Cannabis Use and Traffic Injuries

In Brief

SUPPLEMENTAL DIGITAL CONTENT IS AVAILABLE IN THE TEXT.

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 (http://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: TABLE. Relative Risk of Unintentional Injuries Requiring Medical Care While Driving a Car or Motorcycle During the 1 or 2 Hours After Cannabis Use

Our 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 (http://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