A growing number of drugged drivers are testing positive for prescription drugs, a new study suggests. More drivers involved in fatal crashes tested positive for prescription medications than for any other drug type.

The study, published in Public Health Reports, also found the share of drugged drivers in fatal motor vehicle crashes who tested positive for marijuana reached 36.9 percent in 2010.

Drivers ages 50 and up account for an increasing share of drugged drivers, and for the highest proportion of prescription drug users, the study found. The findings come from an analysis of data from the National Highway Traffic Safety Administration.

The researchers from the University of Nebraska Medical Center also found an increasing number of drivers are using more than one drug at once, according to U.S. News & World Report. “In 1993, about one in eight drivers were using multiple drugs concurrently. By 2010, it was closer to one in five. That’s a large increase in drug usage,” study author Fernando Wilson said in a news release.

“These trends are likely to continue into the future given the aging U.S. population, an increasing reliance on prescription medications by medical providers, and increasing initiatives to legalize marijuana,” Wilson said. “However, it is unclear whether current state policies are completely up to the challenge of addressing the growing issue of drugged driving.”

About half of drugged drivers were also under the influence of alcohol, the study found. Approximately 70 percent of drivers who tested positive for cocaine had also been consuming alcohol, and almost 55 percent of drivers who tested positive for marijuana also had alcohol in their systems.

The researchers suggested a range of strategies, including reducing prescription drug use by drivers through counseling by medical professionals, and increasing affordable access to mass transit.