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Cannabis intoxication and fatal road crashes in France: population based case-control study

BMJ 2005; 331 doi: (Published 08 December 2005) Cite this as: BMJ 2005;331:1371
  1. Bernard Laumon (bernard.laumon{at}, senior researcher1,
  2. Blandine Gadegbeku, research engineer2,
  3. Jean-Louis Martin, senior researcher,2,
  4. Marie-Berthe Biecheler, senior researcher,the SAM Group
  1. 1French National Institute for Transport and Safety Research (INRETS), Epidemiological Research and Surveillance Unit in Transport, Occupation and Environment (UMRESTTE), 25 avenue François Mitterrand, F-69675 Bron Cedex
  2. 2INRETS/Université Claude Bernard Lyon 1 (UCBL)/Institut de Veille Sanitaire (InVS)/UMRESTTE
  1. Correspondence to: B Laumon
  • Accepted 27 September 2005


Objectives To evaluate the relative risk of being responsible for a fatal crash while driving under the influence of cannabis, the prevalence of such drivers within the driving population, and the corresponding share of fatal crashes.

Design Population based case-control study.

Participants 10 748 drivers, with known drug and alcohol concentrations, who were involved in fatal crashes in France from October 2001 to September 2003.

Main outcome measures The cases were the 6766 drivers considered at fault in their crash; the controls were 3006 drivers selected from the 3982 other drivers. Positive detection of cannabis was defined as a blood concentration of Δ9tetrahydrocannabinol of over 1 ng/ml. The prevalence of positive drivers in the driving population was estimated by standardising controls on drivers not at fault who were involved in crashes resulting in slight injuries.

Results 681 drivers were positive for cannabis (cases 8.8%, controls 2.8%), including 285 with an illegal blood alcohol concentration (0.5 g/l). Positive cannabis detection was associated with increased risk of responsibility (odds ratio 3.32, 95% confidence interval 2.63 to 4.18). A significant dose effect was identified; the odds ratio increased from 2.18 (1.22 to 3.89) if 0 < Δ9tetrahydrocannabinol < 1 ng/ml to 4.72 (3.04 to 7.33) if Δ9tetrahydrocannabinol 5 ng/ml. The effect of cannabis remains significant after adjustment for different cofactors, including alcohol, with which no statistical interaction was observed. The prevalence of cannabis (2.9%) estimated for the driving population is similar to that for alcohol (2.7%). At least 2.5% (1.5% to 3.5%) of fatal crashes were estimated as being attributable to cannabis, compared with 28.6% for alcohol (26.8% to 30.5%).

Conclusions Driving under the influence of cannabis increases the risk of involvement in a crash. However, in France its share in fatal crashes is significantly lower than that associated with positive blood alcohol concentration.


  • Contributors BL designed the study, carried out the statistical analyses, contributed to the interpretation of the results, wrote the paper, and is the guarantor. BG and JLM carried out further statistical analyses and contributed to the study design, interpretation of the results, and writing of the paper. MBB contributed to the interpretation of the results and critical review of the paper. Other members of the SAM Group research team who contributed to obtaining the results are: L Campione, R Driscoll, T Hermitte, T Phalempin, D Villeforceix (CEESAR); JY Forêt-Bruno, Y Page (Lab PSA Peugeot Citroën/Renault); F Facy, M Rabaud (INSERM); P Chapuis, C Filou, K Fouquet, Y Gourlet, E Perez, J F Peytavin, P Van Elslande (INRETS); H Martineau (OFDT); C Got.

  • Funding The French Ministry of Health (DGS) funded this study, with additional funding from the National Institute for Health and Medical Research (INSERM) and the French National Institute for Transport and Safety Research (INRETS). The Ministry of Justice funded the screening process. The Home Office and the Ministry of Defence financed the data collection.

  • Competing interests None declared.

  • Accepted 27 September 2005
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