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4. Cannabis the drug
4.9 Intoxication and levels of cannabinoids
Ingestion of cannabis produces a dose related impairment of a wide range of cognitive and behavioural functions. Since there is evidence that cannabis intoxication adversely affects skills required to drive a motor vehicle (see below), it would be desirable to have a reliable measure of impairment due to cannabis intoxication that was comparable to the breath test of alcohol intoxication. For this reason, a reliable measure for determining the degree of impairment due to cannabis has been particularly sought after.
While the degree of impairment from alcohol can be determined from a single blood alcohol estimate, a clear relationship between blood levels of THC or its metabolites and degree of either impairment or subjective intoxication has not been demonstrated (Agurell et al, 1986). The estimation of the degree of intoxication from a single value of blood THC level is difficult, not only because of the time delay between subjective high and blood THC, but also because of large individual variations in the effects experienced at the same blood levels. The difficulty is compounded by the distribution of THC to body tissues, and its metabolism to other psychoactive compounds.
Blood levels of THC metabolites, such as 11-hydroxy-THC, correlate temporally with subjective effects but are not readily detectable in blood after smoking cannabis, while blood levels of THC correlate only modestly with cannabis intoxication, in part because of its lipid solubility (Barnett et al, 1985; McBay 1988; Ohlsson et al 1980). The level of intoxication could only realistically be related to the total sum of all the psychoactive cannabinoids present in body fluids and in the brain and various tissues.
Due to large human variability, no realistic limit of cannabinoid levels in blood has been set which can be related to an undesirable level of intoxication. Tolerance also develops to many of the effects of cannabis. Hence, a given dose consumed by a naive individual may produce greater impairment on a task than the same dose consumed by a chronic heavy user. THC may also be active in the nervous system long after it is no longer detectable in the blood, so there may be long-term subtle effects of cannabis on the cognitive functioning of chronic users even in the unintoxicated state. To date, there is no consistently demonstrated correlation between blood levels of THC and its effect on human mind and performance. Thus, no practical method has been developed as a forensic tool for determining levels of intoxication based on detectable cannabinoids. A consensus conference of forensic toxicologists has concluded that blood concentrations of THC which cause impairment have not been sufficiently established to provide a basis for legal testimony in cases concerning driving a motor vehicle while under the influence of cannabis (Consensus Report, 1985).