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Does marijuana use affect driving?

A person driving a car with impaired driving ability

Marijuana significantly impairs judgment, motor coordination, and reaction time, and studies have found a direct relationship between blood THC concentration and impaired driving ability.6,7 Marijuana is the illicit drug most frequently found in the blood of drivers who have been involved in accidents, including fatal ones8 (although it is important to note that marijuana can remain detectable in body fluids for days or even weeks after acute intoxication). A meta-analysis of multiple studies found that the risk of being involved in an accident roughly doubles after marijuana use.9

Accident-involved drivers with THC in their blood, particularly higher levels, are three to seven times more likely to be responsible for the accident than drivers who had not used drugs or alcohol. The risk associated with marijuana in combination with alcohol appears to be greater than that for either drug by itself.7

This page was last updated September 2015

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