Driving and THC.  OR Blood Drunk or Blood Stoned is a dilemma in Boulder and elsewhere .

First, a warning and some abbreviations. For the cannabinoid primarily responsible for getting us buzzed we look to  Delta-9 Tetrahydrocannabinol, I’ll use the shorthand “THC.” No longer a great abbreviation because Delta-8 and Delta-10 are becoming more a part of the language. “Cannabinoid” is simply a chemical found in the cannabis plant. Not every one of the 105+ cannabinoids are found in every plant, and percentages vary. Finally, this blog may take an extra bit of concentration because of the nature of the topic. I am explaining something that is counter-intuitive, and that is the heart of the blog. So prepare your mind as appropriate and I suggest that this is not one to skim. It is however VERY important. So many who work in the field do not appreciate the import of current and past science on understanding the blood numbers. That severe ignorance has invaded the law, which as a lawyer, is tragic.

The news has been flooded with stories about new devices to detect blood levels of THC (the one we know and love). The newest claims to detect how recently the THC was ingested. The devices look at breath, at saliva, and more. But do they yield any meaningful results regarding possible impairment? While they may in fact accurately detect THC in the body, and accurately even disclose the amount of THC, does that tell us anything useful? NO NO NO!  The results are basically meaningless! Beyond one thing. A blood test showing the presence of Delta-9 THC does indicate that at some time in the probably recent past the blood donor consumed Delta-9 THC. AH, but there’s the rub!

We are used to the “alcohol model.” It is drilled into our heads. It is scientifically validated and valid. Of course there is a range of people on either side of the “norm.” Generally we KNOW that the higher the BAC blood alcohol content, the more impaired a person’s driving becomes. We KNOW that if a person has a BAC of 0.08 or higher, it is highly likely that they have no business driving, are impaired, and are properly subject to the DUI legal limits. While some may be “fine” at higher BACs, and some impaired at much lower levels, many people at 0.08 are breaking the law and for many, the law is fair and reasonable.  The “legal limit” is not unreasonable. Anything but. While not perfect, it passes legal muster by providing due process notice of what is illegal, with a sound scientific underpinning.

The assortment of THC measuring devices may yield correct results as to the measurements they are taking, BUT does that tell us anything about whether a specific individual is impaired by the marijuana? NO NO NO I repeat repeatedly. I repeat it to try to overcome the common misconception that pot and alcohol behave the same way. The science is overwhelming that alcohol, etoh, ethyl alcohol, drinking alcohol, fits its model, and it is also overwhelmingly clear that this does NOT hold true for THC. One person’s 5 nanograms per milliliter (a very small weight per a small amount of blood) might have us with a “trashed” subject for our experiment. For another, typically a more experienced marijuana enjoyer, they might be “sober,” showing no impact or impairment from marijuana at very high levels. For example, a person with 200 ng/ml in their blood may well be impacted the same as another with 3 ng/ml in their blood. That is NOT the alcohol model. Technically, while BAC blood alcohol level does correlate with behavior changes, THC blood levels do not correlate to impact on behavior.

So if one person is at 75 ng/ml, and they smoke more, they will be more impacted. But if two people have the same level, it is well established by many experiments that the relative levels tell us nothing about the relative degrees of impairment or impact.

As a practical matter, we should be looking at behavior, not at blood, for pot. We should look at balance, reaction time perhaps, short term memory, failure to maintain lateral positioning (weaving), driving too slowly, and more. All behavior, no chemistry. And do we really care if someone is behaviorally impaired whether it is caused by a joint, or by lack of sleep, or by the combo of prescription meds taken as prescribed along with some lack of sleep? Personally I do not want to share the road (or the sidewalk) with someone who cannot meet a minimum standard of physical and mental functioning. Driving is NOT a legal right. It is a legal privilege. Will that mean that more people cannot get a license to drive? Sorry, it does. Does it drive the need for better public transportation? Yes! And that is generally a laudable goal.

CAUTION! Remember that a mixture of a joint and a beer works synergistically. The impact on behavior is greater than the sum of the parts. They enhance each other. The combination is far more dangerous and far more impairing that either alone. That may be fine for sitting in a lounge chair at home and watching Harry Potter, but it is a deadly combo for driving or using a table saw. Also do not send emails in that conditions. “Un-send” does not work well at all.

