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Responses to "Cannabis intoxication and fatal road crashes in France"

Presentation of the results is misleading

Franjo Grotenhermen

14 December 2005

Dear Sirs:

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.

Sincerely,

Franjo Grotenhermen, M.D.
nova-Institut, Goldenbergstrasse 2, 50354 Huerth, Germany

References

1. Drummer OH, Gerostamoulos J, Batziris H, Chu M, Caplehorn J, Robertson MD, Swann P. The involvement of drugs in drivers of motor vehicles killed in Australian road traffic crashes. Accid Anal Prev 2004;36(2):239-48.

2. Grotenhermen F, Leson G, Berghaus G, Drummer O, Krüger HP, Longo M, Moskowitz H, Perrine B, Ramaekers J, Smiley A, Tunbridge R. Developing Per Se Laws for Driving Under the Influence of Cannabis (DUIC). Abstract presented at the 17th International Conference on Alcohol, Drugs and Traffic Safety (ICADTS), 8-13 August 2004, Glasgow, UK.

3. Laumon B, Gadegbeku B, Martin JL, Biecheler MB. Cannabis intoxication and fatal road crashes in France: population based case-control study. BMJ 2005 Dec 2 [electronic publication ahead of print]

4. Longo MC, Hunter CE, Lokan RJ, White JM, White MA. The prevalence of alcohol, cannabinoids, benzodiazepines and stimulants amongst injured drivers and their role in driver culpability: part ii: the relationship between drug prevalence and drug concentration, and driver culpability. Accid Anal Prev 2000;32(5):623-32.

Competing interests: None declared

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