WASHINGTON – The level of cannabis that causes driving impairment in occasional marijuana users is actually lower than the residual cannabis levels found in regular users even when they haven’t consumed cannabis recently. These new findings, published today in AACC’s Clinical Chemistry journal, add to a growing body of evidence suggesting that no legal driving limit for cannabis can catch impaired recreational users without unfairly penalizing unimpaired regular or medicinal users.

As more U.S. states legalize medicinal and recreational marijuana use, marijuana-related car accidents are also likely to rise—a trend that Washington, Oregon, and Colorado have already seen since approving recreational cannabis. In light of this, there is a growing need for reliable roadside testing that can identify marijuana-impaired drivers. One of the biggest hurdles to developing such a test, however, is that unlike for alcohol, there is currently no agreed-upon blood concentration of delta-9-tetrahydrocannabinol (THC)—the active ingredient in marijuana—that can be used to determine if a person is impaired. Six states have established legal THC driving limits so far, but these limits vary widely, with most states setting the limit at either 2 or 5 ng/mL, and one state using 1 ng/mL. In Europe, THC driving limits are only slightly more uniform, with most countries using cutoffs between 1-3 ng/mL.

In an effort to determine a unifying THC limit for identifying marijuana-impaired drivers, a team of researchers led by Jean Claude Alvarez, PharmD, PhD, of Hôpital Raymond-Poincaré, AP-HP in Garches, France, looked at the effects of THC on driving performance in 15 regular cannabis consumers and 15 occasional consumers (who typically either smoked 1-2 joints per day or 1-2 per week, respectively). The researchers collected participants’ blood at numerous intervals over a 24-hour period before and after the participants imbibed (via smoking) either a placebo, 10 mg of THC, or 30 mg. The participants’ samples were then tested for THC and its metabolites. Also at intervals throughout the 24-hour period, study participants underwent reaction time tests and driving performance evaluations in a driving simulator.

All participants exhibited the most driving impairment around 5 hours after cannabis administration, with more significant impairment observed in occasional users versus regular users. Strikingly, by the 5-hour time point, occasional users had less than 1 ng/mL of THC in their blood regardless of starting dose—which means that existing legal THC limits would fail to identify these individuals as impaired. Furthermore, Alvarez’s team found that THC lingered in the bloodstream of regular users for much longer than in occasional users. After 24 hours—long after impairment had ended—regular users still had about 1 ng/mL of THC in their blood. This indicates that if legal THC driving limits were lowered enough to catch impaired occasional users, unimpaired regular users could get unfairly charged with driving under the influence of drugs.

“Our results showed that the duration of THC’s effect on driving performance was around 8 hours in chronic consumers and 13 hours in occasional consumers. This new information is important so that people do not drive during this time after consumption of marijuana,” said Alvarez. Beyond this, however, he said that the study “showed that it is not possible to set a legal THC limit for driving. We showed no relationship between the pharmacokinetics of THC in blood and the effects on reaction time or driving performance.”

About AACC

Dedicated to achieving better health through laboratory medicine, AACC brings together more than 50,000 clinical laboratory professionals, physicians, research scientists, and business leaders from around the world focused on clinical chemistry, molecular diagnostics, mass spectrometry, translational medicine, lab management, and other areas of progressing laboratory science. Since 1948, AACC has worked to advance the common interests of the field, providing programs that advance scientific collaboration, knowledge, expertise, and innovation. For more information, visit www.myadlm.org.

Clinical Chemistry is the leading international journal of clinical laboratory science, providing 2,000 pages per year of peer-reviewed papers that advance the science of the field. With an impact factor of 8.636, Clinical Chemistry covers everything from molecular diagnostics to laboratory management.