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Marijuana use can distort the senses, slow reaction time, and impair judgment and coordination, making it highly dangerous to drive under the influence of the drug.
It is a mind-altering drug, and as such, it should not be combined with operating a motor vehicle.
A marijuana high generally lasts about two hours; however, the National Highway Traffic Safety Association (NHTSA) warns that marijuana can impair driving for up to three hours. Issues can include:
Marijuana is considered an illegal drug on the federal level; however, several states have passed laws legalizing its use for medical and recreational purposes. It is harder to measure the level of impairment on marijuana as opposed to impairment due to alcohol. Alcohol impairment is measured by a breathalyzer or blood test, which analyzes a person’s blood alcohol concentration, or BAC. All 50 states cite anything over a 0.08 g/dL BAC as being legally impaired. If a driver is found to have a 0.08 g/dL BAC or above, they are issued a driving under the influence (DUI) ticket and often arrested. Alcohol and marijuana are metabolized differently, however, and such a test does not exist for marijuana intoxication.
Laws are different regarding driving under the influence of drugs, depending on the state and local government. For example, some states have a zero-tolerance policy, NIDA reports, and if any amount of THC (the active psychoactive chemical in marijuana) is present then a DUI may be issued. Other states, like Colorado where recreational pot use is legal, have a limit of five nanograms of THC per milliliter of blood, NPR publishes. Anything above this is considered impaired, and drivers may be arrested and issued a DUI. Still other states may issue a DUI if impairment is even suspected.
This is not to say that someone who has less THC in their bloodstream is not impaired, and the opposite can also be true. THC can stay in the bloodstream for longer than alcohol even when it is not actively impairing a person. THC is stored in fat cells, and a person can test positive for the drug up to a week after using it, even longer after the negative effects have worn off. An infrequent user may be very impaired with five nanograms of THC per milliliter of blood, while a regular user may not be. In a regular user, THC is likely to stay in the bloodstream longer as well.
At this point, there are no consistent measures to test marijuana impairment as it relates to driving. New methods are being researched and tested for accuracy, but they are not ready yet.