US states as different as California, West Virginia and Hawaii share one thing in common - half of automobile fatalities involving young drivers, ages 16 to 25 years, involved pot or alcohol. And those results were from years before the cultural push to make marijuana legal.

The scholars from Columbia University's Mailman School of Public Health used data from California, Connecticut, Hawaii, Illinois, New Hampshire, New Jersey, Rhode Island, Washington State and West Virginia because those nine states all routinely perform toxicological tests on the blood or urine specimens of drivers who die in car crashes. Information was drawn from the Fatality Analysis Reporting System, a census of fatal traffic crashes occurring within the U.S. More than half of the crashes (54.7 percent) for the period between 1999 to 2011 occurred in California. 

Katherine M. Keyes, PhD, assistant professor of Epidemiology, and colleagues analyzed 7,191 fatal accidents involving drivers between the ages of 16 and 25 from 1999 to 2011 who died within one hour of the crash. Overall, 50.3% of the deceased tested positive for alcohol, marijuana or both. Of these, 36.8 percent were under the influence of alcohol, 5.9 percent used only marijuana and 7.6 percent used both substances. 

The researchers further tested whether there were any changes in patterns of alcohol and marijuana use among those aged 21 years and older who were legally allowed to consume alcohol, versus those less than 21. It was found that alcohol consumption indeed increased by 14 percent, but such prominent changes in marijuana use were not seen. Dr. Keyes does note that marijuana use decreased among those aged 21 years and older who used this drug alone. After reaching age 21, use of alcohol in combination of marijuana increased slightly.

"Policies related to the use of substances in the United States remain in flux; the rapid changes in marijuana use policy are a good example of this," said Dr. Keyes. "It's imperative to know whether there will be unintended consequences of changes in policies, including increases or decreases in harm related to other substances that are not the focus of the policy."

"Taken together, we found no significant substitution effect between alcohol and marijuana," noted co-author Guohua Li, MD, DrPH, Mailman School professor of Epidemiology and director of the Center for Injury Epidemiology and Prevention. "Rather, an uptick in availability seems to increase the prevalence of concurrent use of alcohol and marijuana."

Published in Injury Epidemiology