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|Major Studies of Drugs and Drug Policy|
|Canadian Senate Special Committee on Illegal Drugs|
|Volume I - General Orientation|
According to a number of the witnesses we heard, more than 40% of people whose driving abilities are impaired would drive under the influence of cannabis. Others have said that approximately 12% of accidents causing injury could be attributed to the use of cannabis. What do the studies reveal?
Data on the frequency of driving under the influence of cannabis (whether on its own or together with other substances) are, for obvious reasons, difficult to obtain. First, for drivers involved in an accident, a positive breathalyzer test means most of the time that no other measurements are taken because a blood alcohol level above the legal limit is enough to take legal action. Second, the methods available to detect the presence of THC are intrusive (blood, urine), unlike the breathalyzer, and hence pose specific legal and ethical problems. Other forms of measurements, such as saliva samples, do not, for the time being, allow roadside detection. Lastly, in studies of all drivers, the consent of drivers is required to take a blood or urine sample, thus limiting the possibility of generalizing results. Nevertheless, we will summarize the main points of a number of studies conducted in recent years.
Two types of studies were conducted: surveys of all drivers selected at random from the flow of traffic at various times of the day and week, and studies where it was presumed that the people were driving under the influence during police checks. The following table, drawn from the various data available from INSERM, summarizes these studies.
Epidemiological studies on youth
In recent years, epidemiological studies on youth in the school environment have asked questions about the frequency of driving under the influence of psychoactive substances, cannabis in particular. In Ontario, the 2002 OSDUS study described in Chapter 6 shows that 19.3% of the students had driven their car one hour or less after having taken cannabis at least once in the past twelve months. More interesting is that this compares with 15% who said they had taken their car less than an hour after one or two drinks. In Manitoba, the survey of youths in school reveals that almost 20% see nothing wrong in driving after taking cannabis.
Finally, the Cohen and Kaal study on long term consumers had shown that no less than 42% of the respondents in Amsterdam and 74% in San Francisco had driven their car under the influence of cannabis.
Given the difficulties of conducting reliable epidemiological studies on driving under the influence of cannabis, a number of authors have analyzed the probability of responsibility and the risk ratio involved in the use of cannabis. These studies distinguish between drivers who are responsible for accidents and those who are not. The former are the subjects and the latter the control group. Comparisons are then made of their intoxication to various substances. Clearly, placing drivers into the two categories of responsible/not responsible may depend on an investigator’s perception of whether or not psychoactive substances are present.
The following table, which is reproduced from the Ramaekers et al. report (2002) for the International Scientific Conference on Cannabis summarizes the results of various studies. It should be pointed out that the probability of responsibility for drivers showing traces of cannabis (D9THC and/or D9THC–COOH, whether measured in blood or urine) is compared to the responsibility of drivers involved in an accident not testing positive to any substance (including alcohol). The risk ratio for drivers not testing positive to any substances is 1.0 and is used as a point of comparison to determine the statistical significance of observed change in the risk level of impaired drivers. When the reference value is above the statistical confidence level of 95%, the obvious conclusion is that the drug is 95% associated with an increased risk of responsibility.
The study findings show that cannabis alone does not increase the likelihood of responsibility in an accident. However, most of the studies used a measurement of THC-COOH, an inactive metabolite that can remain in urine for several days. When the authors separated out THC alone, the risk ratio was slightly higher, even though it did not reach the required level of significance. In addition, as the concentration of THC increases, the more the ratio increases, once again suggesting a dose-response relationship. Furthermore, the cannabis and alcohol combination significantly increases risk. Without being able to draw any definite conclusions, there are some signs that their effects are in synergy and not merely additive.
Studies on injured drivers (Terhune (1982) and Hunter (1998)) have ratios somewhat higher than in the other studies on fatal accidents. According to Bates and Blakely (1999), the apparent reduction in the risk of a fatal accident stems from the fact that drivers under the influence of cannabis drive less dangerously, for example by reducing their speed.
To conclude, we are rather in agreement with INSERM concerning these studies:
[translation] The findings definitely confirm the significant risk of alcohol, but generally fail to demonstrate that there is an effect of cannabis alone on the risk of being responsible for a fatal accident or an accident involving serious injury. The methodological difficulties that make such a demonstration difficult contribute considerably to the absence of statistically indisputable results. Analyses of responsibility nevertheless suggest that the association between alcohol and cannabis increases the risk of being responsible for an accident, compared to drinking alone; however, this finding needs to be consolidated. Lastly, the most recent data tend to show that there is a risk of becoming responsible at heavy concentrations of D9THC. This involves using cannabis immediately before driving, and perhaps applies also to chronic users. 
In all, it was observed that the detection rates for the presence of cannabis varied between 1% and 5% when there was no presumption of impaired driving. However, the missing data, which likely resulted from refusals to supply a sample, made it impossible to draw clear conclusions. The studies with presumption of driving under the influence of drugs had clearly higher results: between 10 and 26%. These results do not necessarily reveal a much higher prevalence of driving under the influence of psychoactive substances, but rather a higher level of vigilance by the police. Indeed, as we shall see immediately, the prevalence of cannabis detection in fatal accidents is no higher in Norway (7.5%) than in other countries.
