Toxicology: The Science

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1.0  Forensic Toxicology, Pharmacokinetics and the Science of Impairment

Toxicology is the study of the adverse effects of drugs and chemicals on biological systems. Forensic toxicology deals specifically with the application of toxicological science to cases where the adverse effects of a drug on a human body have had administrative or medico-legal consequences, and where the results are likely to be used in court [1].

Pharmacokinetics is the essentially the study of what the body does to a drug. This includes studying and identifying the processes by which a drug is absorbed, distributed, metabolized and eliminated by the body. Pharmacokinetics is often studied in conjunction with pharmacodynamics, which is the study of the reverse—what a drug does to the body.

In the criminal law context, these sciences are most commonly used to detect whether or not an individual is impaired by drugs or alcohol. The detection of impairment is most relevant in impaired driving cases, and the overview of the science presented here focuses on that area.

1.1     Criminal Impairment

Impairment, in the context of criminal law, is the alteration and/or deterioration of both the judgment and physical ability of an individual.

Although being in an impaired state is not always a criminal offence, Canadian law explicitly prohibits the driving or operation of a motor vehicle while impaired. S. 253 of the Criminal Code states (slightly paraphrased):

Every one commits an offence who operates a motor vehicle or vessel or operates or assists in the operation of an aircraft or of railway equipment or has the care or control of a motor vehicle, vessel, aircraft or railway equipment, whether it is motion or not, while the person’s ability to operate the vehicle, vessel, aircraft or railway equipment is impaired by alcohol or a drug [2].

In the case of alcohol impairment,there is a specific statutory measure used to determine whether the degree of impairment has exceeded the legal limit. S.253(b) states that it is an offence to operate a motor vehicle having consumed alcohol in such a quantity that the concentration in the person’s blood exceeds 80mg of alcohol in 100ml of blood [3]. There is no statutory measure for drug impairment.

2.0  The Properties and Transmission of Alcohol

2.1     The Properties and Effects of Alcohol

Alcohol is a general term covering a family of organic chemicals with common properties, including ethanol, methanol, isopropanol and others. The most commonly ingested of these, and the chemical found in alcoholic beverages, is ethanol. Ethanol is water-soluble.

Alcohol (ethanol) is a central nervous system depressant. The central nervous system is composed of the brain and the spinal cord.

As a central nervous system depressant, the ingestion of alcohol slows down or “depresses” the normal activity that goes on in the brain and spinal cord. This slowing down of the central nervous system causes confusion and dizziness, as well as the impairment of the normal functioning of memory, intellectual performance, judgment and motor coordination (i.e. balance, response times etc.) [4]. The degree to which the central nervous system function is impaired is directly proportional to the amount of alcohol ingested and the subsequent concentration of alcohol in the blood [5]. 

The spinal cord conducts sensory information from the peripheral nervous system to the brain (peripheral denotes all the nerve systems that lie outside of the domain of the central nervous system) [6]. The spinal cord also conducts all motor information from the brain to our “effectors”—all of the muscles, glands, nerves and organs in the body that are capable of responding to mental stimulus. In addition, the spinal cord is a minor reflex center—an area in the body that is responsible for turning a sensory impression into an actual motor stimulus [7]. 

The brain receives sensory input from the spinal cord as well as the olfactory and optical nerves, and sends motor stimulus [8].   The observable signs of alcohol impairment are derived from the effect alcohol has on the various areas of the brain. When the cerebral cortex is affected, which processes information from the senses as well as controls conscious thought processing and voluntary muscle movement, the individual commonly loses their normal inhibitions, experiences confusion of the senses and loses their ability to process thoughts normally, leading to a decrease in “good judgment” and clear thinking [9]. The cerebellum coordinates muscle movement, particularly the fine muscle movements required to keep your balance or touch your finger to your nose with your eyes closed.When alcohol affects the limbic system, which controls emotions and memory, the individual is often subject to exaggerated emotional states (anger, aggressiveness, increased affection, misery) and memory loss. The hypothalamus and pituitary gland are also located in the brain. When affected by alcohol, these areas are responsible for one of the more well-known effects of alcohol impairment—the increase of sexual drive but a decease in the ability to perform [10]. 

Absent external factors, the effect of alcohol on the medulla, or brain stem, is most dangerous to the intoxicated individual. The medulla controls all unconscious bodily functions, such as breathing, heart rate, temperature and consciousness. As alcohol starts to influence the upper centers in the medulla, the individual will begin to feel sleepy and may even become unconscious. If the blood alcohol concentration becomes high enough to influence the breathing, heart rate and temperature centers, the individual may stop breathing completely, and both blood pressure and body temperature will fall. This condition can easily be fatal [11]. 

Alcohol also affects other body systems. It irritates the linings of the stomach and intestine, which leads to vomiting. It increases blood flow to the skin, causing the characteristic sweaty skin and flushed appearance. It also reduces blood flow to muscles, lessening muscle tone and causing aches [12].

