Please make yourself comfortable and welcome back to part 3 of this mini-series dealing in the world of DUIs and cannabis tolerance. If you have not read parts 1 or 2 yet I highly suggest you check those out first, as what I will be talking about in this post builds off that past content. In part 1, the goal was to introduce the idea of cannabis tolerance, and take a look at how frequent cannabis smokers were operating with less cognitive impairment than infrequent users. In part 2, I wanted to break this idea down into the neurobiology of our mesolimbic reward pathway and endocannabinoid system. Now in part 3, I want to broaden the scope back out to discuss the real-world implications, safety, and laws surrounding frequent users driving after consuming THC.

A couple years ago a group of friends of mine pitched in to buy a gift for one specific friend of ours who would always drink too much. We got them a breathalyzer, and they would get some teasing everytime their BAC level went over the legal driving limit of .08. This gift would not have been possible for a friend who smokes to much. There is, unfortunately, nothing of the sort for cannabis (except for research articles every year promising that we’re getting closer to a working THC breathalyzer). 

Even if there was a working breathalyzer, this only solves one part of the problem. After reading blog posts 1 and 2, you can expect that frequent cannabis consumption of THC will not scale linearly with impairment in the way you would see in occasional users, or in alcohol consumption. And seperatly, metabolites of THC can remain in the bloodstream weeks after the last intoxication (Karschner et al., 2009). But broader issue is there is a desperate sense of urgency in coming up with a solution, any solution, when THC is being legalized nationwide and its consumption is growing exponentially. We know its an issue, because if we look up north to Canada (who did a mass legalization of cannabis in 2018), researchers found a 2x increase of moderately injuryed drivers with at least some cannabis in their system post-legalization (Brubacher et al., 2022). As said by a marjuana policy researcher, “You can’t legalize a substance and not have a coherent policy for controlling driving under the influence of that substance.” As is the case with all American policy, the science is decades ahead of the law, and everyone is now left scrambling for a quick-fix “answer.” In looking for that answer, discussion in the realm of cannabis impairment almost always eventually flows back to one question:

What is the threshold of THC at which a driver is impaired by cannabis?

The answer is no.

At least for now. There is no one threshold that has significant scientific support as a ‘point of no return’ when it comes to impairment. A huge study by the AAA Foundation for Traffic Saftey (Logan et al., 2016) was conduced for the sole purpose of answering this exact question. The authors peeled through data for 4,799 drivers arrested for DUIs with THC was present in their system and analyzed their comprehensive toxicology reports, looking for both the THC concentration in their system and performance on the Standarized Field Sobreity Test (SFST) battery. What they found is that any THC concentration they might have picked as a “threshold” would have misclassified a substantial amount of drivers – either classifying them as impaired even if didn’t demonstrate impairment on the SFST, or the other way around. Another confounding variables brought up was the time factor of knowing what a driver’s blood THC concentration was at the exact time of the event as compared to the time of law enforcement sampling. This is again, a factor that differs between occasional and frequent cannabis users, as in frequent users THC accumlates in our adipose tissues (fat cells) where it redistrubutes into the blood stream at a much slower rate (Balíková et al., 2014). 

This concept of a THC threshold is called a “per se” law, which is Latin for “by itself.” In the context of the DUI laws, it means that being over the threshold by itself is enough for a violation. There are currently 5 states that have “per se” cannabis DUI laws, with the cutoff ranging between 2 ng/mL THC to 5 ng/mL THC. They are in blue on the map below:

These “per se” states are known to range from poor to inconsistent in their efforts to correctly detect cannabis impairment in the biological samples of drives (Arkell et al., 2020). These are very similar to the “Zero Tolerance” states (in green), which like the name says prohibits driving with any amount of THC in the body. States marked in yellow do not measure for THC at all, and leaving the ruling of a DUI is solely based upon the discretion of the officer that pulls you over. This is alarming for multiple reasons, but there is one primary one: When gray area is left within the law, police enforcement will consistently and systematically reveal the ugliness of prejudice and racial bias in its selective, discriminatory, and vicious application. There have been reports upon reports, all finding similar conclusions that black and brown Americans are incarerated for cannabis-releated charges at rates fire higher than white Americans. I’m personally from New York City, and these spreadsheets of NYC cannabis arrests each year never fail to shock me. If you’re looking for an overview of the long history of racial disparity in drug enforcement, this Stateline article does a bang up job summing it up well in less than 2000 words.