It is difficult to compare studies between countries because the detection methods, even in an accident context, varies widely from country to country. We wish to note once again that simply finding traces of cannabis in drivers involved in accidents is not necessarily a sign that its use was the cause of the accident. Nor does the absence of any screening result mean that no one was driving under the influence of cannabis.
The following table, adapted from INSERM results, refers to a number of recent studies in Europe, America and Australia.
Three of these studies are particularly interesting. The Mura et al. study (2001) shows a significant difference by driver age: among 18-20 year olds, the D9THC was present in 18.6% of drivers, and in 50% of cases it was present alone (without alcohol). An earlier study by Mura (1999) had shown that cannabis was particularly common among young drivers: from 35% to 43% in the under 30 age group, with an even higher prevalence (43%) for the under 20s, whereas past the age of 35, the prevalence drops to 3%.
The study by Kintz et al. (2000) is interesting primarily because it clearly shows that, after alcohol (13.6%) cannabis is the substance most frequently present among drivers involved in accidents (9.6%). This study also shows that in the whole sample, the incidence of cannabis as measured by taking a blood sample (9.6%) is close to the level of driving under the influence of alcohol (10.6%).
Then, Longo’s study is of special interest because of the size and representativeness of the sample and the fact that separate analyses were done of D9THC and D9THC-COOH. The study detected the presence of cannabinoids in 10.8% of drivers: 8% for D9THC-COOH alone and 2.8% for D9THC-COOH and D9THC together, thereby showing a lower percentage of positive tests for D9THC than the other studies. Furthermore, as in the other studies, subjects testing positive to D9THC were younger and more often men.
Closer to home, Mercer and Jeffery examined the toxicological analyses for 227 drivers killed in traffic accidents in British Columbia between October 1999 and September 1991. Samples had been taken during autopsies within 24 hours of death, which according to the authors, may indicate an under-estimation of the presence of alcohol or drugs. Of the 227 people killed, 186 (43%) showed no signs of either alcohol or drugs, 83 (37%) alcohol only, 23 (11%) alcohol and drugs, and 21 drugs only. As for cannabis, 29 of the people killed (13%; 26 men and 3 women) tested positive to D9THC-COOH, showing an average concentration of 15.9 ng/ml. In the +alcohol/+drugs group, (23 subjects), 17 tested positive to THC metabolites and 8 were also positive to D9THC (13%). For the 0alcohol/+drugs group, (21 subjects), 8 (all men) were positive to D9THC–COOH, and 4 to D9THC. Even though the authors concluded that D9THC /D9THC-COOH was present in 13% of cases, which is a percentage comparable to most of the other studies, only 12 subjects killed tested positive to D9THC with or without alcohol and only 4 without alcohol.
Lastly, a more recent epidemiological study dealt with 1,158 cases of fatal accidents (391) or of cases of driving under the influence of psychoactive substances when the percentage of alcohol in the blood was below 0.1 (767) reported in Canadian forensic laboratories on November 12, 1994. The most frequent substances identified were benzodiazepines (590 cases), alcohol (580), cannabis (551), stimulants (224), opiates (176) and barbiturates (131). For cannabis, we get the following table:
In all, cases in which D9THC without alcohol was present accounted for 13% of the total, which is close to the figure found in the other studies.
Out of all the studies, it was found that the presence of cannabis among drivers who were injured or killed varies between 3.6% (confirmed by blood analysis) and 13% (unconfirmed). Where there was confirmation of the presence of D9THC compared to D9THC-COOH, the presence of the active substance decreases by half. In addition, the risk of testing positive is much higher for young men than other drivers. These conclusions are largely shared by other authors.
 Adlaf, E.M. et A. Paglia (2001) Drug Use among Ontario Students 1997-2001. Findings from the OSDUS. Toronto: Centre for Addiction and Mental Health, page 134.
 Patten, D., et coll., (2000) Substance Use among High School Students in Manitoba. Winnipeg: Addictions Foundation of Manitoba.
 Cohen, P.D.A. et H.L. Kaal (2001) The Irrelevance of Drug Policy. Patterns and careers of experienced cannabis use in the populations of Amsterdam, San Francisco and Bremen. Amsterdam: University of Amsterdam, CEDRO, page 68.
 Ramaekers et al. (2002), op.cit., page73.
 Cited in INSERM (2001), op. cit., page 192.
 INSERM (2001), op. cit., page 194.
 Table reproduced from INSERM (2001), op. cit., page 175.
 Adapted from INSERM (2001) op. cit., pages 171 and 174.
 See INSERM, (2001), op. cit., page 172.
 Mercer, W.G. and W.K. Jeffery (1995) “Alcohol, Drugs and Impairment in Fatal Traffic Accidents in British Columbia” Accid. Anal. And Prev., 27 (3), pages 335-343.
 Jeffery, W.K. et al. (1996) “The involvement of drugs in driving in Canada: An update to 1994.” Can. Soc. Forens. Sci. J., 29 (2), pages 93-98.
 Including the INSERM report (2001), op. cit.; Ramaekers, J.G. et al., (2002) “Performance impairment and risk of motor vehicle crashes after cannabis use” in Pelc, I. (ed.) International Scientific Conference on Cannabis, Brussels.
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