Plasma concentrations of delta-9-tetrahydrocannabinol and impaired motor function

Fifty-nine volunteer subjects were allowed to smoke marijuana cigarettes until a satisfactory level of 'high' was obtained. They then had blood samples taken 5, 30, 90 and 150 min following smoking after which they were tested with the roadside sobriety test. Attempts to correlate passing or failing on three coordination tests with plasma concentrations of delta-9-tetrahydrocannabinol (THC) showed that if concentrations measured at 5 min were ignored, failures were almost inevitably associated with plasma concentrations of THC above 25-30 ng/ml. Overall, 94% of subjects failed to pass the test 90 min after smoking and 60% after 150 min, despite the fact that by then plasma concentrations were rather low. It would seem that establishing a clear relation between THC plasma concentrations and clinical impairment will be much more difficult than it has been for alcohol.