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Cannabis Use and Traffic Injuries

2011; Lippincott Williams & Wilkins; Volume: 22; Issue: 4 Linguagem: Inglês

10.1097/ede.0b013e31821db0c2

1531-5487

José Pulido, Gregorio Barrio, Pablo Lardelli, María J. Bravo, M. Teresa Brugal, Albert Espelt, Luís de la Fuente, José Pulido, Gregorio Barrio, Pablo Lardelli, María J. Bravo, M. Teresa Brugal, Albert Espelt, Luís de la Fuente, Mireia Ambrós, María José Belza, Yolanda Castellano, Antònia Domingo‐Salvany, Fermín Fernández‐Calderón, Gemma Molist, Albert Sánchez‐Niubò, Sara Santos, Luis Sordo, Fernando Vallejo,

Alcohol Consumption and Health Effects

To the Editor: Cannabis, even at low doses, affects cognitive and psychomotor abilities required for driving.1 However, the causal relationship between cannabis use and traffic injuries has not been definitely established. Previous observational studies (mainly case-control designs) only partially control the effect of potential confounders, such as concurrent use of other psychoactive drugs or psychologic factors.2,3 Moreover, in case-control designs, it is difficult to obtain a valid control group of drivers from the same population where the cases were recruited. The case-crossover design, in which cases and controls are the same subjects in 2 different periods (hazard period and control period), is useful for assessing the effect of transient exposures on acute outcomes.4 In this design, the self-matching guarantees a valid control group as well as complete adjustment for all confounders that remain stable over time, such as personality traits, driving abilities, or physical limitations. We used a case-crossover design to estimate the transient effect of cannabis on the risk of unintentional driving-related injuries during 60 or 120 minutes after use. Some 720 young regular cocaine users were recruited from nontreatment settings in Spain during 2004–2006, using targeted sampling and chain-referral methods.5 All completed a baseline questionnaire, and 503 (70%) a follow-up questionnaire 12 months later. Case-crossover analysis was applied to all 68 subjects who reported having suffered in the previous 12 months a nonfatal, unintentional, driving-related injury that required medical care, and using the “usual-frequency-model” approach.6 Mantel-Haenszel rate ratios (RRs) of injury were used to compare self-reported cannabis exposure in a hazard period (60 or 120 minutes preceding the injury) with their expected cannabis exposure in an average control period. This latter exposure was estimated from the usual frequency of cannabis use in the last 30 days before the injury. RRs of injury were also obtained for cases without concurrent exposure to alcohol, cocaine or any illicit drugs immediately before the injury and for concurrent use of alcohol. See methodological details in the eAppendix (https://links.lww.com/EDE/A486). The RR of driving-related injury in the 60 minutes following cannabis use was 7.0 (95% confidence interval = 3.1-16). This value increased to 11 (1.3-88) for concurrent exposure to alcohol, and decreased to 6 (2.4-14) for nonconcurrent exposure to any other psychoactive drug. The RRs were considerably lower for a hazard period of 120 minutes (Table).TABLE: Relative Risk of Unintentional Injuries Requiring Medical Care While Driving a Car or Motorcycle During the 1 or 2 Hours After Cannabis UseOur results show a strong effect of cannabis on the risk of driving-related injuries during the first 60 minutes after use. This effect is independent of concurrent use of other psychoactive drugs, and seems to increase with concurrent use of alcohol. Furthermore, the cannabis effect tends to disappear soon after the first 60 minutes of use. These results agree with previous case-control studies, which gave odds ratios between 2 and 4 for the effect of cannabis.1,2,7 In contrast, the protective effect of cannabis reported by Gmel et al8 in a case-crossover study may probably be explained by the wide hazard period (6 hours) considered. Perhaps the most important limitation of our study is the small number of cases, which led to somewhat imprecise RR estimates. We also did not measure concurrent exposure to cannabis and driving (effective exposure) during the control period. Therefore, we assumed that all cannabis exposure during this period was effective exposure. Other limitations are discussed in the eAppendix (https://links.lww.com/EDE/A486). The strong association between cannabis use and driving-related injuries in our study and others, along with the high prevalence of cannabis use among young drivers observed in several studies,1,9,10 support the prevalent risk of serious auto accidents shortly after cannabis use. ACKNOWLEDGMENTS We thank all the participants and those persons and institutions who were involved in recruitment and follow-up, who allowed us the use of their centers for the interviews, and who helped with project management in Madrid, Barcelona, and Seville. We also thank Kathryn M. Fitch for translation and suggestions. José Pulido Centro Nacional de Epidemiología Instituto de Salud Carlos III Madrid, Spain CIBER Epidemiología y Salud Pública (CIBERESP) Spain Gregorio Barrio CIBER Epidemiología y Salud Pública (CIBERESP) Spain Escuela Nacional de Sanidad Instituto de Salud Carlos III Madrid, Spain Pablo Lardelli CIBER Epidemiología y Salud Pública (CIBERESP) Spain Departamento de Medicina Preventiva y Salud Pública Facultad de Farmacia Universidad de Granada Granada, Spain María J. Bravo Centro Nacional de Epidemiología Instituto de Salud Carlos III Madrid, Spain CIBER Epidemiología y Salud Pública (CIBERESP) Barcelona, Spain M. Teresa Brugal Albert Espelt CIBER Epidemiología y Salud Pública (CIBERESP) Barcelona, Spain Agència de Salut Pública de Barcelona Barcelona, Spain Luis de la Fuente Centro Nacional de Epidemiología Instituto de Salud Carlos III Madrid, Spain CIBER Epidemiología y Salud Pública (CIBERESP) Barcelona, Spain José Pulido Gregorio Barrio Pablo Lardelli María J. Bravo M. Teresa Brugal Albert Espelt Luis de la Fuente Mireia Ambrós M. José Belza Yolanda Castellano Antonia Domingo-Salvany Fermín Fernández Gemma Molist Albert Sánchez-Niubó Sara Santos Luis Sordo Fernando Vallejo The Itínere Project Group