Effects of Marijuana: Driving-Related Skills

Marijuana Impairs Driving-Related Skills and Workplace Performance.Marijuana use impairs driving-related functions and is linked to a pattern of behaviors that leads to poor job performance, according to two NIDA-supported studies on the effects of marijuana on human performance. Findings from the studies were presented at NIDA’s first National Conference on Marijuana Use.
At NIDA ‘s National Conference on Marijuana Use, Dr. Stephen Heishman presented data from laboratory studies showing that marijuana impairs functions important to driving. Figures from previous studies of automobile accident victims show that from 6 to 12 percent of nonfatally injured drivers and 4 to 16 percent of fatally injured drivers had tetrahydrocannabinol (THC), the psychoactive ingredient in marijuana, in their bloodstream. One study showed that 32 percent of drivers in a shock trauma unit in Baltimore had marijuana in their bloodstream. However, in most of these studies, the majority of subjects who tested positive for THC also tested positive for alcohol, making it difficult to single out THC’s effect on driving.
In a laboratory study at NIDAs Addiction Research Center in Baltimore that controlled for alcohol’s confounding effect, Dr.Stephen Heishiman,a research psycologist in clinical pharmocology, tested marijuana’s effects on the functional components of driving. Study subjects smoked a marijuana cigarette, waited 10 minutes. then smoked another cigarette. Both cigarettes contained either 0. 1.8. or 3.6 percent THC. Twenty minutes after smoking the cigarettes. the subjects were given a standard sobriety test similar to a roadside sobriety test. The test showed that marijuana significantly impaired their ability to stand on one leg for 30 seconds or touch their finger to their nose. As the dose of THC increased, the subjects swayed more, raised their arms, and had to put their feet down in an attempt to maintain their balance. Subjects also committed 2.5 times more errors when they attempted to touch their nose with their finger.
The data from these laboratory studies show that marijuana impairs balance and coordination – functional components important to driving – in a dose-related way, said Dr. Heishman. These effects may be related to reported marijuana-induced impairment of automobile driving, he stated.
Highway and urban driving studies conducted in the Netherlands show less impact on actual driving. However. these driving studies used very low doses of marijuana for safety reasons, Dr. Heishman said. Future research using appropriate safety measures should test the effect of higher doses of marijuana on driving as well as the combined effect of marijuana and alcohol on driving, he concluded.

Source:NIDA conference on Marijuana use,Reported in NIDA notes vol 11.

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