THC (delta-9 tetrahydrocannabinol) is known to be the primary psychoactive ingredient in marijuana that causes driving impairment. For years, legislators and some scientists have sought a per se level of THC in a driver’s blood that is equivalent to alcohol per se levels, .08 BAC in the United States and .05 BAC in most other countries.
The fact is that there is no level of THC in blood above which everyone is impaired, and below which no one is impaired. This is not because we need more studies; it’s because of human biology. Consider the following three factors.
First, blood is not impaired by either THC or alcohol. These substances impair only the brain. We test blood for alcohol levels simply as a surrogate for testing what’s in the brain. Blood testing is more convenient than brain testing, since the subject can remain alive for the former, but not for the latter.
For alcohol, blood is a very good surrogate for what’s in the brain. Alcohol is a small, water-soluble molecule that rapidly equilibrates its concentration throughout the body. Therefore, what’s in the blood is in the brain and vice-versa.
For THC, that’s just not the case. THC is a large, fat-soluble molecule whose concentration in the blood rapidly drops as it is sequestered into the body’s fat stores, including the brain. Immediately after smoking a joint, the THC level will be very high in the blood and very low in the brain. The THC level in the brain climbs rapidly at the same time as it is declining in the blood. At some point, the concentrations cross, and the concentration continues to rise in the brain while it is still declining in the blood, since the brain acts like a sponge, soaking up the partially insoluble THC from the blood.
Second, marijuana addicts and other daily users can develop a tolerance to some, but not all of the impairing effects of THC. They can drive safely with THC levels that would seriously impair others. A per se level that is appropriate for addicts and heavy users (like Colorado’s 5 ng/ml limit) is a license to drive stoned for the occasional and inexperienced users.
Third, normal delays in taking blood samples mean that the THC in the blood sample can never represent the THC level in the blood at the time of the incident. The normal time from arrest to taking a blood sample is an hour, two hours in case of a serious crash, and three or more hours if a warrant is required. Since the blood level of THC drops as much as 90% within the first hour after smoking a joint, blood test results will dramatically understate the amount of THC at the time of arrest.
So while the marijuana lobby asks us to regulate marijuana like alcohol, the scientific fact is that marijuana is not like alcohol chemically, biologically, or metabolically. Laws that mimic alcohol regulations cannot change the facts.
Even if these problems with a THC per se level did not exist, we are faced with the fact that intoxicating substances like alcohol and THC are frequently taken together and their impairing effects are additive. A non-impairing dose of alcohol, when combined with a non-impairing dose of THC can render someone impaired, even if the blood levels of each intoxicant are below individual legal limits.
Courts like per se levels because they are scientific and unambiguous. They work for alcohol. But they are meaningless for drugs like marijuana. And the widespread acceptance of impairing drugs is making existing alcohol per se levels less meaningful when alcohol and drugs are combined.
Until a Star Wars-like scanner that measures physical and mental impairment is invented, we must rely upon trained officers’ ability to observe and document symptoms of drug impairment and use rapid roadside biological assays to merely identify the presence of drugs responsible for the observed impairment. Rapid roadside biological assays based on oral fluids are in common use outside the United States, and other assays based on the breath may eventually be proven feasible as well.