The legalization of marijuana—now okayed in 28 states plus D.C. for medical use, and in eight plus D.C. for recreational—has ignited controversy over the drug’s safety and effects. And as we reported in the past, the whole picture on pot isn’t exactly clear yet.
Now, a new study suggests that pot might not be as harmless as you think in the short-term: Marijuana use—especially in combination with alcohol—can increase your risk of causing a fatal car accident, the research from Columbia University Mailman School of Public Health suggests.
In the study, researchers analyzed data on 14,742 drivers who were at fault and 14,742 non-culpable drivers from the same fatal car accidents from 1993 to 2014. They found that 28 percent of the drivers at fault tested positive for alcohol, 10 percent for marijuana, and 4 percent for both substances. On the other side, 10 percent of the non-blameworthy drivers tested positive for alcohol, 6 percent for marijuana, and 1 percent for both.
When compared to drivers who tested negative for both pot and booze, those who had both drugs in their system were six times as likely to initiate a fatal crash.
Both drugs separately raised the risk, too: Having just alcohol in your system can raise your risk of causing a fatal wreck by fivefold; testing positive for pot can make you 62 percent more likely to be responsible, according to their estimates.
The three most common driving errors that led to these deadly crashes were failure to stay in your lane, failure to yield to right of way, and speeding.
The researchers do point out one limitation to their study, especially in regard to pot’s culpability: Testing positive for marijuana only indicates past marijuana use—not necessarily impairment and the time of the crash. It’s also possible that some people tested positive for pot because they may have been using it for medical conditions which may have affected their motor or sensory functions, which itself could up the risk of crash.
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Still, prior research has shown that recent marijuana use—like within the past few hours can increase your risk of causing a crash, likely because it can impair your motor skills and driving performance. (Want more health news delivered to your inbox each day? Sign up for our Daily Dose newsletter.)
More research is needed to determine how dosage of marijuana affects crash rate, the researchers say. In the meantime, remember these stats next time you’re tempted to get behind the wheel right after you smoke up.
