The simple fact that THC is fat soluble. 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."  The height of your intoxication isn't at the moment when blood THC levels peak, and the high doesn't rise and fall uniformly based on how much THC leaves and enters your bodily fluids, says Marilyn Huestis, who headed the chemistry and drug metabolism section at the National Institute on Drug Abuse.