Drivers who test positive for marijuana or reported using marijuana and then driving have double the risk of being involved in a vehicle accidents, compared to drivers who are not under the influence, researchers from Columbia University’s Mailman School of Public Health, New York, reported in the journal Epidemiologic Reviews. The authors add that over ten million individuals aged at least 12 years are thought to have driven under the influence of illegal drugs during the last twelve months in the USA.

As background information, the researchers explained that marijuana is the most commonly detected non-alcohol drug in vehicles drivers. However, what impact it has on road accidents had been unclear.

Guohua Li, MD, DrPh. and team had set out to determine whether there might be a link between marijuana use and vehicle accident risk. The team carried out a study of nine previous epidemiologic studies – a meta-analysis.

They found that people who tested positive for marijuana or reported having just used it had over twice the risk of being involved in a car crash compared to people who had consumed no drugs (including alcohol). They also reported an association between levels of marijuana concentration found in urine and crash risk – the higher the concentration the higher the risk.

Eight of the nine studies they examined reported that marijuana users are considerably more likely to crash their vehicles than individuals who are not under the influence. The exception was one small study carried out in Thailand, where marijuana use prevalence is lower than elsewhere.

In 28% of driver fatalities the driver tested positive for non-alcohol drugs, the researchers wrote. Over 11% of the general driving population (regardless of accidents) tested positive to non-alcohol drugs. Marijuana is by far the most common substance detected.

Dr. Li and team warn about inferring causality from these epidemiologic studies, even though there is clear evidence of an association between marijuana consumption and crash risk.

The authors wrote:

“If the crash risk associated with marijuana is confirmed by further research, this is likely to have major implications for driving safety and public policy. It also would play a critical role in informing policy on the use of medical marijuana. Given the ongoing epidemic of drug-impaired driving and the increased permissibility and accessibility of marijuana for medical use in the U.S., it is urgent that we better understand the role of marijuana in causing car accidents.”

Written by Christian Nordqvist