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.