Alcohol is well-known for the increased risk of traffic crashes it confers on drivers. The effects of other psychotropic drugs on crash risk are not so familiar, and have been harder to study, as have the risks faced by people dependent one more than one substance. We have been able to examine the driving records of samples of subjects beginning treatment for substance abuse at the Centre for Addiction and Mental Health (CAMH) during 1994. The sampling scheme resulted in approximately 90 people in each of seven treatment groups: alcohol, cannabis, cocaine, and all possible combinations of these substances. A control group of 518 drivers was selected randomly from the provincial driver records. Crash rates per year of driving were computed for 1985-1993 and 1995-2000. Adjusted relative risks (ARR) were computed for each substance and combination of substances using Poisson regression to control for differences in age and sex. A significant interaction was found for cocaine and cannabis (P = 0.010) pre-treatment, such that subjects dependent on both substances had a relative risk substantially lower than what is expected from each substance on its own. Alcohol, cocaine and cannabis all were associated with significant increases in crash risk. A separate model, which excluded the interaction term, gave estimates of ARR consistently below those obtained in the correct model, raising the possibility of mis-interpretation when the interaction is not recognized. A separate model, fitted on the collision frequencies in the post-treatment time interval, had no evidence of interaction; in this interval, no substance was associated with an increase in risk. Only being male and being younger were associated with higher crash (Author/publisher) For the covering abstract of the conference see ITRD Abstract No. E201067.
Abstract