Cannabis intoxication and fatal road crashes in France: population based case-control studyBMJ 2005; 331 doi: https://doi.org/10.1136/bmj.38648.617986.1F (Published 08 December 2005) Cite this as: BMJ 2005;331:1371
All rapid responses
There is one thing about the findings of the study by Laumon et al.
that seems to be at odds with the overall claim. The study states that
2.9% of French drivers are found to have significant cannabis levels,
whilst cannabis is a causal factor in 2.5% of fatal accidents. This
suggests that cannabis use causes no greater proportion of accidents than
would be expected if it did not increase risk. This discrepancy may be due
to unknown or unmeasured confounding variables. It is a fundamental
limitation of case-control methodology that it cannot exclude confounding
variables that we either are not aware of or cannot measure. Even the
dose-response effect does not prove causality if the confounding variable
is correlated with both effect and putative cause. Is it possible, for
example, that increased cannabis levels are correlated with personality
factors that are independently associated with greater accident risk?
This study certainly moves us closer to demonstrating the risk caused by
cannabis, but the case is not yet proven.
Competing interests: No competing interests
The article by Laumon et al. (2005) supports the assumption of a low
risk of cannabis use to cause accidents and a dose-effect relationship
observed in previous studies (Drummer et al. 2004, Longo et al. 2001). The
results have been cited in the popular media stating that cannabis users
face a three times greater risk of being responsible for a fatal road
crash. But the results do not support this conclusion. The presentation of
the results in the abstract is somewhat misleading, which may have caused
this misinterpretation. The figures for the unadjusted odds ratios suggest
a more than threefold risk increase for all THC positive drivers and a
more than twofold increase even for drivers with a THC blood concentration
of less than 1 ng/ml. However, closer review of the results shows that two
other factors contributed to the higher accident risk, i.e., alcohol
consumption and the younger age of the THC positive drivers, compared to
the whole cohort.
About 42% (285/681) of THC positive drivers also tested positive for
alcohol, with a blood alcohol concentration (BAC) of 0.05%, which was
associated with an increased risk of 8.5. Even a BAC below 0.05% was
reported to be associated with an odds ratio of 2.7 in the study, but no
data were given on the percentage of THC positive drivers with an
additional BAC <0.05%. Thus, no information is available on drivers
who had only THC in their blood and on their risk of causing an accident,
which has been used as a standard way of reporting in previous studies
(e.g. Drummer et al. 2004, Longo et al. 2001). Laumon et al. (2005) were
careful not to over-adjust the risk for cannabis use, but they may have
under-adjusted it and it remains unclear if there is indeed an increased
risk for THC blood concentrations below 3 ng/ml in their cohort.
Previous epidemiological data and results from experimental studies
prompted an international working group to make a suggestion for legal THC
limits in blood comparable to BAC limits for alcohol to separate impaired
from non-impaired drivers (Grotenhermen et al. 2004). The study by Laumon
et al. (2005) is well in agreement with the proposal of a legal limit of a
THC blood concentration in the range of 3-5 ng/ml (corresponding to about
6-10 ng/ml in blood serum) made by this group.
Franjo Grotenhermen, M.D.
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Competing interests: No competing interests