Drivers who smoked marijuana and then got behind the wheel of a vehicle nearly tripled the risk of crashing when compared to others not under the influence, a new study from Columbia University's Mailman School of Public Health finds.
There's an even higher risk should he driver have alcohol in his or her system, the study published online on Oct. 4 in Epidemiologic Reviews noted.
These findings come at a time where there are moves in many states in the U.S. to legalize medical marijuana.
Based in a large survey conducted in 2009, researchers estimated that more than 10 million people who are 12 years and over have driven under the influence of illegal drugs during the previous year.
The researchers also found that 28 percent of drivers who die from a crash tested positive for drugs that are not alcohol. More than 11 percent of the general driving population also tested positive for drugs that are not alcohol. Marijuana was found to be the most common substance detected in drivers after alcohol, according to the study.
As more and more states consider medical use of marijuana, there could be health implications, the study's senior author Dr. Guohua Li told USA Today.
Alcohol use has decreased over the past 40 years, however, illegal used of non-alcoholic drugs like prescription medications and marijuana have increased, Li said.
Li is a professor of epidemiology at Columbia University Mailman School of Public Health in New York City.
What exactly was the impact marijuana use had on road accidents was unclear. So Li and the team decided they wanted to to find if there was any link between marijuana use and the rish of a vehicular accident.
The team therefore, assessed nine previous epidemiologic studies in six countries looking at marijuana use and motor vehicle accidents. This is called a meta-analysis.
The studies the team looked at focused on different time frames. Some also assessed marijuana use one hour before driving while others looked at use one year or more.
One of the studies cited noted that a person's driving skills are acutely affected for three to four hours after marijuana use.
The Columbia University researchers also found that people who tested positive for marijuana or reported having just used it had more than twice the risk of being involved in a car crash when compared to people who took no drugs including alcohol. The risk of a crash was almost 2.7 times higher.
Experts say that marijuana use may affect a person's reaction times and coordination to name a few.
The team also noted that there is an association between the levels of marijuana concentration found in urine and crash risk i.e. the higher the concentration the higher the risk.
None of the studies the group assessed specifically looked at medical marijuana. Medical marijuana is now legal in 16 U.S. states, according to USA Today, as well as the District of Columbia in the United States.
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, the authors wrote. It also would play a critical role in informing policy on the use of medical marijuana.
The team also noted that 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 to better understand the role of marijuana in causing car accidents.
One expert told USA Today that there must be caution used against inferring too much from Li's study, as it wasn't designed to capture cause and effect.
We can't really say yet that marijuana increases the risk by two or three times, Chuck Farmer, director of statistics at the Insurance Institute for Highway Safety in Arlington, Va., said. Most of their studies pointed to a very strong bad effect of marijuana on driving, but there are other studies out there that actually go the other way.