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September 18, 2016

What level of THC in blood causes driving impairment?

Let us provide a rational answer to a nonsensical question. It is a nonsensical question because blood is never impaired by THC. Never. Alcohol doesn’t impair blood either. These drugs only impair the brain, not the blood. We can only test for drug content in the brain by means of an autopsy, something most drivers would reasonably object to. We test blood as a surrogate for what’s in the brain. For alcohol, blood is a very good surrogate. Alcohol is a tiny, water-soluble molecule that rapidly crosses the blood-brain barrier and quickly establishes and maintains an equilibrium concentration between what’s in the blood and what’s in the brain.

Blood is a terrible surrogate for learning the amount of THC in the brain. It’s used because we blindly follow the precedence set by alcohol, perhaps even believing the pot lobby’s mantra that marijuana should be regulated like alcohol. It’s also used because we haven’t proven anything else that’s any better. Oral fluid likely is somewhat better, but that may only be because it can be collected more quickly at the roadside.

Blood is a terrible surrogate because unlike alcohol, THC is a very large fat-soluble molecule. This results in three major differences in behavior compared to alcohol:

Perhaps this explains why researchers agree that marijuana impairs driving, but none claim there is a good correlation between blood levels of THC and impairment.

The fact is that there is no level of THC above which, everyone is impaired, and below which, no one is impaired.

The same is true of alcohol. In spite of common belief, the .08 BAC limit wasn’t determined by science. It can’t be, due to the reality of biological variability. The .08 BAC limit was determined by politicians, using scientific input as well as societal input. That explains why the alcohol per se limit varies from .02 to .08 gm/dl in various developed countries of the world, and those countries based their decision all on the same science! It’s other societal inputs such as risk tolerance and desire for freedom that come into play to make that decision.

None of this proves it’s safe to drive after smoking pot. It’s not. It simply explains why a defined per se limit of THC in blood that proves someone is impaired can never be supported by science.

This also may explain why the preferred means to deal with drug impaired driving is not to establish per se limits, but rather to establish a zero tolerance policy for mind altering drugs in a driver that has been shown to be impaired.

2 Comments

  1. HARD TO BELIEVE SCIENCE HAS NOT DEVISED A WAY OF DETERMINING A LEVEL OF IMPAIRMENT THAT MAKES IT UNSAFE TO DRIVE A VEHICLE.

    MAYBE THE ANSWER IS IN PURSUING A LEVEL OF REACTION TIME AND ALERTNESS
    TO A DRIVER’S SURROUNDINGS!!!!

  2. Richard says:

    We don’t need to establish the link between levels of THC in the blood and in the brain, and we don’t test blood as a surrogate to what’s in the brain. We test THC levels in the blood and corrolate this with driving impairment by measuring THC in the blood and comparing it to the crash risk increase relative to people with no THC in the blood. THC peaks in the bloodstream after 15 minutes of smoking and drops after 4 hours (8 hours for eating it after ingestion). After this period researchers show no increase in risk of crashing when below 5 nanograms/ml. Consumption of antihistamines, antidepressants and benzodiazepines increases crash risk by 45%, alcohol over 0.08grams/dl had a 600% increase. DON’T DRIVE UNTIL 4 HOURS AFTER SMOKING AND 8 HOURS AFTER INGESTING!

    Actually Richard, according to the author of this blog, you are incorrect. Richard is incorrect. There is no correlation between forensically-determined blood THC levels and crash risk. Smoked THC’s rapid redistribution from blood to fatty tissues means the blood THC level is very highly dependent upon the time between apprehension and blood draw. Dose and time since consumption are secondary factors. Hartman demonstrated that the maximum blood THC levels declined an average of 76% within the first 25 minutes after beginning to smoke a joint. Time between apprehension and taking a blood sample can vary from less than 30 minutes to over 6 hours. Consequently, forensically-determined blood THC levels are far too erratic to establish a correlation with crash risk. Laboratory experiments can do so, but their results are meaningless in the real world of law enforcement. The pharmacokinetics of ingested THC behave so differently from smoked of vaped THC that there is no correlation between consumption time and blood THC levels between the two dosing methods.

    Ed

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