The speed of absorption of alcohol may change depending on the concentration of alcohol in the beverage; the type of drink ingested; the size, weight and gender of the individual; and whether the stomach is full or empty. However, once a certain level of blood alcohol concentration (BAC) has been reached, people tend have similar reactions.

Blood Alcohol Stages & Behaviour [13]. 

Euphoria (BAC = 0.03 to 0.12 percent)

  • More self-confident or daring; poor judgment i.e. not moderating speech
  • Flushed complexion; trouble with fine motor movements such as writing

Excitement (BAC = 0.09 to 0.25 percent)

  • Trouble understanding or remembering things, even recent events
  • Slower reaction times; uncoordinated body movements and loss of balance
  • Blurry vision; sleepiness; sense confusion

Confusion (BAC = 0.18 to 0.30 percent)

  • Confusion as to where they are/what they are doing
  • Dizziness and staggering, slurred speech, blurred vision, heightened pain tolerance
  • Prone to highly emotional states—aggressive, maudlin or overly affectionate

Stupor (BAC = 0.25 to 0.4 percent)

  • Person may be barely able to move or respond to stimuli
  • May vomit and lapse in and out of consciousness

Coma (BAC = 0.35 to 0.5 percent)

  • Person is likely unconscious and has depressed reflexes (i.e. pupils do not change in response to light)     
  • Body temperature is lower, breathing is slower and more shallow, heart rate is slow

Death (BAC = to or greater than 0.50 percent)

2.2      The Transmission

2.2.1     The Transmission of Alcohol Through the Body

When alcohol is ingested, it first enters the stomach. From there, the gastrointestinal tract absorbs about 20% of it. The remaining 80% travels into the small intestine, where, as a water-soluble chemical, it is rapidly absorbed into the water content of the blood stream and distributed throughout the body by entering and dissolving in the water of each tissue of the body (with the exception of fat tissue, as alcohol cannot dissolve in fat) [14].  Once the absorption of alcohol is complete equilibrium occurs, meaning that blood at all points throughout the human body will contain approximately the same concentration of alcohol [15].  Because the distribution occurs so quickly and thoroughly, even small concentrations of alcohol can seriously and quickly affect the functioning of the central nervous system.

2.2.2     Factors Affecting the Transmission of Alcohol

In general, the less an individual weighs the more he or she will be affected by a given amount of alcohol. In the case of two individuals with similar body compositions and different weights, the larger individual will achieve lower alcohol concentrations than the smaller if ingesting the same amount of alcohol [16]. Blood only makes up about 6% of the total amount of water found in the human body, so 94% of alcohol will be located in the water of other tissues. At the most basic, an individual’s blood alcohol concentration is the result of the total amount of alcohol in one’s system divided by total body water.

The amount of water found in the human body is influenced not only by weight and size, but also by age and gender. Generally speaking, young men will have considerably more total body water than older women. For example, a male weighing 220 lbs must theoretically consume twice the amount of alcohol as a man weighing 110 lbs in order to attain the same BAC. Gender is also important—a 110 lb man will generally have to consume more alcohol than a 110 lb woman in order to attain the same BAC.

The presence of food in the stomach either prior to the ingestion of alcohol or ingested along with alcohol will not stop absorption, but it will slow the process down somewhat by slowing the emptying of the stomach into the small intestine.

The rate of consumption also affects blood alcohol concentration. Because the body metabolizes alcohol at a fairly constant rate (somewhat more quickly at higher and lower alcohol concentrations), ingesting alcohol at a rate higher than the body’s rate of elimination results in a cumulative effect and an increasing blood alcohol concentration [17]. 

2.2.3     The Elimination of Alcohol

Alcohol is eliminated from the body primarily by the liver, which—through the oxidation of ethanol—rids the body of about 90% of ingested alcohol. The remainder of the alcohol is eliminated through excretion of alcohol in breath, urine, sweat, feces, breast milk and saliva [18]. Healthy people metabolize alcohol at a fairly consistent rate. Generally speaking, a person will eliminate one average drink or .5 oz (15 ml) of alcohol per hour, although several other factors may influence this rate. For example, the rate of elimination tends to be higher when the blood alcohol concentration in the body is very high. In addition, chronic alcoholics may (depending on liver health) metabolize alcohol at a significantly higher rate than the average person. Finally, the body's ability to metabolize alcohol tends to diminish significantly with age and disease.

Contrary to many myths, the only thing that can decrease blood alcohol concentration is the passage of time during which the body goes through the above processes. Folk remedies such as exercise, sweating, drinking large amount of fluids or coffee, sleep and cold showers have no effect.

3.0  Alcohol Impairment versus Alcohol Intoxication

Alcohol impairment is the alteration and/or deterioration of both the judgment and physical ability of an individual due to the effects of alcohol on the body. Intoxication is a term for the observable physicals signs and effects of alcohol consumption, such as red glassy eyes, flushed face, slurred speech and staggering. Intoxication and impairment are not the same thing.

Signs of alcohol intoxication are not always reliable indicators of alcohol impairment, as they can be caused by numerous other factors. In addition, the physical signs of intoxication exhibited by an individual do not always correlate with the individual’s actual blood alcohol concentration and his or her degree of real impairment. Furthermore, impairment can occur without any of the outward and common signs of intoxication.

