In fatal two-car crashes, drivers deemed to have caused the wreck were almost twice as likely as those who were not at fault to test positive for opioids, a U.S. study finds.

The most common error leading to fatal crashes, whether or not opioids were involved, was drivers veering out of their lanes, according to the results, published last week in JAMA Network Open.

"This shows that the ongoing national opioid epidemic has spilled over to our national highway system with deadly consequences," said study coauthor Dr. Gouhua Li, founding director of the Center for Injury Epidemiology and Prevention at the Mailman School of Public Health of Columbia University. "Drivers who caused the crashes were about twice as likely to have taken prescription opiates as the non-initiators of the crashes."

To take a closer look at the impact of drugs on fatal vehicle crashes, the researchers turned to the Fatality Analysis Reporting System, which is compiled and maintained by the National Center for Statistics and Analysis at the National Highway Traffic Safety Administration.

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The fatality census keeps detailed data on all motor vehicle crashes that occur on U.S. public roads and result in at least one fatality. The researchers analyzed data from 18,321 fatal two-car crashes. The most common driver error causing those crashes, "failure to keep in lane," occurred in 7,535 instances, the researchers found.

Drivers who were found to have caused the crashes were more likely than those who were not at fault to test positive for prescription opioids (918 versus 549) and alcohol (5,258 versus 1,815). As a sign that the problem has been growing, the proportion of crash initiators with prescription opioids in their system increased from 2 percent in 1993 to 7.1 percent in 2016.

Among the 1,467 drivers testing positive for prescription opioids, 32 percent were positive for hydrocodone, 27 percent for morphine, 19 percent for oxycodone, 14 percent for methadone and 9 percent for other prescription opioids.

Because of its design, the study can only show an association rather than proof that opioids cause crashes, said Dr. Andrew Stolbach, a medical toxicologist and emergency medicine physician at the Johns Hopkins Hospital in Baltimore.

Moreover, it does not suggest that people who are taking opioids for chronic pain are having accidents because of the drugs, said Stolbach, also an associate professor of emergency medicine at the Johns Hopkins University School of Medicine. "You can have psychomotor and cognitive impairment in people who are not used to taking them," he explained.

"But there is scientific literature, using driving simulators, for example, that shows that when a person builds up tolerance many of the psychomotor and cognitive effects that would affect driving are not detected. And that suggests that people on a chronic and stable opioid prescription can drive safely," Stolbach said.

The same is not true for people taking opioids for an acute injury such as a burn or a broken limb, he noted.

Dr. Ajay Wasan, who wasn't involved in the study, questioned its focus on any use of prescription opioids among drivers involved in crashes, describing it as "very misleading, since this is really abuse, not use."

Given that studies have shown that people taking prescription opioids chronically are not impaired when driving, these kinds of accidents, especially those where people drifted out of their lanes, are most likely due to opioid abuse, said Wasan, an anesthesiology professor at the University of Pittsburgh and president elect of the American Academy of Pain Medicine.

Moreover, Wasan expects that the number of accidents involving people taking prescription opioids will turn out to have gone down in more recent years because of the decline in the number of prescriptions written for opioids.

"This sensationalizes an issue when there is a need for better science and clarity," Wasan said.