Seniors who get behind the wheel soon after starting to use narcotic pain relievers have twice the risk of getting into a serious car crash as their peers who use non-opioid painkillers, Swedish researchers say.

Senior drivers who’d been using opioid painkillers regularly for several months also had higher odds of getting into accidents, but not as high as the new users did, according to Joel Monarrez-Espino of Karolinska Institute in Stockholm and colleagues.

Their study included 4,445 drivers between the ages of 50 and 80 who had been involved in a single car crash between 2005 and 2009 in which at least one person suffered an injury that required medical care, plus more than 17,000 similar drivers who had not been in crashes.

As reported in the journal Age and Aging, study participants were considered new to opioid painkillers if they had been given a prescription within one month before the crash. Regular users were those given at least three prescriptions in the last six months, with at least one prescription within a month of the crash.

“New, but also frequent opioid analgesic use, resulted in an increased probability of single vehicle crashes,” the research team wrote.

Specifically, the risk was 100 percent higher for the new opioid users, and 60 to 70 percent higher for the regular users, compared to the risk in people of the same age taking one or two non-opioid painkillers.

“While more epidemiologic evidence is needed, patients could be advised to refrain from driving when using opioid analgesics,” the authors wrote.

They did not respond to a request for comment.

Whether the “regular users” in this study had a lower risk because their bodies were accustomed to the opioids is hard to know, said Dr. Thomas Meuser, a specialist in aging at the University of Missouri who was not involved with the study.

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“The study doesn’t show if the participants took their medications consistently,” Meuser told Reuters Health by phone.

“Another reason for the drop in risk (among regular users) could be that some stopped or reduced taking their medications due to side effects, even though they continued being prescribed,” said Meuser.

Paul Atchley, who studies the human brain, vision and attention to driving at the University of Kansas, told Reuters Health the findings should serve as a wake-up call for doctors to have better conversations with their patients about the true risks of taking these medications.

“Driving is the riskiest thing we do on a daily basis,” said Atchley, who wasn’t involved in the study.

“We need to understand what’s at risk, so that we as drivers can make better choices,” he said.

“What’s unique about this study isn’t just painkiller use, but the pattern of use,” Atchley pointed out.

The risk of being injured or killed in a car crash increases with age, according to the Centers for Disease Control and Prevention (CDC).

Meuser noted that older adults generally have more diagnoses and take more medications than younger adults. “There’s always a risk for side effects for someone taking five or more medications,” he said. “Side effects that affect the brain and nervous system are especially worrisome for older drivers.”