Impairment in drivers due to cannabis in combination with other drugs.

Blood samples from 425 suspected drugged drivers who were clinically impaired and negative for alcohol were analysed. Fifty-six percent of the samples were positive for tetrahydrocannabinol (THC). Tetrahydrocannabinol-positive blood samples were analysed for amphetamines, barbiturates, benzodiazepines, cocaine metabolites and opiates. Eighty-two percent of the samples were found to be positive for one or more drugs in addition to THC, and the concentrations of these drugs were often high. Thus, THC in combination with other drugs seems to be a much more frequent reason for impairment than THC alone among Norwegian drugged drivers.