Addiction Research Center, IRP, NIDA, Baltimore, MD, USA
* Center for Human Toxicology, University of Utah, Salt Lake City, UT, USA

In the first study, 6 healthy research volunteers smoked on separate sessions one marijuana cigarette containing either 0, 1.75, or 3.55% THC. Plasma THC and 11-nor-9-carboxy-D⁹-tetrahydrocannabinol (THCCOOH) were measured by GC-MS, and a battery of performance measures was assessed before and after marijuana smoking. At 20 min postsmoking, mean plasma THC levels were 27.5 and 48.8 ng/ml for 1.75 and 3.55% THC, respectively, and at 30 min postsmoking, mean THC levels were 17.3 and 29.7 for 1.75 and 3.55% THC, respectively. At these same times, accuracy on a test of grammatical reasoning was significantly impaired.

In the second study, 16 healthy research volunteers smoked on separate sessions two marijuana cigarettes that contained either 0, 1.75, or 3.55% THC. Plasma THC and THCCOOH were measured by GC-MS, and subjects performed four field sobriety tests (FST) 20 min after smoking ended. Mean plasma THC levels obtained immediately before FST testing were 15.4 and 28.3 ng/ml for 1.75 and 3.55% THC, respectively. Marijuana produced dose-related impairment on two of the FST, One Leg Stand and Finger to Nose.

In both studies, although significant marijuana-induced performance impairment was documented, there was no consistent correlation between any of the performance measures and plasma THC, THCCOOH, or the ratio of THC/THCCOOH across both marijuana doses. Thus, during the early distribution phase after marijuana smoking, there is no correlation between pharmacokinetic measures and performance impairment. Additionally, large inter-individual variation precluded identifying a threshold plasma THC level for impairment.