3.1  Tolerance

Chronic alcohol consumption can lead to the drinker developing a certain degree of functional tolerance to at least some of alcohol’s effects. Functional tolerance occurs when the brain functions of chronic drinkers adapt to compensate for the disruption caused by alcohol in both their behaviour and bodily functions. Chronic heavy drinkers display functional tolerance when they show few obvious signs of intoxication even when their blood alcohol concentration is at a level that for others would be seriously incapacitating of even fatal. However, chronic drinkers usually develop tolerance toonly some of the effects of alcohol, not all.

However, there are obvious limitations to the development of tolerance, and it cannot compensate for the skills required for safe driving. If a driver is exhibiting no outward signs of intoxication, but fails to pass a breath test, he or she will be guilty of the offence of driving while impaired by alcohol, and with a blood alcohol concentration of “over 80”.

4.0  Alcohol Impairment and Operating a Motor Vehicle—Making Out the Offence

The relationship between alcohol and motor vehicle crashes is well known. Alcohol has been implicated in a great number of automobile, railroad, boating, and aircraft accidents. The subtlety and complexity of the skills required to operate these vehicles make them susceptible to impairment by even low doses of alcohol.

Although impaired driving laws have been in place for decades, law enforcement were faced with the challenge of determining whether someone was actually impaired enough to make out the offence of impaired driving.

Out of this challenge developed a number of tools and processes by which to both identify alcohol impairment and quantify its degree. The most common of these are blood and breath tests. Both serve to identify whether the suspect driver was driving with a BAC that was in either the “WARN” range of 0.05% to 0.08%, or in the illegal range of a BAC of over 0.08%. The 0.08% limit is also referred to as 80mg/dL, or simply as “over 80”. 

4.1 Breath Tests for Alcohol


Breath testing theory is based on the measurement of alcohol contained in deep-lung or “alveolar” air. Alcohol shows up in the breath because it is not digested upon consumption and absorption, nor is it chemically changed once it joins the bloodstream. As blood moves through the lungs, some of the alcohol will be transferred through the membrane of the lung’s air sacs, or alveoli, and as alcohol is a volatile organic compound, it will then evaporate. As the person breathes, the alcohol in the alveolar air is exhaled and can be detected by a breath alcohol testing device. The concentration of the alcohol in the alveolar air is related to the concentration of the alcohol in the blood. Although an immediate measure of the blood alcohol concentration would be ideal, the benefits of the breath test are obvious. Instead of having to draw a driver's blood to test his alcohol level, an officer can test the driver's breath on the spot and instantly know if there is a reason to arrest the driver and administer further testing [19].

The ratio of breath alcohol to blood alcohol is 2100:1. This means that 2100 milliliters (ml) of alveolar air will contain the same amount of alcohol as 1 ml of blood [20]. The underlying basis for this ratio is predicated on a phenomenon known as Henry’s Law, which states:

In a closed system, at a given temperature and pressure, for each chemical compound that is dissolved in another liquid compound (e.g., ethanol in water) the concentration of the volatile substance dissolved in the liquid is directly proportional to the vapor pressure of the volatile substance above the liquid [21].

Henry’s Law is accepted as basic science, in much the same way as the laws of gravity or physics are accepted as facts of science. It is not a regularly contested issue in breath testing—contested issues tend to involve the proper breath to blood ratio for use as an evidentiary test, determining the temperature of the exchange process, and ascertaining whether the human respiratory system is in fact a “closed” system [22].

The 2100:1 ratio is used to estimate the amount of ethanol in the blood by measurement of the amount of ethanol in an expired breath sample, and is based on a 1972 study by the American National Safety Council that determined 2100 cubic centimeters of lung air at 34 degrees centigrade will closely equal the amount of alcohol present in one cubic centimeter of blood [23].

For many years, the value of 2100:1 was accepted as the population average, and breath testing equipment in Canada still is calibrated using a ratio of 2100:1. More recent studies have indicated that a more appropriate average venous blood:breath ratio is approximately 2300:1 for subjects in the post-absorptive state. The United Kingdom has recently abandoned the 2100:1 ratio in favour of the newer 2300:1 ratio. However, in Canada the ratio remains at 2100:1. Individuals with ratios greater than 2100:1 could have their BAC levels underestimated, whereas individuals with ratios less than 2100:1 would have their levels overestimated [24].

4.2  Roadside Breath Tests

In order to evaluate whether a driver stopped at the roadside is impaired and should be subject to further testing, police may use an approved screening device to conduct roadside breath test. According to s.254(1) of the Criminal Code, an approved screening device is a device of a kind that is designed to ascertain the presence of alcohol in the blood of a person [25]. Approved screening devices are used as a preliminary screening test only—they are not evidentiary devices and the results obtained from them cannot be used as evidence at trial.

