Guohua Li, a physician who was the study’s lead researcher, said the findings demonstrate the ripple effect of the national opioid crisis.

“The impact of the opioid epidemic goes far beyond the body count from overdoses,” Li said in an interview Thursday. He said the findings should stand as a warning to clinicians and patients about the risks of getting behind the wheel when using prescription narcotics such as Vicodin, OxyContin, morphine or codeine.

Before opioid use became widespread, only 1 percent of all fatally injured drivers were found to have prescription narcotics in their systems. But that figure rose to more than 7 percent in the past two decades. While the percentage of Americans using prescription opioids has declined in recent years, the overall rate of use of medicines such as remains high.

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The study, conducted by Li and colleagues at the Columbia University Mailman School of Public Health, looked at more than 18,000 fatal two-car crashes that occurred between Jan. 1, 1993, and Dec. 31, 2016.

The researchers, using data from the National Highway Traffic Safety Administration (NHTSA), first weeded out crashes involving tractor trailers or other commercial vehicles and crashes in which toxicology reports were missing or incomplete. They analyzed only those that involved passenger car crashes and in which only one driver was deemed to be at fault.

Then researchers compared the two drivers, using a number of variables, including age, sex, driving records and prescription opioid use, among others. The researchers found that the drivers who were at fault in a two-car fatal crash were more likely to test positive for prescription opioids (5 percent) than the driver who was not at fault (3 percent).

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Of the drivers who tested positive for opioids, nearly 32 percent were using hydrocodone, the active ingredient of Vicodin and other brand prescription drugs; nearly 27 percent were using morphine; and more than 18 percent were using oxycodone, which is marketed as OxyContin and other brands.

When controlling for all factors except opioid use, the researchers found that the odds of triggering a two-car fatal crash for drivers who tested positive for opioids were more than twice the odds for the person who tested negative.

The study also found that the most common driving error cited in fatal crashes was failure to stay in one’s lane (41 percent). That was followed by failure to yield the right of way, such as by running a stop sign or a traffic light (25 percent) and speeding (17 percent). Among drivers who tested positive for opioids, failure to stay in one’s lane was the most common error cited (55 percent).

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“That was a surprise even to us,” Li said. “We didn’t expect those kinds of striking difference in the two groups.”

The study was published by JAMA Network Open.