Screening for drug use among Norwegian drivers suspected of driving under influence of alcohol or drugs

Two hundred and seventy blood samples selected at random from Norwegian drivers apprehended on the suspicion of drunken or drugged driving were screened for the presence of amphetamine, benzodiazepines, cannabinoids, tetrahydrocannabinol (THC) and cocaine. Of the samples tested, 223 were from drivers suspected of driving under the influence of alcohol only (A-cases). In the rest (n = 47) of the cases, the police also suspected drugs as a possible reason for driving impairment (D-cases). In the A-cases, benzodiazepines were found in 17%, cannabinoids in 26%, THC in 13% and amphetamine in 2% of the blood samples. One or more drugs besides ethanol were found in 38% of the A-samples. In the D-cases, benzodiazepines were found in 53%, cannabinoids in 43%, THC in 43%, amphetamine in 13% and 77% of these samples contained one or more drugs. Cocaine was not detected in any sample. Blood alcohol concentrations (BAC) above the legal limit of 0.05% were found in 80% of the drug positive A-cases and in 28% of the drug positive D-cases. The frequency of drug detection in A-samples was similar (40%) in samples with BAC above and below 0.05%, while this frequency was much higher (above 90%) in D-samples with BAC below 0.05% than in D-samples with BAC above 0.05% (53%). Benzodiazepines were most frequently found among drivers above 25 years of age, while cannabinoids were most frequently found among drivers below 35 years. For about 15-20% of the A-cases with BAC below 0.05%, other drugs were detected at concentrations which may cause driving impairment. It was concluded that analysis of alcohol only might often be insufficient in A-cases to reveal driving impairment.