Driving under the influence of marijuana is associated with an increased risk of a motor vehicle crash, especially for fatal collisions, an analysis of nine studies concludes.

The analysis found driving under the influence of marijuana was associated with almost twice the risk of a motor vehicle crash compared with unimpaired driving, CNN reports. The studies in the analysis included nearly 50,000 people.

The results are published in the British Medical Journal. According to a press release issued by the journal, this is the first review to look at observational studies concerned with the risk of vehicle collision after the use of marijuana. “Previous studies have failed to separate the effects of alcohol and other substances from the use of cannabis, resulting in a lack of agreement,” the release notes.

Lead researcher Mark Asbridge of Dalhousie University in Halifax, Nova Scotia, said while alcohol impairs drivers’ speed and reaction time, marijuana affects spatial location. He said drivers who have recently smoked marijuana may follow cars too closely, and swerve in and out of lanes. He added that while people who are drunk often recognize they are impaired by alcohol, those under the influence of marijuana often deny they are impaired.

A 2009 report from the National Highway Traffic Safety Administration (NHTSA), based on blood, breath and saliva tests collected on weekends from drivers in 300 locations nationally, found that 16.3 percent of drivers at night were impaired from legal or illegal drugs, including 9 percent of drivers who had detectable traces of marijuana in their system.