Cannabis legalization poses considerable risks for drivers and policy makers, with multiple investigations converging on the conclusion that cannabis intoxication increases road traffic accidents and fatalities, likely due to a reduction in attention, reaction speed, and judgement of drivers (for a Lift review of the research see here).
Countries around the globe have implemented a range of policies regulating driving under the influence of cannabis. For instance, Australia prohibits driving under any detectable level of THC, whereas Switzerland has a more lax limit of 2.2 ng/ml. Across the U.S., states have imposed limits ranging from zero to 5 ng/ml.
Arguments for a zero-level tolerance posit that exact measurement of cannabis blood levels is not possible due to technological limitations and a great variability in how individuals metabolize the drug, added to the fact that even low levels of cannabinoids can impair driving significantly (especially if other substances like alcohol are also ingested).
Arguments for specific limits, on the other hand, are centered around pragmatic factors and findings that THC and other cannabis metabolites can remain detectable for nearly a week, presumably long after any psychoactive effects can still be felt. Some concerns shared by both sides revolve around the need for blood samples for adequate ruling of intoxication, due to the associated medical risks.
A recent investigation from the University of Marseille and the Service de Pharmacocinétique Toxicocinétique provides new important data for this ongoing debate. The team, led by Dr. Amélie Marsot, described in great detail the pharmacokinetic profiles of THC and its metabolites in saliva, blood plasma, and urine during the first 72 hours. Their report can be accessed for free in the Journal of Pharmacy and Pharmaceutical Sciences.
Eighteen male tobacco smokers and occasional cannabis users, aged 20 to 45 years, were recruited from the local community. Each participant underwent two treatments: a) tobacco cigarette as a control, and b) tobacco cigarette with 500 mg of cannabis (20mg of THC). To reduce differences in smoking technique, the participants followed a computerized procedure under medical supervision that explained how they should inhale the cigarette. The two treatments were separated by a 4-week washout period and randomly sorted in order to blind participants. Finally, they were asked to refrain from cannabis during the entire experiment as well as during the 28 days leading up to it.
In total, blood and oral fluids were collected 16 times each and urine was collected 6 times. The samples were tested for the presence of THC, 11-OH-THC (main psychoactive metabolic product of THC) and THC-COOH (inactive metabolite) with a minimum detection level of 1ng/ml.
The analysis yielded a wide variability in the levels of absorption, metabolism, and excretion of these metabolites. The maximum concentrations of THC ranged from 55.4 to an incredible 120000 ng/ml in saliva and 1.6 to 160 ng/ml in plasma, whereas the maximum plasma concentrations of the two metabolites varied 3 to 10 times less than this. Overall, THC salivary content was in no way indicative of THC blood content, nor were urinary samples. Other parameters such as time to maximum concentration and to last detectable concentration also varied widely across individuals.
The authors argue that oral and urinary samples can still be taken as evidence of recent use, but fail to specify what they subtend by it. In the study, half of the participants reached THC subthreshold levels in plasma in just 2 hours (maximum took 12 hours) against 18 hours in saliva (maximum took 48 hours). Urinary samples remained positive for even larger periods of time, and previous studies have shown that detection levels subsist up to 18 days after the administration of a single marijuana joint. It seems unclear what driving regulatory policy could be adopted that does not rely on blood sampling.