We are so used to two things. First, that the number, the BAC, has regarding impairment,  and second, are people generally impaired to similar degrees with similar BACs? tje answer is again “yes.” These things are true, proven, accepted, and form a rational scientifically sound basis for the law. I refer to blood testing, which if done properly, is accurate. Breath testing is nonsense IMO in my opinion. NOT LEGAL ADVICE. Nothing in this blog is legal advice. Blood, absent operator error, is pretty reliable scientifically. A GCMS gas chromatography mass spectrometer yields amazingly reliable results if used properly. And it yield a number for the blood level of etoh drinking alcohol that has meaning. Due process is happy, and sober drivers can be happy also.

Remember that this blog is about pot vs alcohol and measuring impairment. If we measure the reaction time of people who have imbibed Delta-9 THC we may assume that they are not driving as well as they would if their reaction time was normal. Same for alcohol (etoh for ethyl alcohol). For etoh, most will have the same impact on their reaction time as others at the same BAC. So if we measure the PBL (pot blood level), certain levels should produce similar changes in reaction time. Same for balance. Same for short term memory. Same for hand-eye coordination. Same for failure to maintain lateral positioning (fancy for weaving). As many state laws mandate, a Delta-9 THC blood level greater than 5 ng/ml is illegal. While some states permit a defendant to argue that they were not impaired regardless of the blood result, other states have a “non-rebuttable presumption.”  The driver is presumed impaired based upon the number alone. But is that scientifically valid? Simple. NFW! (Not Fairly Witnessed).

We are so used to alcohol etoh model that we apply it to marijuana very naturally. The etoh model is intuitively perfect. It makes sense. The problem is that the premise, that blood level of Delta-9 THC indicates degree of impairment is simply this. WRONG!!!  One person’s pleasure is another’s poison. One person’s 0.05 ng/ml might have them trashed, completely stoned, fit to do many things not including driving. BUT another person’s 5 ng/ml might mean NOTHING as to whether or not they are impaired. An individual might have a marijuana blood level of 200 ng/ml and show zero impairment.clear drinking glass on white table

Speaking of impairment, should we perhaps use the word “impacted?” Some changes in behavior might be just that. Changes in behavior might be just that, with no impact on driving.

Back to the subject. The key, the Cliff Notes, are simple. The impairment of alcohol DOES correlate well with BAC. BAC levels and impairment ARE similar, person to person. That makes sense. As with so many things about being stoned, that does NOT hold true for marijuana (by which I refer to the plant and primarily Delta-9 THC). One person’s blood level does NOT predict the level of impairment on another person. They simply do not correlate Counterintuitive as it is, the science is quite clear. There are many peer reviewed studies, and more all the time, which establish and confirm that the blood level of Delta-9 THC does NOT correlate with the degree of impairment.

What does it all mean? As a defense attorney of almost 50 years it means to me that people can be charged and convicted of DUID pot, driving while impaired by marijuana even if they are NOT impaired at all. It is like saying you can be convicted of DUID pot if your cholesterol measures high. The blood level simply has nothing to do with impairment.


  • Senior Counsel Emeritus to the Boulder Law firm Dolan + Zimmerman LLP : (720)-610-0951
  • Former Judge
  • Photographer of the Year, AboutBoulder 2023
  • First Chair and Originator of the Colorado Bar Association’s Cannabis Law Committee, a National first.
  • Previous Chair, Boulder Criminal Defense Bar (8 years)
  • Twice chair Executive Counsel, Colorado Bar Association Criminal Law Section
  • NORML Distinguished Counsel Circle
  • Life Member, NORML Legal Committee
  • Life Member, Colorado Criminal Defense Bar
  • Board Member Emeritus, Colorado NORML
  • Chair, Colorado NORML, 7 years including during the successful effort to legalize recreational pot in Colorado
  • Media work, including episodes of Fox’s Power of Attorney, well in excess of many hundreds media interviews, appearances, articles, and podcasts, including co-hosting Time For Hemp for two years.
  • Board member, Author, and Editor for Criminal Law Articles for the Colorado Lawyer, primary publication of the Colorado Bar Assoc. 7 Years, in addition to having 2 Colorado Lawyer cover photos, and numerous articles for the Colorado Lawyer monthly publication.
  • LEAP Speaker, multi-published author, University lectures Univ. of Colorado, Boulder, Denver University Law School, Univ. of New Mexico, Las Vegas NM, and many other schools at all levels.
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