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Major Studies of Drugs and Drug Policy
Canadian Senate Special Committee on Illegal Drugs
Volume I - General Orientation

Chapter 8 - Driving Under the Influence of Cannabis

Forms of testing


There are five known media for testing the presence of cannabinoids in the organism: blood, urine, saliva, hair and perspiration.

Blood is the most appropriate medium for detecting recent cannabis use because only a blood analysis can distinguish between the active ingredients of cannabis and metabolites that have no psychoactive effects. However, as we have already seen, blood concentrations of D9THC peak 9 minutes after smoking; after 10 minutes only two-thirds of the concentration remains, and it is down to 5 to 10% at the end of an hour; after two hours, it becomes difficult to detect. Thus not all methods are appropriate for testing because of the strong possibility of obtaining false negatives and false positives. The most reliable method, gas chromatography using mass spectrometry for detection, is extremely sensitive and can also estimate the time that has elapsed between the most recent use and the taking of the blood sample.

We saw in Chapter 7 that there was a dose-response relationship: 25 puffs affect cognition more than do 10 puffs, and 10 have more of an effect than 4. But not much data is available on the relationship between concentration and effects on people, and the ability to answer the key road safety question, namely at what concentration can one consider that faculties are impaired? In France, the D9THC level that constitutes testing positive has been set at 1ng/ml[1][7] for drivers involved in fatal accidents. Another author has come up with a formula that establishes a relationship between D9THC, 11-OH D9THC and D9THC-COOH to determine a cannabis influence factor with a positive threshold of 10ng/ml. An equal concentration of D9THC and COOH suggest use approximately 30 minutes beforehand, and hence a very high probability of psychoactive effects, whereas a higher concentration of COOH than D9THC suggests that use was more than 40 minutes beforehand. However, a concentration of COOH in excess of 40 μg/l would indicate a chronic user, and hence it becomes impossible to determine when the last use occurred. Other research has established that a blood concentration of 10 to 15 ng/ml suggests recent use, without however being able to give an exact figure.[2][8]

Urine tests are also frequently employed and remain the most appropriate method for rapidly determining whether subjects have been using. On the other hand, traces of cannabis can remain in urine for weeks. Furthermore, the traces that remain are of D9THC-COOH, an inactive metabolite. Consequently, urinalyses are primarily useful for epidemiological measurements of cannabis use, and cannot contribute to information about impaired driving.

The levels of concentration of D9THC-COOH in urine are very high: for someone who smokes a joint a day, the level is between 50 to 500 ng/ml and may reach several thousands ng/ml in heavy users; the currently recommended threshold level for testing positive is 50ng/ml urine. 

Saliva is a very promising option for road safety because it is non intrusive and can indicate recent use with some accuracy. The presence of D9THC in saliva essentially results from the phenomenon of bucco-dental sequestration during inhalation. Concentrations are very high in the few minutes following absorption, varying between 50 and 1,000 ng/ml, but then decline very quickly in the hours that follow, though they remain detectable for an average of four to six hours. The European ROSITA project compared the reliability of samples taken from urine, perspiration and saliva compared to that taken from blood. Saliva is by far the most reliable, showing an exact correlation in 91% of cases. However, the low level of concentration during the period when the psychoactive effects are active means that sensitive analytical methods are essential. There is unfortunately not yet a sufficiently accurate and reliable rapid detection tool that can be used in driving situations. Hence the driving detection tools correctly identified only 18 to 25% of cases and led to many false negatives.[3][9]

Perspiration is generally considered poor for detection purposes, because of the persistence of D9THC in sweat, and the fact that it is also excreted into sweat in small quantities.

Hair looks very promising because the significant amount of D9THC can determine time since and level of use (low, moderate, high). However, concentrations are only a few ng per mg of hair, which requires highly efficient testing.

The following table, taken from the INSERM report, summarizes the main characteristics of the various biological testing media; where available, we have added the threshold detection level adopted.



Main Characteristics of Biological Testing Media


Primary cannabinoids

Maximum detection period

Useful for

Methodologies available

Threshold for positive test
























THC (active)













Occasional use: 2 to 7 days

Regular use: 7 to 21 days


2 to 10 hours



Highly variable







2 to 10 hours


Identifies use





Identifies recent use



Not useful



Identifying & monitoring regular user


Confirmation, identification, dosage


Yes, many rapid tests




No, no rapid tests


No, no rapid tests








50ng of D9THC-COOH per ml



not determined



not useful



not determined




1ng/ml (France)



In all instances, the handling and transportation of samples and the toxicological dosages are essential to the quality of the analyses.

There is still considerable uncertainty about thresholds that make it possible to affirm that the presence of D9THC would impair the driver. Furthermore, there is still no reliable rapid screening test to identify recent use (urine tests cannot do this). Moreover, other drugs besides alcohol, including many types of prescription medicines, may have an impact on driving. That is why many authors, and a number of witnesses, suggested to us that Canada adopt the Drug Evaluation and Classification Program (DEC) and recognize police officers trained as Drug Recognition Experts. This practice has now been adopted in most U.S. states (at least 34, as well as the District of Columbia), British Columbia, Australia, Norway and Sweden.

