Does marijuana use affect driving?
Marijuana significantly impairs judgment, motor coordination, and reaction time, and studies have found a direct relationship between blood THC concentration and impaired driving ability.6–8
Marijuana is the illicit drug most frequently found in the blood of drivers who have been involved in accidents, including fatal ones9 Two large European studies found that drivers with THC in their blood were roughly twice as likely to be culpable for a fatal accident than drivers who had not used drugs or alcohol.10,11 However, the role played by marijuana in accidents is often unclear, because it can remain detectable in body fluids for days or even weeks after intoxication and because users frequently combine it with alcohol. 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
Several meta-analyses of multiple studies found that the risk of being involved in an accident significantly increased after marijuana use12—in a few cases, the risk doubled or more than doubled.13–15 However, a large case-control study conducted by the National Highway Traffic Safety Administration (NHTSA) found no significant increased crash risk attributable to cannabis after controlling for drivers’ age, gender, race, and presence of alcohol.16
This series of reports simplifies the science of research findings for the educated lay public, legislators, educational groups, and practitioners. The series reports on research findings of national interest.