Occasional users can be very impaired at one microgram per liter, and chronic, frequent smokers will be over one microgram per liter maybe for weeks." "And it's almost impossible to come up with one number. "Everyone is looking for one number," she says. The instinct, Huestis says, is to come up with a law that parallels the 0.08 BAC standard for alcohol.
While scientists continue to bang their heads over how to draw up a biological measurement for marijuana intoxication, legislators want a way to quickly identify and penalize people who are too high to drive. This all translates into a colossal headache for researchers and lawmakers alike. The marijuana advocacy group NORML emphasizes that driving high can be dangerous, and advises people to drive sober. But in her studies, she found that being blazed enough, as when a smoker's blood THC level peaks at 13 nanograms per milliliter, could be just as a dangerous as driving drunk.
Pot smokers, she says, "tend to be more aware they're impaired than alcohol users." Drunk drivers are more aggressive, and high drivers are slower. The attitude difference between stoned drivers and alcohol drivers seems clear, Huestis says. They were cognitively impaired for up to 28 days after their last use, and their driving might also still be impaired for that long. "We found brains had changed and reduced the density of cannabinoid receptors," she says. It gets trickier when you try to factor in the chronic effect of smoking weed, Huestis says. These chronic, frequent users will also experience a rapid loss of THC from their blood after smoking, but they will also have a constant, moderate level of blood THC even when they're not high, Huestis says. Huestis says that heavy smokers build up so much THC in their body fat that it could continue leaching out for weeks after they last smoked. It's a hundredfold difference," she says.īut daily users are different. "With oral THC, it takes several hours for to peak, but it remains very low compared to the smoked route, even though they're very high. "The 0.08 standard in alcohol is from decades of careful epidemiological research," says Andrea Roth, a professor of law at the University of California, Berkeley.Īnd if you eat the weed instead of smoking it, Haney says, your blood never carries that much THC. Eventually, decades of study helped formulate the 0.08 blood alcohol limit as too drunk to drive safely. That made it possible to do the science on alcohol and crash risk back in the mid-20th century. Measuring the volume of alcohol in one part of your body can predictably tell you how much is in any other part of your body - like how much is affecting your brain at any given time. It evenly saturates your lungs and blood. When you drink, alcohol spreads through your saliva and breath. That makes it absorbed in a very different way and much more difficult to relate behavior to, say, levels of THC or develop a breathalyzer."
"It's really difficult to document drugged driving in a relevant way," says Margaret Haney, a neurobiologist at Columbia University, " the simple fact that THC is fat soluble. But the decades of experience the country has in setting limits for alcohol have turned out to be rather useless so far because the mind-altering compound in cannabis, THC, dissolves in fat, whereas alcohol dissolves in water.Īnd that changes everything.
Law enforcement officials would love to have a clear way to tell when a driver is too drugged to drive. The psychoactive ingredient in marijuana is a fat-soluble compound called THC.