S. 254(1)  lists the device currently approved for the purposes of s.254 of the Criminal Code:

(a) Alcolmeter S-L2;

(b) Alco-Sûr;

(c)  Alcotest® 7410 PA3;

(d) Alcotest® 7410 GLC;

(e) Alco-Sensor IV DWF;

(f)  Alco-Sensor IV PWF;

(g) Intoxilyzer 400D;

(h) Alco-Sensor FST; and

(i)   Dräger Alcotest 6810.

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4.2.1     How Do Approved Screening Devices Work?

Most of the above instruments use a fuel cell. The fuel cell, or electrochemical detector, consists of two electrodes separated by an acid electrolyte. When breath containing alcohol is drawn into the cell, the alcohol is oxidized and an electrical voltage is produced that is proportional to the concentration of alcohol in the breath sample just received. The greater the alcohol concentration, the greater the voltage that is generated [26].

The modern fuel cells are more alcohol specific—they do not react to acetone. However, as with any breath test, mouth alcohol due to oral ingestion of any alcohol-containing material within 10-15 minutes of testing can also produce an apparently falsely high sample, as can burping or vomiting after having ingested alcohol [27]. A police officer using this instrument must ensure that the driver has been observed for 10-15 minutes prior to taking the test, and has not burped, vomited or ingested any other alcohol-containing material. In addition, an alveolar or deep lung sample must be collected and analyzed in order to obtain a reliable blood alcohol concentration or BAC. If the subject provides a poor breath sample, the results may be falsely low [28].

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The result of the test will be shown in one of three possible categories:

1)   Pass or Ready: this indicates a BAC of 0.0% to 0.049% and is usually shown on the machine interface as a number

2)   Warn: this indicates a BAC of 0.05% to 0.099%. The machine will show the word “WARN” or possibly the letter “A” [some of the machines are German—the A stands for the German word “Achtung” or Attention].

Note:This result does not allow the police to demand that the subject return to the station for approved testing, but in Ontario it does allow for the temporary suspension of the subject’s license for 3, 7 or 30 days. Consequences become stricter for repeat offenders. The exception to this is novice drivers—drivers under the age of 21. As of August 1, 2010, novice drivers in Ontario who are caught with any amount of alcohol in their blood will receive an immediate 24-hour roadside driver licence suspension, and if convicted, will face a fine of $60-$500 and receive a suspension period as per the Novice Driver Escalating Sanction scheme, up to and including cancellation of the novice licence. The novice driver will also have to return to the start of the Graduated Learning System [29].

3)   Fail: this indicates a BAC of 0.100% and over. The machine will show an “F” or “Fail”. Failing the roadside screening test is not an offence under the Criminal Code—what it does it give police the reasonable grounds to make a demand for a second breath or blood sample under s.258(3) of the Criminal Code. The breath machines found in police stations are designed to operate and analyze a breath sample in a “laboratory” fashion.

Note: Approved roadside screening devices register a Fail at a BAC of 0.100% even though the legal limit is actually exceeded at 0.08%. There are two reasons for this: first, it allows for a small margin of error in the device; and second, it avoids the problem of a person presenting with a BAC of 0.08% on the road but—because of the elapsed time in getting to the police station—is below that level upon getting there.

4.3  Evidentiary Breath Alcohol Testing Devices

The main distinctions between evidentiary breath test machines and the approved screening devices used at the roadside are the degree of exactness of the results and the “laboratory-like” procedures that are used when doing the tests. These distinctions allow for the results of machines like the Breathalyzer or the Intoxilyzer to be given “evidentiary” status and relied on in court. Due to their size and complexity, these machines are usually located at police stations, although some have been adapted to fit in cruisers.

Operators of any breath alcohol testing device must be trained in the use and calibration of the device, especially if the results are to be used as evidence in court. Past trials have turned on the perceived accuracy of a breath test, and although recent legislation and case law has made it more difficult to challenge their accuracy, it is still important that police and prosecutors be able to rely on the results obtained from full-size devices.

In all the machines, each test is repeated twice, fifteen minutes apart. The degree of precision required for approved breath-alcohol testing instruments is not explained by the Criminal Code. However, the Recommendations of the Alcohol Test Committee of the Canadian Society of Forensic Science, the principal scientific advisor to the government of Canada on matters relating to breath and blood alcohol testing, indicate that if the two tests are within 20 milligrams in 100 milligrams of blood (mg/dL), they should be regarded as being in agreement with each other and can be used as evidence in court. If the results of the two tests are greater than 20 mg/dL apart, a third test should be performed [30].

There are three major types of breath alcohol testing devices, all of which are based on different scientific principles and processes:

1)   Breathalyzer: uses a chemical reaction involving alcohol that produces a color change

2)   Intoxilyzer: detects alcohol by infrared (IR) spectroscopy

3)   Alcosensor III or IV: works like an approved screening device by detecting the chemical reaction of alcohol in a fuel cell

Regardless of the type, each device has a mouthpiece, a tube through which the suspect blows air, and a sample chamber where the air goes. The rest of the device varies with the type.

4.3.1     The Breathalyzer

Note: the Breathalyzer is no longer used in Ontario.

