It is very difficult to test whether a driver has been using marijuana.
The reason is that the psychoactive ingredient in marijuana, THC, dissolves in fat, unlike alcohol, which dissolves in water, experts tell NPR.
“It’s really difficult to document drugged driving in a relevant way, [because of] the simple fact that THC is fat soluble,” said Margaret Haney, a neurobiologist at Columbia University. “That makes it absorbed in a very different way and much more difficult to relate behavior to, say, [blood] levels of THC or develop a breathalyzer.”
When a person drinks, alcohol spreads through the saliva and breath, and evenly saturates the lungs and blood, the article notes. That means measuring the volume of alcohol in one part of the body reliably indicates how much is in other parts, including the brain.
Marilyn Huestis, who headed the chemistry and drug metabolism section at the National Institute on Drug Abuse, explained marijuana does not work the same way in the body. THC is fat soluble, meaning it moves easily from water environments, such as blood, to fatty environments. She said fatty tissues absorb THC. “And the brain is a very fatty tissue. It’s been proven you can still measure THC in the brain even if it’s no longer measurable in the blood,” she said.
THC leaves the blood within several hours in occasional marijuana users. A lab test would find only a trace amount of THC in the blood of occasional marijuana users after a few hours. “You could have smoked a good amount, just waited two hours, still be pretty intoxicated and yet pass the drug test [for driving],” Haney noted.
In contrast, people who use marijuana daily may end up with constant, moderate levels of THC in their blood even if they are not high, because heavy smokers build up so much THC in their body fat that it can continue leaching out for weeks after they last smoked.