A Toxicology Primer
The Psychomotor Impairments of Drugs of Abuse and Alcohol and How They Contribute to Motor Vehicle Accidents
Thursday, March 24, 2022
by: Christopher Spaeth, Ph.D., Senior Consultant, Rimkus Consulting Group, Inc., Toxicology and Food Safety Practice

Section: Winter 2022

My casework involves forensic investigation of neuronal deficits caused by drugs or toxins, drug toxicology, and alleged injuries caused by consumer products, especially cosmetics. My previous training includes investigating the basic functions of neurons; drug, toxin, or genetically induced neuronal alterations; and toxicological safety and regulatory compliance of consumer ingredients.

Driving is complicated, and principally involves balancing multiple tasks and goals simultaneously while constantly calculating and recalculating the risk of each action. Compounding the risk of driving, drivers can be operating under the influence of one or more mind-altering drugs at the time of an accident. In 2018, 20.5 million drivers admitted to driving under the influence of alcohol and an additional 12.6 million people admitted driving under the influence of illicit drugs, with marijuana accounting for 8 million of those drivers. SAMHSA, 2019. Approximately 20 percent of drivers who tested positive for drugs during moving violations had opioids in their system. NHTSA, 2017. Since some drugs, such as marijuana, are not completely eliminated from an individual for days or weeks after use, evaluating acute or chronic impairment based on the levels that are present is very difficult without additional testing or evidence. Furthermore, many drivers may have more than one drug in their system, thus complicating the extent for the attribution of impairment from the presence of a drug, or multiple drugs, at the time of an accident.

Forensic Toxicology for Drugged Driving

A forensic toxicology expert evaluates toxicological data. From the perspective of a motor vehicle accident involving one or more drivers with drugs present in their systems, the toxicologist can assess the evidence for the identification of the drug(s) associated with the accident. They can then determine whether the drug was at a sufficient concentration to cause impairment for a given individual. When multiple drugs are present, novel chemical combinations might be formed that are more potent than either individual drug alone, leading to additive or synergistic effects.

A significant part of the evaluation by the toxicologist involves understanding how the drug was administered to an individual; how it distributes once in the body and if it is modified to more or less toxic chemicals; the current science behind the physiological effects of a drug(s) once it gets to its site of action; and, ultimately, how and at what rate these chemicals or metabolites are eliminated from a given individual. This can change depending on age, gender, length of use of a given drug, or the presence of other concurrent medical or even environmental conditions.

A toxicologist can then assess how the drug interacts in the body, if it has a specific target molecule(s) in the body, and the impairment that happens when the drug interacts with the target. Based on this target interaction, the toxicologist can then assess any cellular dysfunction and how the affected cells repair themselves or adapt to the changes caused by the drug. One example of adaptation is addiction, wherein an individual must take progressively increasing amounts of a drug to achieve the euphoric effect, due to physical and chemical changes in the brain caused by the drug. Another consideration is how an individual can modify his or her behavior to hide typical behaviors associated with impairment. This commonly occurs with addiction, such as highly functioning alcoholics. After taking the available chemical, physiological, genetic, and other data accumulated about the individual, before, during, and after the accident, the toxicologist can assess the systemic impairment caused by that drug and determine whether the impairments from the drug could potentially be the primary cause of, or contribute to, the accident.


The most commonly abused drug that affects driving is alcohol. SAMHSA 2019. The alcohol molecule acts differently than many drugs as it does not have a specific binding site. As a small molecule, alcohol easily distributes throughout the entire body and strongly and non-specifically affects neurons in the brain to alter behavior. Garriot, 2015.

