A study of young drivers who died in crashes finds that half were under the influence of alcohol, marijuana or both at the time the accident occurred, HealthDay reports.

Researchers from Columbia University examined car-crash data from nine states between 1999 and 2011. The 7,200 drivers in the study were between the ages of 16 and 25. The states in the study routinely collect blood and urine tests of drivers in fatal accidents, the article notes.

The study found 36.8 percent tested positive for alcohol, 5.9 percent tested positive for marijuana, and 7.6 percent tested positive for both. Drivers older than 21 were 14 percent more likely to test positive for alcohol than those under the legal drinking age. They were also slightly more likely to test positive for both marijuana and alcohol.

The findings appear in Injury Epidemiology.

“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,” study leader Katherine Keyes said in a news release. “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.”

Added study co-author Dr. Guohua Li, “Taken together, we found no significant substitution effect between alcohol and marijuana. Rather, an uptick in availability seems to increase the prevalence of concurrent use of alcohol and marijuana.”