Cannabis Myth #2: THC Tests can Indicate Impairment Level

As Canada moves toward legalization, law enforcement agencies are demanding a roadside THC test to assess impairment, similar to a breathalyzer. Even if such a thing existed, THC levels in the body cannot indicate impairment. The Roadside Sobriety Test that these same law-enforcement people have been using for decades are a far better measure of impairment than any type of body chemistry. This is the only test we have that can take into account the complexity of how THC is affecting the body.

Cannabis contains almost 500 active compounds that we know of. Each active compound has its own effect on the body, which often counteracts the effects of another compound. For example, CBD is the antidote to THC’s euphoria and hunger, while THC is the antidote to nausea that CBD sometimes induces. And those are just a few of the interactions between the two most well-known cannabinoids.

Looking at hundreds of compounds, there are infinite interactions that we will probably never fully understand. Your body produces its own complement of endo-cannabinoids, which create further interactions that vary from person to person. Since other plants contains some of the same compounds, even the food people eat can influence how THC affects the individual on a particular day.

The high (and level of impairment) that people get from THC specifically is not just dependent on how much they consume in a day but over a long period of time. What matters in the short-term is how the level of THC has changed in your body. When a frequent THC user consumes more, it might double the THC in the body. For new and infrequent users, the change in THC is infinitely greater, so the effects will also be greater.

Even for people trying to be responsible, we cannot predict the THC level in the body. With alcohol, we always know how much is getting into the body and we have a good idea of how long it will take to eliminate it from our bodies. There is no way to know how many milligrams of THC you receive or whether they are affecting you at a point in time:

  • THC levels that producers quote are actually “potential” levels. Depending on how users prepare it, they may or may not actually achieve those levels.
  • THC breaks down into CBN over time, so the amount of time since harvest affects actual THC levels.
  • If you smoke marijuana, you exhale some portion of the cannabinoids, and there is no way to know how much you retained.
  • THC levels in the body rise and fall on different timelines depending on how you consume it, because your liver and lungs receive and extract it differently, but these timelines are poorly understood.

Furthermore, other compounds can also increase the level of impairment. CBN causes sleepiness, but a THC test cannot determine the cognitive effects of CBN.

We also need to think about how such tests affect the people who need cannabis. The policies that police forces are trying to impose would effectively prevent medical users from ever driving. Medical users tend to choose strains high in CBD, which reverses the high. Medical users will go for months or even years with a relatively constant level of THC in their system, so individual doses have little effect. These people will always test with the highest levels of THC, while experiencing the least impairment.

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