The Breathalyzer device contains:

  • A system to sample the breath of the suspect;
  • Two glass vials containing the chemical reaction mixture; and
  • A system of photocells connected to a meter to measure the color change associated with the chemical reaction [31].

To measure alcohol, a subject breathes into the device. The breath sample is bubbled in one vial through a mixture of sulfuric acid, potassium dichromate, silver nitrate and water. The principle of the measurement is based on the following chemical reaction [32].

  1. The sulfuric acid removes the alcohol from the breath into a liquid solution.
  2. The alcohol reacts with potassium dichromate to produce: chromium sulfate potassium sulfate acetic acid water

The silver nitrate is a catalyst, a substance that makes a reaction go faster without participating in it. The sulfuric acid, in addition to removing the alcohol from the air, also provides the acidic condition needed for this reaction [33].

During this reaction, the reddish-orange dichromate ion changes color to the green chromium ion when it reacts with the alcohol. The degree of the color change is directly related to the level of alcohol in the expelled air [34].

To determine the amount of alcohol in that air, the reacted mixture is compared to a vial of non-reacted mixture in the photocell system, which produces an electric current that causes the needle in the meter to move from its resting place. The operator then rotates a knob to bring the needle back to the resting place and reads the level of alcohol from the knob—the more the operator must turn the knob to return it to rest, the greater the level of alcohol [35].

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The Breathalyzer 900A.

4.3.2     The Intoxilyzer ®

The Intoxilyzer 5000 and 8000 machines both work by exposing alveolar air in a sample chamber to infrared light.

1)   A lamp generates a known amount of infrared light, which passes through the sample chamber and is focused by a lens onto a spinning filter wheel and sensor [36].

2)   Alcohol molecules are known to absorb a certain amount of infrared radiation, so if alcohol is present in the air of the sample chamber, then a certain amount of the infrared light will not reach the sensor. The machine then compares the amount of light sent with the amount of light received in order to calculate an alcohol concentration. The more infrared radiation that is absorbed, the more alcohol is present in the breath sample [37].

3)   The degree of light is converted to an electrical pulse, which is relayed to the microprocessor of the machine. The microprocessor interprets the pulses and calculates the BAC of the subject in question.[xxxviii]

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The Intoxilyzer 8000C

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The Intoxilyzer 5000C

4.3.3     Alco-Sensor III or IV

The Alco-Sensor III or IV works in much the same way as an approved screening device, by using a fuel cell to oxidize the alcohol present in a sample. The fuel cell has two platinum electrodes with a porous acid-electrolyte material between them. As the exhaled air from the subject sample flows from one side of the fuel cell to the other, the platinum electrode oxidizes any alcohol present in the breath sample to produce acetic acid, protons and electrons [34].

The electrons flow through a wire from the platinum electrode. The wire is connected to an electrical-current meter and to the platinum electrode on the other side. The protons move through the lower portion of the fuel cell and combine with oxygen and the electrons on the other side to form water. The more alcohol that becomes oxidized, the greater the electrical current. A microprocessor in the machine measures he electrical current and calculates the BAC of the subject.

The Alco-Sensor IV, image courtesy of

4.4     Biological Fluid Analysis—Blood and Urine Alcohol Tests

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A Headspace Gas Chromatography set-up for the analysis of biological fluids including blood and urine.

4.4.1     Blood Alcohol Tests

A blood alcohol test measures the amount of alcohol (ethanol) in the body. Alcohol is quickly absorbed into the blood and can be measured within minutes of having an alcoholic drink. The amount of alcohol in the blood reaches its highest level about an hour after drinking, although food in the stomach may increase the amount of time it takes for the blood alcohol to reach its highest level. A blood alcohol test is done on blood taken from a venous blood sample—blood taken directly from the vein by a qualified health professional.

A biochemical analysis will then be done on the blood sample in order to determine the amount of alcohol present in the blood at the time the sample was taken.

4.4.2     Urine Alcohol Tests

Urine samples are referred to a “pooled samples” and will indicate the subject’s BAC at some time prior. Urine contains about 1.3 times as much water as blood does, and therefore the amount of alcohol in urine is referred to the urine alcohol concentration or UAC.

For example: a UAC of 0.302 mg/100mL indicates a BAC of 0.232 mg/100mL sometime prior (302/1.3=232).

An alcohol urine test works by measuring the level of alcohol metabolites in urine, which indicates how much alcohol the subject had consumed in approximately the last 48 hours. The test measures the level of ethylene glycol, which is a by-product of the breakdown of ethanol alcohol by the liver. This test is referred to as the EtG (Ethyl Glucuronide) test.

However, the American Substance Abuse and Mental Health Services Administration has recently issued a warning that the EtG test is too sensitive to be reliable, as it and other similar highly sensitive tests are not able to distinguish between alcohol absorbed into the body from exposure to many common commercial and household products containing alcohol, or from the actual consumption of alcohol.  The Advisory, titled “The Role of Biomarkers in the Treatment of Alcohol Use Disorders”, made it clear that the test—while acceptable in clinical settings—should absolutely not be used as a stand-alone test in a forensic situation [40]. 

