|
Abstract
|
In their article on the cannabis debate, Strang et al raise the issue of cannabis and its effects on driving, suggesting that “a clearer understanding will be required of the extent to which a particular concentration of the drug (or its metabolites) can reliably be taken as evidence that an individual's driving ability was consequently impaired.”3-1 A review of the literature, however, suggests that defining an acceptable level of cannabis consumption for driving is unlikely to be possible for several reasons.
Firstly, there is a poor correlation between plasma concentrations of trans-Δ-tetrahydrocannabinol (THC) (the constituent of cannabis responsible for the production of most of the psychoactive response) after smoking and subjective, self reported psychological effects; plasma concentrations of THC decline long before peak effects are felt.3-2 Secondly, the relation between psychological testing and performance in experiments of real driving is complex because the two modalities of testing are different. Impairment in driving has been shown experimentally with acute intoxication by cannabis3-3; however, attempts to correlate driving performance with concentrations of THC will be severely affected by the observed time lag between THC concentrations and peak effects.
The issue is further complicated in people who use cannabis heavily for prolonged periods. This group has been shown to develop tolerance to the somatic and psychological effects of THC; this tolerance cannot be correlated with any drop in concentration below that of users who are not tolerant.3-4 Chronic heavy users of cannabis do, however, show subtle impairment in memory, organisation, and attention, and the effect becomes more pronounced the longer the duration of use.3-5 Whether these effects diminish the ability to drive is not clear, but since they are unrelated to acute intoxication, it is unlikely that it will ever be possible to correlate them with concentrations of THC. Furthermore, it is not clear whether these abnormalities are reversible with prolonged abstinence.
The pursuit of a definition of a “safe” amount of cannabis that can be consumed and still allow for driving is unlikely to be successful.
Keywords: Cannabis impaired driving
Language: en |