The typical scenario for driving under the influence of psychoactive substances other than alcohol is as follows: a vehicle attracts the attention of a police officer, who pulls the vehicle over and questions the driver; if there are reasonable grounds to believe that the driver is intoxicated, a breathalyser test is administered; however, when the test yields a result below the legal limit, the police officer may still not be convinced that the driver is capable of driving, but how is this to be proven? Before, more often than not, the police officer had to release the driver. As we have just seen, there are no equivalents to the breathalyser test for drugs and medicines, and, for cannabis in particular, traces found in urine in no way establish that use was recent. It was in this context that the police officers working for the Los Angeles Police Department developed the Drug Recognition Expert System (DRE) in the early 1980s. Police officers are given specific training in the detection of people driving under the influence of psychoactive substances and in the use of the DEC.

The system allows police officers who have reason to believe that drivers are intoxicated to call on an officer specially trained in drug recognition, who can then evaluate the driver on the basis of a set of systematic and rigorous factors that are recognized as signs of the presence of drugs. The process involves 12 steps:

··                Breath alcohol test: This test will have been conducted by the police officer who stopped the vehicle. The Drug Recognition Expert is only called in when the test is negative.

··                Interview by the arresting officer: The DRE asks the arresting officer a series of conventional questions: in what condition did he or she find the suspect, what he or she had observed, if he or she found drugs in the vehicle, suspect’s statement, etc.

··                Preliminary examination (the first of three pulse measurements): This involves determining whether there are reasonable grounds to suspect the presence of drugs, and hence eliminate the possibility that there is a medical condition. The DRE observes the suspect’s overall condition, and questions the suspect about health, examines the pupils and gaze, and takes the first of three pulse measurements. If the DRE feels that there are no signs, the suspect is released. If the condition is medical, a medical evaluation is requested. However, if drugs are suspected, the examination continues

··                Examination of the eyes: This consists of three tests: horizontal gaze, vertical gaze and convergence. Apparently when under the influence of any drug, it is impossible to have an involuntary jerky movement of the pupils on the vertical axis without first provoking such movements on the horizontal axis. Thus if there are only vertical jerky pupil movements, it is likely a medical condition (e.g. brain damage). If there is horizontal jerkiness, there are likely drugs involved. To determine horizontal movements, the DRE moves a pen or other object horizontally in front of the suspect’s eyes. For vertical movement, the pen is moved from top to bottom. Furthermore, as certain drugs prevent eyes from being able to converge towards the bridge of the nose, the DRE performs a convergence test by placing the pen or object on the person’s nose and asking the suspect to look at it

··                Divided attention psychophysical tests: The tests include balancing, walking, standing on one leg and the finger-to-nose test

··                Vital signs examination: This is the second of three pulse measurements, as well as a measurement of blood pressure and body temperature

··                Dark room examination: This involves examining the pupils under four different lighting conditions: room lighting, darkness, indirect light and direct light

··                Examination of muscle tone: arm movements

··                Examination for injection sites

··                Questions about suspect’s drug use and living habits

··                Opinion: On the basis of all the evidence, the DRE forms an opinion based on a reasonable amount of certainty

··                Toxicological examination: The purpose of this examination is to corroborate the analysis by the DRE officer. The decision concerning prosecution is made only when the analyses are returned.


The system was standardized in the early 1980s with the assistance of the U.S. National Highway Traffic Safety Administration. It was first tested in a laboratory study.[4][10] In the study, four Drug Recognition Experts evaluated subjects who had received either a placebo or a dose of drugs. Neither the subjects nor the officers knew who had received the drugs. In 95% of cases, the officers correctly identified the subjects who had not been given drugs. In 97% of cases, they correctly identified the subjects who had been given drugs and in 98.7% of cases, they were able to determine which subjects were under the influence of drugs.

A field study was then conducted in 1985, once again with the assistance of the Highway Traffic Safety Administration.[5][11] In the study, blood samples of 173 drivers arrested for driving under the influence of drugs were analyzed by an independent laboratory. The study showed that the analyses carried out by the Drug Recognition Expert officers correctly predicted the presence of drugs other than alcohol in 94% of cases. In 79% of the cases, the analyses of the officers identifying the presence of a specific drug turned out to be accurate.

The most complete study was carried out in Arizona in 1994. In this study, the files of over 500 persons arrested for driving under the influence of drugs were analyzed, and toxicological analyses were conducted. The study showed that the toxicological analyses corroborated the conclusions of the officers in 83.5% of cases. Similar studies conducted in other states yielded comparable results: 81.3% in Texas, 84.5% in Minnesota, 88.2% in California, 88.2% in Hawaii and 88% in Oregon.

With respect specifically to cannabis, the expected signs listed in the system are generally the following: no horizontal or vertical shaking, but no convergence in gaze, dilated pupils, accelerated pulse and high blood pressure.

In short, given the limits of detection in the field of the influence of cannabis and the results of these studies, it would appear that it would be highly desirable to adopt the DEC and train police officers in drug recognition.



[1][7]  In this chapter, ng means nanogram (i.e. one billion of one gram) and μg means microgram (one million of one gram)

[2][8]  INSERM (2001), op. cit., pages 152-153.

[3][9]  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, page 81.


[4][10]  Bigelow, G.E. (1985) Identifying types of drug intoxication; laboratory evaluation of a subject procedure. Cited in Sandler, D. (2000) “Expert and Opinion Testimony of Law Enforcement Officers Regarding Identification of Drug Impaired Drivers.” University of Hawaii Law Review 23 (1), 150-181.

[5][11]  Compton, P.R. (1986) Field Evaluation of the Los Angeles Police Department Drugs Detection Procedure. Cited in Sandler, op. cit., page 151.

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