5.0  Drug Impairment and The Operation of a Motor Vehicle—Making Out the Offence

Drugs, both legal and illegal, can cause impairment and compromise an individual’s ability to safely operate a motor vehicle. For this reason, drug impairment is specifically included under s. 254 of the Criminal Code.

Every one commits an offence who operates a motor vehicle or vessel or operates or assists in the operation of an aircraft or of railway equipment or has the care or control of a motor vehicle, vessel, aircraft or railway equipment, whether it is motion or not, while the person’s ability to operate the vehicle, vessel, aircraft or railway equipment is impaired by alcohol or a drug [41]

Like with alcohol, drug use has been implicated in a great number of motor vehicle accidents. However, unlike modern alcohol impairment investigations, law enforcement today still faces a significant challenge in determining whether or not an individual was actually impaired by a drug while he or she was operating or assisting in the operation of a motor vehicle.

This is because there is no reliable and effective scientific roadside tool that can be used to identify or quantify the degree of drug-impairment a person is suffering from, or even what drug(s) may be involved in their impairment. In addition, while later blood or urine tests may be able to tell whether the suspect had taken a drug, pinpointing when the drug was actually taken and in what dose, when it was active, and what affect it had is impossible using these tests.  Instead, police must rely on non-quantifiable symptoms of drug-impairment, such as erratic driving behaviour and witness testimony. Drug tests are admissible as evidence in court only if the driver participates voluntarily.

In an effort to face this challenge, a relatively new program has been developed—the drug recognition expert (DRE) program.

A drug recognition expert (DRE), sometimes referred to as a drug recognition evaluator, is an individual, usually a police officer, who has successfully completed all phases of the Drug Evaluation and Classification Program’s (DECP) training requirements for certification as established by the International Association of Chiefs of Police (IACP) and the National Highway Traffic Safety Administration (NHTSA). A DRE is skilled in detecting and identifying persons under the influence of drugs and in identifying the category or categories of drugs causing the impairment [42]. 

In evaluating drug impairment, the focus is not on legal status of the drug, but rather its effects.  Drugs that impair depth perception, attention span, concentration, decision-making, and reaction time are all considered to be “impairing”. This includes “street drugs” such as cocaine and marijuana, but also a wide variety of prescription drugs such as morphine, oxycodone, valium and other painkillers.

Drugs that cause hallucinations, distort the user’s perception of time and distance, cause confusion or make it hard to distinguish between fantasy and reality are also clearly extremely impairing. This would include a variety of hallucinogenic street drugs such as magic mushrooms, LSD, acid, and also amphetamines such as ecstasy, crystal meth, methylenedioxymethamphetamine (MDMA) and dexamphetamine.

5.1 The History and Purpose of the DRE Program

***Content from the International Drug Evaluation & Classification Program Website at *** [43]. 

The Los Angeles Police Department (LAPD) originated the DRE program in the early 1970s. Back then LAPD officers noticed that many of the individuals arrested for driving under the influence (DUI) had very low or zero alcohol concentrations. The officers reasonably suspected that the arrestees were under the influence of drugs, but lacked the knowledge and skills to support their suspicions. In response, two LAPD sergeants collaborated with various medical doctors, research psychologists, and other medical professionals to develop a simple, standardized procedure for recognizing drug influence and impairment. Their efforts culminated in the development of a multi-step protocol and the first DRE program.

The LAPD formally recognized the program in 1979.

The LAPD DRE program attracted NHTSA’s attention in the early 1980s. The two agencies collaborated to develop a standardized DRE protocol, which led to the development of the DEC Program. During the ensuing years, NHTSA and various other agencies and research groups examined the DEC program. Their studies demonstrated that a properly trained DRE can successfully identify drug impairment and accurately determine the category of drugs causing such impairment.

In 1987, NHTSA initiated DEC pilot programs in Arizona, Colorado, New York and Virginia. The states of Utah, California, and Indiana were added in 1988. Beginning in 1989, IACP and NHTSA expanded the DEC Program across the country. Currently, 43 states, the District of Columbia, three branches of the military, the Internal Revenue Service (IRS), and several countries around the world, including Canada, participate in the DEC Program.

In October 1995 the program was brought to British Columbia. This training is now being offered to police officers from all agencies in Canada under the umbrella of the Canadian Association of Chiefs of Police [44].

5.2  What Does a DRE Do?

***Content taken directly from the International Drug Evaluation & Classification Program Website at*** [45].

A DRE conducts a detailed, diagnostic examination of persons arrested or suspected of drug-impaired driving or similar offenses. Based on the results of the drug evaluation, the DRE forms an expert opinion on the following:

  1. Is the person impaired? If so, is the person able to operate a vehicle safely? If the DRE concludes that the person is impaired…
  2. Is the impairment due to an injury, illness or other medical complication, or is it drug-related? If the impairment is due to drugs…
  3. Which category or combination of categories of drugs is the most likely source of the impairment?

DREs conduct their evaluations in a controlled environment, typically at police precincts, intake centers, troop headquarters or other locations where impaired drivers are transported after arrest. The drug evaluation is not normally done at roadside and is typically a post-arrest procedure.

