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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.


  1. J says:

    Would a THC-COOH urine test result of 529 ng/ml (by LC/MS/MS) necessarily indicate impairment?
    Logic would dictate yes, however I have no technical background so would be good to hear from someone that knows.
    I have found it difficult to find the answer online.
    Thanks. John

    • Candace Lightner says:

      John you should consult a toxicologist for the answer to this however, the most important factor would be your driving behavior. It really is up to the police and the courts to determine if you are impaired.

  2. brian pelton says:

    great answer, ed. thank you. i was only on this site because of reading about the decline and eventual suicide of rashaan salaam. such a tragedy. he had 55 ng of thc in his blood, as well as a pretty high bac of 0.25%. i am not sure how long these tests were performed after he was discovered in the park parking lot. hard to watch the decline of such a gifted athlete. have not been able to find any data to know if 55 ng is a lot, a fair amount, or a little. i know in the long run it doesn’t matter, but i was just curious, as i have nothing to compare with that amount.

  3. Nancy Berry says:

    We have a situation that is court related regarding driving children. The court judge wants some type of verification that a 6’3″ 220lbs 40 year old man who smokes Marijuana daily, is not impaired while driving his children for his visits every other weekend. His blood level is 1000 when the ex-wife has him drug tested. There are no other drugs noted. He has been doing this for the last 3 years with no incidents, but the ex wants to make his life miserable because of a new girlfriend. Where can I find some verification that you noted in your assessment.

    • Candace Lightner says:

      I strongly suggest you contact a toxicologist in this case. However, I will tell you that people drive impaired in many cases for years and never get caught or crash and then they do, often with deadly consequences. Because nothing has happened doesn’t mean it is safe behavior. To be on the safe side wouldn’t it be better to just use a taxi when transporting the children?

  4. Susan Jacobs says:

    I have a marijuana card and got pulled over. I had a e pen, sitting in my door. The cop put me in cuffs and took me down to their station for blood draw. Towed my car. I had a severe panic attack during whole thing. I am being charged with dui because there was marijuana in my system.

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