When a person drinks, alcohol spreads through their saliva and breath evenly saturating their lungs and blood. It is therefore possible to tell how much alcohol is affecting the brain at any given time simply by measuring the volume of alcohol elsewhere. This made it possible for scientists to study alcohol and crash risk, and to formulate a 0.08 blood alcohol limit as too drunk to safely drive.
But marijuana doesn’t work like that. The height of intoxication does not occur when blood THC levels peak, nor does the high rise and fall uniformly based on how much THC leaves and enters a person’s bodily fluids.
Unlike alcohol which dissolves in water, THC is dissolved in fat. This means that it moves from water environments (blood) to fatty environments like the brain. And once THC has reached these fatty tissues, it stays there. You’re still able to measure THC in the brain even if it’s no longer measurable in the blood.
What’s more, research has found that THC leaves the blood of occasional users within a couple of hours. A lab test would therefore only find a trace amount of THC in the blood of these occasional users after just a few hours. Occasional users are able to smoke a large amount, wait a couple of hours, and pass a drug test for driving even if they are still intoxicated.
This isn’t true of daily users though. A heavy smoker builds up enough THC in his/her body that it could continue to trickle out into the blood for weeks after their last smoking session. Frequent users will experience a rapid loss of THC from their blood after smoking, but even when these users are not high they will have a constant level of blood THC. The brains of chronic smokers actually change, reducing the density of cannabinoid receptors. Research has found that these users can be cognitively impaired for nearly a month after their last use, and their driving could potentially impaired for that long as well.
And then there are the differences between smoking weed and eating it. When a person eats weed their blood never carries that much THC and it can therefore take several hours for blood THC to peak. Even then, blood THC is very low compared to when weed has been smoked, even though a person is still very high.
Because of these variations scientists have not yet found a way to determine a biological measurement for marijuana intoxication. There simply is no easy way to come up with a standard like the 0.08 for alcohol.