In some cases, the person evaluated will be a driver the DRE personally arrested. In many cases, however, the DRE will be called upon to conduct the evaluation after the driver was arrested by another officer. The DRE is requested to assist in the investigation because of his special expertise and skills in identifying drug impairment.

The DRE drug evaluation takes approximately one hour to complete. The DRE evaluates and assesses the person’s appearance and behavior. The DRE also carefully measures and records vital signs and makes precise observations of the person’s automatic responses and reactions. The DRE also administers carefully designed psychophysical tests to evaluate the person’s judgment, information processing ability, coordination and various other characteristics. The DRE will systematically consider everything about the person that could indicate the influence of drugs

5.3  The DRE Tests and Process

When conducting an investigation to elevate suspicion of impairment to probable grounds, a police officer can use a variety of divided attention tests known as the Standardized Field Sobriety Tests or SFSTs. This battery of tests has undergone a number of field validation studies in the United States that have shown that it produces accurate indicators of a blood alcohol level of 80mg% (.08) or greater. In addition, the tests do show impairment that can be caused by other things. The SFST test battery consists of Horizontal Gaze Nystagmus, the Walk and Turn test and One Leg Stand test [46]. 

The evaluation of a suspected drug impaired driver is conducted by an evaluator who is accredited by the International Association of Chiefs of Police. The Drug Recognition Expert uses a 12-step procedure in performing the evaluation [47].

5.3.1     The Twelve Step Procedure

1)   Breath Alcohol Test

The arresting officer reviews the subject’s BAC test results to determine if the subject’s apparent impairment is consistent with his or her BAC. If so, the officer will generally not call a DRE. However, if the impairment is not explained by the BAC, the officer will request a DRE evaluation under the newly amended s.254(2) of the Criminal Code [48]. 

2)   Interview of the Arresting Officer

The DRE begins the investigation by reviewing the BrAC test results and discussing the circumstances of the arrest with the arresting officer. The DRE asks about the subject’s behavior, appearance, and driving. The DRE also asks if the subject made any statements regarding drug use and if the arresting officer(s) found any other relevant evidence consistent with drug use [49]. 

3)   Preliminary Examination and First Pulse

The DRE conducts a preliminary examination, mainly to ascertain whether the subject may be suffering from an injury or other condition unrelated to drugs. Accordingly, the DRE asks the subject a series of standard questions relating to the subject’s health and recent ingestion of food, alcohol and drugs, including prescribed medications. The DRE observes the subject’s attitude, coordination, speech, breath and face. The DRE also determines if the subject’s pupils are of equal size and if the subject’s eyes can follow a moving stimulus and track equally. The DRE also looks for horizontal gaze nystagmus (HGN—the jerking or bouncing of the eyeball that can occur when the central nervous system is depressed by a drug) and takes the subject’s pulse for the first of three times. The DRE takes each subject’s pulse three times to account for nervousness, check for consistency and determine if the subject is getting worse or better. If the DRE believes that the subject may be suffering from a significant medical condition, the DRE will seek medical assistance immediately. If the DRE believes that the subject’s condition is drug-related, the evaluation continues [50].

4)   Eye Examination

The DRE examines the subject for HGN, vertical gaze Nystagmus (VGN) and a for a lack of ocular convergence. A subject lacks convergence if his eyes are unable to converge toward the bridge of his nose when a stimulus is moved inward. Depressants, inhalants, and dissociative anesthetics, the so-called "DID drugs", may cause HGN. In addition, the DID drugs may cause VGN when taken in higher doses for that individual. The DID drugs, as well as cannabis (marijuana), may also cause a lack of convergence [51]. 

5)   Divided Attention Psychophysical Tests

The DRE administers four psychophysical tests: the Romberg Balance, the Walk and Turn, the One Leg Stand, and the Finger to Nose tests. The DRE can accurately determine if a subject’s psychomotor and/or divided attention skills are impaired by administering these tests [52]. 

6)   Vital Signs and Second Pulse

The DRE takes the subject’s blood pressure, temperature and pulse. Some drug categories may elevate the vital signs. Others may lower them. Vital signs provide valuable evidence of the presence and influence of a variety of drugs [53].

7)   Dark Room Examinations

The DRE estimates the subject’s pupil sizes under three different lighting conditions with a measuring device called a pupilometer. The device will assist the DRE in determining whether the subject’s pupils are dilated, constricted, or normal. Some drugs increase pupil size (dilate), while others may decrease (constrict) pupil size. The DRE also checks for the eyes’ reaction to light. Certain drugs may slow the eyes’ reaction to light. Finally, the DRE examines the subject’s nasal and oral cavities for signs of drug ingestion [54]. 

8)   Examination for Muscle Tone

The DRE examines the subject’s skeletal muscle tone. Certain categories of drugs may cause the muscles to become rigid. Other categories may cause the muscles to become very loose and flaccid [55]. 

