Proof of driving impairment from marijuana smoking is usually based on the interpretation of cannabinoid levels determined in a single blood sample taken sometime after the event. Most studies of Delta(9)-tetrahydrocannabinol (THC) pharmacokinet- ics have focused on what happens during the acute phase of smoking. It is well known that in occasional users, THC levels declined to values below 1 microgram/L within approximately six hours after smoking, whereas in heavy users, THC levels exceeding 1 microgram/L may be present more than 24 hours after the last use. In fact, the latest study from the National Institute on Drug Abuse (NIDA) showed that chronic smokers may have plasma concentrations >2 mg/mL of plasma one week after use.

A new study was undertaken to compare pharmacokinetic properties of cannabinoids in occasional and heavy users during the eight hours after smoking a high dose of THC, and again, in a second study, where the same 12 heavy smokers were given a placebo to smoke (SW Toennes, et al., “Comparison of cannabinoid pharmacokinetic properties in occasional and heavy users smoking a marijuana or placebo joint,” Journal of Analytical Toxicology, 2008 Sep;32(7):470-7. Blood samples were obtained during both the cannabis and placebo sessions. The placebo condition of the heavy users was given special attention because residual cannabinoid concentrations due to previous use were expected.

The results obtained with occasional users were in contrast to those of the heavy users. The heavy users admitted cannabis use on four to 25 occasions during the previous week. Of the 12 heavy users, 10 exhibited up to 12.3 microgram/L THC prior to smoking. During the eight hours after smoking, the distribu- tion and elimination patterns were comparable to those of the...