Improving the quality of the cannabis debate
EDITOR- In his article Strang (1) raises the issue of cannabis and
driving, suggesting that 'a clearer understanding will be required of the
extent to which a particular concentration of drug (or its metabolites)
reliably taken as evidence that an individual's driving ability was
consequently impaired'. However, review of the literature suggests that
the pursuit of an acceptable level of cannabis consumption for driving is
unlikely to be possible for several reasons.
Firstly there is poor correlation between plasma concentrations of
trans delta tetrahydrocannabinol -THC (the constituent of cannabis
responsible for the production of the majority of the psychoactive
response) after smoking and subjective self reported psychological effects
plasma concentrations decline long before peak effects are felt (2). The
relationship between psychological testing and performance on real driving
experiments is complex as the two modalities of testing are very
Impairment of driving has been shown experimentally with acute
intoxication by cannabis (3), however, attempts to correlate driving
performance with levels of THC will be severely affected by the observed
time lag between THC
levels and peak effects.
The issue is further complicated by subjects who use cannabis heavily
and for prolonged periods. This group has been shown to develop tolerance
to the somatic and psychological effects of THC that cannot be correlated
with any drop in THC levels below that of the non-tolerant user
(4). Chronic heavy users of cannabis do, however, show subtle impairment
of memory, organization and attention with the effect becoming more
pronounced with longer duration of use (5). Whether these effects diminish
drive is not clear, but as they are unrelated to acute intoxication, it is
unlikely that it will ever be possible to correlate them with THC levels.
Furthermore, it is not clear whether these abnormalities are reversible
with prolonged abstinence.
We would propose, therefore, that the pursuit of a 'safe' level of
cannabis consumption for driving is unlikely to be successful.
Dr Sarah Levy
Registrar in Toxicology,
Dr Alison Jones
Consultant Physician and Medical Toxicologist,
National Poisons Information Service (London) Medical Toxicology
Unit, Avonley Road, London.SE14 5ER.
1 Strang J. Witton J. Hall W. Improving the quality of the cannabis
debate: defining different domains. BMJ 2000; 320: 108-10.
2 Perez - Reyes M, DiGuiseppi S, Davis K.H, Schindler V.H, and Cook
C.E, 'Comparison of Effects of Marijuana Cigarettes of Three Different
Potencies'. Clinical Pharmacology and Therapeutics. Vol 215.No.1. Oct
1980. pp 35-44.
3 Smiley A. Marijuana: on road and driving simulator studies. In:
Kalant H, Corrigal W, Hall W, Smart R, eds. The Health Effects of
Cannabis. Toronto: Addiction Research Foundation. 1998.
4 Hunt CA, and Jones, R.T. 'Tolerance and Disposition of tetra hydro
cannabinol in man'. Journal of Pharmacology and Experimental Therapeutics;
Vol.215. No1. Oct 1980. pp 35-44.
5 Solowij N. Cannabis and Cognitve Functioning. Cambridge: Cambridge
University Press, 1998.
Competing interests: No competing interests