9)   Check for Injection Sites and Third Pulse

The DRE examines the subject for injection sites, which may indicate recent use of certain types of drugs. The DRE also takes the subject’s pulse for the third and final time [56]. 

10)   Subject’s Statements and Other Observations

The DRE typically asks the subject a series of questions regarding the subject’s drug use [57]. 

11)   Analysis and Opinions of the Evaluation

Based on the totality of the evaluation, the DRE forms an opinion as to whether or not the subject is impaired. If the DRE determines that the subject is impaired, the DRE will indicate what category or categories of drugs may have contributed to the subject’s impairment. The DRE bases these conclusions on his training and experience and the DRE Drug Symptomatology Matrix [see below]. While DREs use the drug matrix, they also rely heavily on their general training and experience [58]. 

12)   Toxicological Examination

After completing the evaluation, the DRE normally requests a urine, blood and/or saliva sample from the subject for a toxicology lab analysis. The toxicological sample is sent to a forensic laboratory for analyses to confirm or refute the findings of the evaluator. The mere presence of a drug in the sample does not constitute sufficient evidence to charge a person as being impaired by a drug. The evaluation must show impairment, indicia consistent with one or more drug categories, and the evaluator’s findings must be supported by the toxicology [59]. 

5.3.2     DRE Drug Symptomatology Matrix Chart

dre 1.png

DREs may also use a small Drug Recognition and Classification index card for ease in referring to in the field. The chart lists reactions to various classes of drugs. The cards also commonly include a “pupilometer”—a simple way to measure the dilation of a subject’s pupil. The black marks at the bottom of the card pictured are actually holes, and the measurement is visible on both sides.

dre 2.png

Drug Recognition Card and Pupilometer. Image courtesy of


1 The Forensic Toxicology Council, Briefing: What is Forensic Toxicology? (July 2010) at 1, online: . 

Criminal Code, RSC 1985, C-46 s 253.


4 Medical Dictionary, Central Nervous System Depressants, online: The Free Dictionary .

5 “Alcohol and the Human Body: Alcohol’s Properties”, online: Intoximeters Inc   [Intoximeters].

6 Molecular & Cell Biology, The Central Nervous System, online: University of California, Berkeley .


8 Craig Freudenrich,  "How Alcohol Works" (21 December 2000), online: [Freudenrich—Alcohol].


10 bid.

11 Ibid.

12 Ibid.

13 bid.

14 Ibid.

15 ntoximeters, supra note v.

16 Ibid.

17 bid.

18 Freudenrich—Alcohol, supra note viii at “How Alcohol Leaves the Body”.

19 Craig Freudenrich, “How Breathalyzers Work” (20 October 2000), online: HowStuffWorks [Freudenrich—Breathalyzers].

20 Patrick Mahoney, Simplified Theory of Breath Testing & Breath Testing Instruments, online: Mahaney Law [Mahaney.]

21 bid.

22 bid.

23 bid.

24 Forcon Forensic Consulting, Breath Testing, online: Forcon Forensic Consulting .

25 Criminal Code, RSC 1985, C-46 s 254(1).

26  DM Lucas and JG Wigman, “Approved Screening Devices” (1994) 6(2) JMVL 189 at 190, online: Indiana University Borkenstein Course [Lucas & Wigman].

27 Ibid.

28 bid.

29 Ontario Ministry of Transportation, Impaired Driving Fact Sheet, online: Ontario Ministry of Transportation 

30 The Canadian Society of Forensic Science, “Recommended Standards and Procedures of the Canadian Society of Forensic Science Alcohol Test Committee” (2003) 36:3 Can Soc Forens Sci J 101 at 115.

31 Freudenrich—Breathalyzers supra note xix.

32 bid.

33 Ibid.

34 bid.

35 Ibid.

36 Luke Rioux, Intoxilyzer Breath Test: How it Works, How to Read the Result (5 April 2013), online: Luke Rioux .

37 Ibid.

38 bid.

39 Freudenrich—Breathalyzers supra note xix.

40 Center for Substance Abuse Treatment, “The Role of Biomarkers in the Treatment of Alcohol Use Disorders: Substance Abuse Treatment Advisory” (2006) Volume 5, Issue 4, online: Substance Abuse and Mental Health Services Administration .

41 Criminal Code, RSC 1985, C-46 s 253.

42 The International Drug Evaluation & Classification Program, Drug Recognition Experts (DRE), online: The International Drug Evaluation & Classification Program:  .

43 The International Drug Evaluation & Classification Program, History and Development, online: The International Drug Evaluation & Classification Program:  .

44 Royal Canadian Mounted Police, Drug Recognition Expert: Background and Responsibilities, online [RCMP—DRE].

45 The International Drug Evaluation & Classification Program, What They Do, online: The International Drug Evaluation & Classification Program:  [DECP—What DREs Do].

46 RCMP—DRE supra note xliv.

47 DECP—What DREs Do supra note xlv.

48 Ibid.

49 bid.

50 bid

51 bid.

52 Ibid.

53 bid.

54 bid.

55 bid.

56 bid.

57 bid.

58 Ibid.

59 RCMP—DRE supra note xliv.