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Prevalence of Alcohol and Drugs in Blood Samples from Norwegian Drivers Involved in Road Traffic Accidents

AS Christophersen*, KM Beylich*, A Bjørneboe*, S Fosser**, A Glad**, J Mørland*

* National Institute of Forensic Toxicology, P.O. Box 9934 Ila, N-0132 Oslo, Norway

** Institute of Transport Economics, P.O. Box 6110 Etterstad, N-0602 Oslo, Norway


The prevalence of alcohol and drugs in blood samples from drivers involved in non-fatal accidents (n=394) has been investigated. In 62,9% (n=248) of the blood samples, alcohol was found either alone or together with drugs, in 24,1% (n=95) of the cases drugs were found alone or with alcohol, and 11,2% (n=44) of the samples contained both alcohol and drugs. The most prevalent drugs were benzodiazepines (13,7%), cannabis (7,5%), opiates (4,3%) and amphetamine (4,1%). In about 3/4 of the drug positive samples the concentration was so high that it was considered likely or very likely that the driver was influenced by drug(s). The bias due to the police's inability to detect all drivers influenced by drugs was estimated. With bias correction it was estimated that at least 2,7% of the drivers involved in injury accidents will have significant concentrations of drugs in blood, and at least 4,4% will have significant blood alcohol concentrations.

As for other comparable studies, our results showed that the accident risk increases rapidly with increasing blood alcohol concentration (BAC). The prevalence of high dose use of benzodiazepines, use of cannabis and amphetamine among drivers was estimated. Comparing these estimates with the results from our study, showed that drivers who use cannabis, amphetamine or high doses of benzodiazepines run a considerable risk of being involved in an injury accident, comparable to BAC levels of 0,1 - 0,15%.


It is well known that alcohol may impair driving ability and thus cause road traffic accidents. Use of illegal and some legal drugs also affect the central nervous system which may increase the risk of accidents (Woods et al 1992). The prevalence of such drugs among drivers involved in accidents depends on the increase of the accident risk due to the drug and on the prevalence of the drug among drivers on the road.

The National Institute of Forensic Toxicology (NIFT) is a national institute, which receives blood samples from all Norwegian drivers suspected to drive under the influence of alcohol and/or drugs. The main reasons for apprehension by the police are road traffic accidents, irregular or dangerous driving. In 1993, NIFT received 5514 blood samples from drivers suspected to be influenced by alcohol alone and 2966 samples from drivers suspected to be influenced by drugs.

The role of drugs as a risk factor in road traffic accidents has earlier been debated (Skegg et al, 1979, Honkanen et al,1980, Kirby et al, 1992). The aim of the present study was to investigate the incidence and concentration of alcohol and psychoactive drugs among drivers involved in injury traffic accidents during a five months period in 1993.


The study included all blood samples received by NIFT from drivers involved in injury accidents and suspected to be influenced by alcohol and/or drugs in the time period from August through December 1993 (n=394). The samples were analysed for both alcohol and drugs independent of the primary suspicion by the police. Before the sampling period started, information about the study was given to all Norwegian police stations. The police was asked to inform in the protocol following the sample, if an accident was the reason for apprehension and to collect enough blood samples for the extended analytical repertoire, if alcohol only was suspected. When drugs other than alcohol were suspected, blood sampling and clinical examination were performed according to standard procedure for such cases.

According to statistics from Institute of Transport Economics, it was calculated that our material (n=394) from the five months period represented about 7% (Bjørnskau, 1994) of all drivers (n=5600) involved in injury accidents during that time period. For drivers not submitted for blood sampling, the police had not been asked or not suspected driving under the influence. However, it is likely that some of these drivers might have been influenced, mainly by drugs other than alcohol, due to more difficulties to recognize influence by such drugs.


All samples were screened for the following drugs or drug groups by immunological or chromatographic methods: Alcohol, cannabinoids, amphetamines, cocaine, benzodiaze-pines, barbiturates, opiates and other analgetic drugs, muscle relaxants, antiepileptics, neuroleptics, antidepressants and antihistamines.

Positive results after screening were confirmed by alternative methods (GC/MS, GC, HPLC) and the concentrations of the psychoactive drugs or metabolites were determined. The police was asked to give information concerning possible drug treatment after serious accidents, to exclude possible positive results caused by medical treatment after the accident but before blood sampling (mainly morphine).


The total number of blood samples from accident drivers (n=394), included 206 cases from drivers suspected to be influenced by alcohol only and 188 cases where drugs other than alcohol were suspected. The number of positive cases and frequency of alcohol and drug detection in these samples is summarized in Table 1. All samples with blood alcohol concentration (BAC) above 0,01 % were recorded as positive.

