Cannabis and alcohol mix leads to more crash deaths, new study shows

Dec 7, 2021

According to the results of a new study, the percentage of car crash deaths in the U.S. involving cannabis have doubled, while the percentage of deaths involving both cannabis and alcohol, have more than doubled between 2000 and 2018.

The study was published in the American Journal of Public Health and conducted by researchers from Boston Medical Center, Boston University, and University of Victoria. Its results demonstrate that people who died in car crashes involving cannabis had 50% increased likelihood of also having alcohol in their system, suggesting that as more U.S. states have been decriminalizing and legalizing cannabis, it has been increasingly used with alcohol when driving.

While proportion of crash deaths involving alcohol has remained relatively constant over the last years, the proportion of crash deaths involving other substances, including cannabis, has increased. Specifically, the results of this study suggests that cannabis and alcohol are increasingly being used together when it comes to impaired driving, and that cannabis increases the likelihood of alcohol use in crash deaths.

“There has been progress in reducing deaths from alcohol-impaired driving, but our study suggests that cannabis involvement might be undercutting these public health efforts,” says Dr. Timothy Naimi, an adjunct professor at Boston University Schools of Medicine and Public Health, and director of the Canadian Institute of Substance Use Research in Victoria, Canada and senior author of the study. Currently, almost 40% of crash deaths in the U.S. involve alcohol, and 30% of deaths involve a level of alcohol that is above the legal limit for driving.

Researchers analyzed 19 years of data from the Fatality Analysis Reporting System, a national database of fatal crashes on public roads to see what trends, if any, could be seen. They found that the percentage of crash deaths involving cannabis more than doubled from 9.0% in 2000 to 21.5% in 2018, and the percent of deaths involving cannabis and alcohol also more than doubled from 4.8% to 10.3%. In addition, cannabis was demonstrated to be a risk factor for alcohol co-involvement, even at levels below the legal limit.

These results reveal that cannabis-involved car crashes are more likely to involve the deaths of passengers, as well as individuals younger than 35 years, compared to crash deaths that do not involve cannabis. The study’s authors carried out a series of analyses to account for drug testing rates and alcohol policies, and the results remained consistent.

“Our testing methods for cannabis remain suboptimal and individuals can test positive for cannabis weeks after they have consumed it. However, we can say that fatalities from crashes involving cannabis are more likely to have also involved alcohol, even if we don’t know the exact level of cannabis,” said Marlene Lira, an epidemiologist at Boston Medical Center and first author of the study.

“The bottom line is that we have a lot of work to do to reduce deaths and harms from impaired driving from alcohol, cannabis, and other substances,” said Lira.