(−)-trans-Δ9-tetrahydrocannabinol (THC) from the cannabinoids chemical group is the most prevalent psychoactive constituent in marijuana that causes behavioral toxicity or the “high” feeling. It quickly absorbs into lungs then to the brain. The psychoactive effects are usually experienced within minutes of taking the drug. There is solid evidence that THC affects psychomotor functioning, cognition, attention vigilance, tracking reaction time and coordination. Drivers often report requiring conscious effort while driving, they often lower their speed, perform less passing and increase following distance. All of these behavioral effects depend on the dose, method of drug use, time of consumption, metabolic profile, history of use (first-time, occasional or chronic use), driving experience, health and age.
Potential challenges that marijuana pharmacokinetics presents:
Given the complicated chemistry of THC in our body, cannabis driving impairment testing often requires a combination of measures such as:
This two-step test requires positive results for both the recent usage and behaviors along with physiological evidence of impairment.
Oral testing: Oral fluid is suitable for monitoring marijuana exposure and determining if the users are driving under the influence. Due to the non-invasive nature of oral drug testing, it is considered a top choice for roadside screening. Given the instability of THC level in one’s body and the time sensitive constraint in measuring, THC oral testing provides a good solution. There is a reliable correlation between THC level in saliva and THC level in the blood. The correlation is the result of transmucosal absorption of THC into blood. Oral testing is the ideal option for roadside screening.
Urine testing: Urine tests are the most popular drug test due to its ability to test for marijuana for days or even weeks after use. THC is excreted in urine as inactive THC-COOOH which is a good measure of prior use of marijuana but a bad measure of recent cannabis use. Urine testing is ideal for determining prior marijuana use.
Blood testing: Blood THC testing is the ideal testing method for evidential purposes, providing a reliable impairment indication. However, due to its invasive nature, it’s not used on the roadside. Unlike urine testing, blood test results give a useful indication that one is currently under the influence of marijuana. Blood tests are typically used during investigations of accidents, injuries and DUI cases.
Here is a table of THC testing methods in different states: