Cannabis use: a perspective in relation to the proposed UK drug‐driving legislation

Abstract

With regard to THC (Δ(9)-tetrahydrocannabinol), the main psychoactive constituent identified in the plant Cannabis sativa L, several facts are indisputable. Cannabis remains the most commonly used drug in the UK among those who reported driving under the influence of illegal drugs in the previous 12 months. There is a significant dose-related decrement in driving performance following cannabis use; raised blood THC concentrations are significantly associated with increased traffic crash and death risk. When cannabis and alcohol are detected together, there is a greater risk to road safety than when either drug is used alone. Patterns of use are important when interpreting blood concentration data: Smoking infrequently a single cannabis cigarette leads to peak plasma THC concentrations (21-267 µg/L) causing acute intoxication. In habitual, daily users, plasma THC concentrations range from 1.0 to 11.0 µg/L and are maintained by sequestration of the drug from the tissues. These facts undoubtedly make setting thresholds for drug-driving legislation difficult but there is clearly a case for cannabis. Determining minimum blood THC concentrations at which a driver becomes sufficiently impaired to be unable to safely drive a vehicle is of particular concern given the increasing medicinal use of the drug. Internationally legislation for driving under the influence of drugs (DUID) is based on either a proof of impairment or a per se approach. For the latter this can be either zero-tolerance or based on concentration limits such as those used for alcohol. The different approaches are considered against current scientific evidence.

Establishing legal limits for driving under the influence of marijuana

“…Due to its relatively long half-life, THC can be detected in the blood and urine of an individual for hours to days and for days to months following marijuana use, respectively, depending on the frequency of use and other factors (NORML [2014b]; Asbridge [2014]; Grotenhermen et al [2007]; NORML [2014b]). For instance, chronic marijuana users have plasma THC concentrations ranging from 1.0 μg/L to 11.0 μg/L, which are maintained by the continual passage of THC from the tissues into the bloodstream (Wolff and Johnston [2014]). Therefore, habitual users who test positive for THC may not necessarily be impaired at the time of testing (Bedard et al [2007]).…”

“…Therefore, habitual users who test positive for THC may not necessarily be impaired at the time of testing (Bedard et al [2007]). Another major limitation of zero tolerance laws is the risk to convict those with heavy passive exposure to marijuana smoke (Grotenhermen et al [2007]; Wolff and Johnston [2014]). Following passive exposure, the THC of an individual may be detectable in his blood in concentrations <1.0 μg/L within one hour, without inducing concurrent impairment (Grotenhermen et al [2007]; Sharma et al [2012]; Wolff and Johnston [2014]).…”

“…Drugged driving is a safety concern of increasing importance in the United States and in Europe (Asbridge [2014]; Brady and Li [2014]; Li et al [2013]; Whitehill et al [2014]; Wolff and Johnston [2014]). According to the 2012 National Survey on Drug Use and Health, 10.3 million individuals aged 12 years or older operated a motor vehicle under the influence of illicit drugs in 2011 (SAMHSA [2013]).…”

“…Marijuana, the most commonly detected non-alcohol substance in American and European drivers, has been shown to impair driving performance and cognitive functions and increase crash risk (Asbridge [2014]; Downey et al [2013]; Hartman and Huestis [2013]; Li et al [2013]; Ramaekers et al [2004], [2006]; Schwope et al [2012]). While driving under the influence of marijuana is prohibited in all US states (Walsh [2009]), there has been a significant rise in marijuana decriminalization and medical marijuana legalization over the past decade (NORML) [2014c]; Svrakic et al [2012]; Wolff and Johnston [2014]). Recreational marijuana use is also currently legal in Colorado and Washington (Asbridge [2014]; Barry et al [2014]; Kilmer [2014]).…”

“…DEC programs are the principal legal strategy to enforce drugged driving legislation, and train police officers to recognize signs of drug-induced impairment in drivers [e.g., bloodshot eyes, increased nervousness, etc. (Wolff and Johnston [2014])], qualifying them as drug recognition experts (DREs) (ONDCP [2010]; Asbridge [2014]; Talpins and Hayes [2004]; Walsh [2009]). Similar programs also exist in Europe (EMCDDA [2009]; Verstraete et al [2011]), but as of 2009, training was required for traffic police in only 4 countries (Belgium, Portugal, Sweden, and United Kingdom) (EMCDDA [2009]).…”