Alcohol is known to increase crash risk, but the evidence for other drugs is much less clear. Studies have been hampered by measurement issues and the small proportions of drivers found with drugs other than alcohol, especially drugs in the absence of alcohol. Cannabis and benzodiazepines (BZDs) are the two drugs other than alcohol most often found among crash involved drivers. Both have been found to impair driving skills measured in the laboratory, but there are few adequate studies assessing their contribution to crashes, and these studies have mixed results. The weight of the evidence suggests that benzodiazepines increase crash risk, in particular long-acting BZDs prescribed for medical use, at least for the first weeks of use. Further studies of cannabis and benzodiazepines are needed to clarify their contribution to the highway safety problem.
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