Table 1
Frequency of Alcohol and Drugs in Blood Samples from Accident Drivers (n=394)

  Number of cases Frequency (%)
No alcohol or drugs 95 24,1
Alcohol only 204 51,8
Drugs only 51 12,9
Alcohol and drugs 44 11,2
Alcohol - total 248 62,9
Drugs - total 95 24,1
Drugs and alcohol - total 299 75,9

Alcohol was detected with a prevalence of more than 50% among the injury involved drivers. Alcohol was also found in 46% of the samples positive for drugs other than alcohol. More than one drug was detected in 36% of the drug positive samples (alcohol not included). Table 2 shows the distribution by different BAC levels of all samples containing alcohol alone or in combination with other drugs (n=248).

Table 2
Samples Positive for Alcohol Only and Both Alcohol and Drugs Distributed after Different BAC Levels

BAC (%) Alcohol only Drugs and alcohol
  No. of cases Frequency (%) No. of cases Frequency (%)
0,11-0,50 13 6,4 3 6,8
0,51-0,99 25 12,3 6 13,6
1,00-1,49 48 23,5 7 15,9
1,50-1,99 63 30,9 12 27,3
2,00-2,49 36 17,6 9 20,5
2,50 + 19 9,3 7 15,9
Total 204 100,0 44 100,0

The distribution was not significant different (p > 0,05; X2-test) between drivers using alcohol and those using both alcohol and drug, indicating that alcohol consumption among drivers combining alcohol and drugs, is similar to that consumption by drivers using alcohol only. Thus, the total influence for drivers combining both alcohol and drugs was probably higher than for those using alcohol only.

The most prevalent drugs besides alcohol were the benzodiazepines, which were detected in 13,7% of the samples, either alone or in combination with other drugs and/or alcohol. Other commonly detected drugs were tetrahydrocannabinol (THC) (7,6%), opiates (4,3%) and amphetamines (4,1%). Cocaine, antidepressants, neuroleptics and antihistamines were not detected. Muscle relaxants were detected in only two cases. Table 3 summarises the results of samples positive for drugs other than alcohol (n=95) (single drug detectection and multi drug detection). The most prevalent detected benzodiazepines was diazepam and flunitrazepam.

Table 3
Frequency of Drugs Other Than Alcohol Among Accident Drivers (n=394), Single Drug Cases and Multi Drug Cases

  Number of cases Frequency (%)
Benzodiazepines only 12 3,1
Benzodiazepines only or combined with other drugs (except alcohol) 28 7,1
Benzodiazepines - total 54 13,7
THC only 5 1,3
THC only or combined with other drugs (except alcohol) 15 3,8
THC - total 30 7,6
Amphetamines only 6 1,5
Amphetamines alone or combined with other drugs (except alcohol) 13 3,3
Amphetamines - total 16 4,1
Opiates only 5 1,3
Opiates alone or combined with other drugs (except alcohol) 13 3,3
Opiates - total 17 4,3


All cases positive for drugs were evaluated for possible impairment based on blood drug concentrations. Four categories were used: Not impaired, possible impairment, likely impaired and impaired. None of the individuals with drugs other than alcohol in the blood were evaluated as "not impaired". More than 75% of the these cases were evaluated as «likely impaired» or «impaired» based on the blood drug concentrations. For most of the cases positive for more than one drug, the concentrations of each drug probably represented an increasing accident risk.


When using data from a former road side study on the prevalence alcohol among Norwegian drivers (Glad, 1985), the relative risks for drivers to be injured in accidents based on different BAC levels were estimated:

Relative risk for BAC 0,00 - 0,050 % = 1; BAC 0,051 - 0,99 = 4,4; BAC 0,10 - 0,149= 12,7; BAC > 0,15 = 45,2.

Based on the official Norwegian statistics of benzodiazepine sales (Norsk Medisinaldepot, 1993), it could be calculated that 0,0165 % of Norwegian inhabitants are high dose benzodiazepine users. Diazepam above therapeutic levels were found in 17 samples from our study, or about 0,3% of the drivers involved in an injury accident from the same time period (N=5600). Thus, the increase of the relative risk could be roughly estimated to be 19 (0,3/0,016), i.e. close to the risk increase at a BAC of 0,15%.

From official statistics on the illegal drug use in Norway and the number of samples positive for such drugs in this study, the relative risks for being involved in injury accidents after use of cannabinoids and amphetamines, could roughly be estimated to increase by a factor of 10 for both drugs. This is comparable to the risk increase at BAC levels of about 0,1 %.

These estimates indicate that drivers who use high dose of benzodiazepines, cannabinoids and amphetamines run a considerable risk of being involved in an injury accident. The estimates have not taken into account that DUI-drivers may have attitudes and life styles that increase their accident risk (Beirness & Simpson, 1987). The estimated risks are therefore probably not exclusively a results of the influence of alcohol or drugs at the time of the accident. Drunken drivers constitute the main part of the DUI-problem. Nevertheless, the relatively high frequency of drivers influenced by drugs among drivers involved in accidents show that these drivers also constitute a serious safety problem.


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Glad, A. Omfanget av og variasjoner i promillekjøringen. Reviderte resultater fra en landsomfattende promilleundersøkelse i 1981-1982. Oslo, Transportøkonomisk institutt.

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