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(CNN) --
More than 40% of drivers who reported using both alcohol and cannabis in a national survey also reported driving under the influence of one or both substances, according to a new study.
"Alcohol and cannabis are two of the most common substances in drunk driving and traffic accidents" in the United States, Priscila Dib Gonçalves, lead author of the study, said in a news release.
Gonçalves is a postdoctoral researcher in the Department of Epidemiology at Columbia University's Mailman School of Public Health.
Having alcohol or cannabis in your system while driving can impair cognitive function and physical movement related to those mental processes, impacting driving performance, according to previous research.
A person drinks an alcoholic beverage while smoking cannabis.
Adults who reported using cannabis and alcohol also reported driving under the influence of these drugs, according to a new study.
Concomitant use of both substances has produced more serious and fatal outcomes related to driving, compared to the number of results derived from the use of either of the two substances separately, especially among younger people, according to previous research.
But until now, no nationally representative study of these associations had been conducted among adults aged 16 and older, according to the authors of the new study.
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The authors used data from more than 34,000 drivers who responded to questions about their drug use and driving habits for the National Survey on Drug Use and Health between 2016 and 2019. More than half of the participants reported that they had not driven under the influence of alcohol or cannabis, while 42.3% said they had.
More than 8% of the survey participants reported consuming alcohol daily, and nearly 20% used cannabis daily.
About 28% of the participants reported having used both drugs simultaneously, which the study defined as "using marijuana or hashish at the same time or within two hours of their last alcohol consumption."
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"The high prevalence of both alcohol and cannabis users who also reported driving under the influence of alcohol in the past year was surprising," said study lead author Pia Mauro, an assistant professor of epidemiology in the School of Health. Public Mailman of Columbia University.
"Two out of five people in the sample reported driving under the influence, which indicates that it is not uncommon among people who use these substances."
The authors found that people who used cannabis and alcohol simultaneously were almost three times more likely to drive under the influence of cannabis, and three and a half times more likely to drive under the influence of both drugs.
The study was published Tuesday in the
American Journal of Preventive Medicine.
"In the context of increasing daily cannabis use among adults, our findings connecting daily cannabis use and alcohol-impaired driving raise public health concerns," Mauro said in a news release.
"There is a need to continually monitor changes in the population in frequency of cannabis use that may be associated with health hazards, including daily use."
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Avoid unsafe driving
Of all the study participants, between 57% and 68% were male, white, had a household income of $40,000 or less, and lived in a state where medical cannabis was legal.
Since details of drug use and impaired driving were based on self-reports, the authors did not have data on the actual levels of cannabis or alcohol in the participants' bodies, nor about whether any of them had been cited for driving under the influence of alcohol or had been involved in an incident behind the wheel.
However, "when people use alcohol and cannabis (for example, daily) or at the same time, there may be more potential opportunities to drive while intoxicated," Mauro told CNN.
Previous research "has shown that we are making progress in reducing the likelihood of drivers drinking and driving. This study demonstrates that the majority of drunk driving offenses are not solely related to alcohol use," he said by email. Yvonne Terry-McElrath, a research associate at the Institute for Social Research at the University of Michigan.
Terry-McElrath was not involved in the study.
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"There is a great need for accurate information on the extent to which cannabis and its concomitant use impact driving skills and ability," he added.
People who use alcohol or cannabis should be aware that these drugs can cause "a decrease in someone's ability to respond to things that are happening on the road, even at lower levels" of use, said Linda Degutis, a professor at the Yale School of Public Health and former president of the American Public Health Association, who was not involved in the study.
"You don't have to get to the point of feeling drunk for your ability to respond to, say, changes in the road or a pedestrian to cross or any kind of situation where you have to react relatively quickly to be affected," Degutis said.
If you've recently used cannabis or alcohol and are thinking about driving, ask someone who isn't intoxicated to drive, Degutis said.
You can also use public or shared transport, or simply not travel if it is not essential.
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