Marijuana use worsens reaction time, perception, coordination and decision making; thus, one would think it would worsen driving. Marijuana appears to influence automatic performance whereas alcohol appears to impair judgment. Also, some data suggest that marijuana users are more aware of their intoxication than alcohol users and compensate for this by driving slower or more carefully. Moreover, regular users may become tolerant of the intoxicating effects of marijuana. A recent review (Li et al, Epidemiology Reviews 34:65-72, 2012) located nine studies of the risk of automobile crashes. Eight of the nine studies found more accidents in those using vs. not using marijuana (the one negative study was very small). In these studies, marijuana users were 2.7 times more likely to have an accident. Heavier users of marijuana had more accidents than light users. Use of both alcohol and marijuana together increased accidents even more; however, the effect of marijuana on crashes was not due to their greater use of alcohol.
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–Dr. John R. Hughes