drunk driving, other

Results will be published in the May 2015 online-only issue of Alcoholism: Clinical & Experimental Research and are currently available at Early View.

"There has been some disagreement regarding whether using cannabis and alcohol together is more dangerous than using either alone," said Meenakshi S. Subbaraman, corresponding author for the study and associate scientist at the Alcohol Research Group, a program of the Public Health Institute. "My study is the first to compare how simultaneous and concurrent use of alcohol and cannabis relate to drunk driving and other social consequences among adults, and the first to examine differences between simultaneous and concurrent users in terms of demographics and substance use quantity/frequency. In this study, concurrent means having used both alcohol and cannabis within the previous 12 months, but always separately."

"Because of both the number of states permitting medical marijuana, and differing legalizations in Washington and Colorado, as well as efforts in other states for legalization, this is a timely study and indeed it is an understudied issue," added Tom Greenfield, center director of the Alcohol Research Group.

The researchers analyzed data from the 2005 and 2010 National Alcohol Survey (n=8,626; 4,522 females, 4,104 males). This was a Random Digit Dial, Computer Assisted Telephone Interview survey of individuals aged 18 and older from all 50 states and DC. Blacks and Hispanics were over-sampled. The study authors assessed differences in demographics, alcohol-related social consequences, harms to self, and drunk driving across simultaneous, concurrent, and alcohol-only using groups.

"We looked at three groups of adults," explained Subbaraman. "One, those who used only alcohol in the previous 12 months; two, those who used both alcohol and cannabis but always separately, or concurrently; and three, those who used both alcohol and cannabis and usually together, or simultaneously. Please note that the simultaneous users did not necessarily always use cannabis while they drank; the groups were based on how often they drank when using cannabis, and not vice versa.

The study authors found that, compared to adults who solely used alcohol, simultaneous users had double the odds of drunk driving, social consequences, and harms to self. Compared to concurrent users, simultaneous users had double the odds of drunk driving. Simultaneous users also had the heaviest drinking patterns in terms of quantity and frequency.

"If cannabis use becomes more prevalent as U.S. states and other countries continue to legalize it, then we need to be prepared to advise people appropriately," cautioned Subbaraman. "If you use both substances together, your risk of drunk driving, and possibly other consequences, may be higher than if you stick to using one at a time."

While Subbaraman acknowledges that few in the field of alcohol studies would be surprised by these findings, she said it was important to study this issue in a scientific way. "Now people have at least one scientifically rigorous, peer-reviewed study to support what they might have already thought they knew," she said. "Most importantly though, the findings will help clinicians and prevention/treatment specialists advise patients and others in the community regarding the elevated risk of consequences related to simultaneous use. I also hope that as the cannabis industry continues to grow, manufacturers will consider some sort of warning label related to increased risks when mixing alcohol and cannabis."