Blood-alcohol concentration (BAC) measures the amount of alcohol consumed. Olivera et al., 2010. It requires a blood draw that must take place in a hospital or medical facility, sometimes hours after the event. Since alcohol is processed, or metabolized, by the body at a highly variable rate, but within a known range, a toxicologist can use this metabolic range to calculate an estimated BAC at a given time, such as the time of the accident. As a person drinks, the alcohol is first absorbed, which increases the BAC, and then is eliminated, which lowers the BAC. If the BAC is known, a calculation called retrograde analysis can be performed, using a standard range of absorption and elimination rates so that the BAC of an individual can be estimated within a range of confidence at the time of an accident or at the time a person was last served.

Alcohol causes a dose-dependent decrease in driving ability, as measured by the BAC. Studies have shown impairment after intoxication with alcohol for many psychomotor and behavioral categories. These include impairments in reaction time, attention tasks, information processing, and visual function. Not surprisingly, serious impairments are observed in actual driving skills on the road or simulator following alcohol consumption. Garriott, 2015. Of these, the skill that shows the greatest potential for driving impairment in laboratory studies was divided attention performance, followed by visual tracking performance. Id. Since driving requires an individual to focus many different tasks simultaneously, an inability to perform multiple tasks and follow moving objects impairs driving.


Marijuana is the most used illicit drug in the U.S., based on federal law, with an estimated 45.2 percent of people over the age of twelve using at least once in their lifetime in 2018. SAMHSA 2019. Notably, many states have changed their laws on marijuana; however, this law change does not reflect the impairments caused by marijuana. The primary psychoactive chemical in marijuana is tetra-hydro-cannabinol (THC), however marijuana contains hundreds of other chemicals, including cannabidiol (CBD) and other cannabinoid-like chemicals.

A urine test for marijuana assays for a breakdown product or metabolite of THC that is excreted in the urine, 11-nor-delta9-tetrahydrocannabinol-9-carboxycilic acid, or Δ9-THC-COOH. When THC is smoked or eaten, Δ9-THC-COOH can typically be detected in the urine for between four to seven days; however, chronic users could test positive for Δ9-THC-COOH in their urine for up to thirty days. Moeller et al., 2017. Thus, even though an individual tested positive for Δ9-THC-COOH in their urine, he or she may not have been under the acute effects of THC at the time of a given accident. Urine screening is performed using an enzyme-linked immunosorbent assay (ELISA), which directly measures the amount of Δ9-THC-COOH that binds to a protein indicator. Laboratory confirmation and quantification using blood requires a technique known as gas chromatography/mass spectroscopy (GC/MS) or liquid chromatography/mass spectrophotometry (LC/MS). Id.

Acute intoxication from marijuana has many impairing effects on the neurocognitive system. Functions that are impaired by acute marijuana usage that would be used in operating a vehicle include visual tracking and divided attention, similar to alcohol. Other impairments related to driving include increased anxiety, euphoria, sensation of time slowing, decreased pain, paranoia, irritability, worsened short-term memory, poor attention, altered awareness of the passage of time, impaired judgment, decreased coordination and balance, and distorted spatial perception. (Bosker et al., 2013). These impairments were still observed to some degree even after three weeks of abstinence from the drug, which also corresponds to the average time it takes for marijuana chemicals to leave the body in habitual smokers. Id. A growing body of scientific knowledge that indicates that chronic marijuana smokers have a baseline level of impairment that may impair their ability to operate a vehicle properly even when not acutely high. Bosker et al., 2013. Consistent with these described deficits, impaired drivers with THC in their blood were roughly twice as likely to be involved in a fatal motor vehicle accident than sober drivers. SAMHSA 2019; Capler et al., 2017).

Not all individuals with measurable THC or its metabolites in their system are impaired. Capler et al., 2017. Chronic users of THC exhibit tolerance, in that they might be less impaired by a dose of THC compared to someone who does not use THC as often. The effects of THC tolerance are difficult to measure and are highly variable, as exhibited in driving tests. Id.

Stimulants – Cocaine, Methamphetamine, Amphetamines

Drugs that act as stimulants to the brain include both the illicit drugs cocaine and methamphetamines, as well as amphetamines that are prescribed for medical conditions such as ADD/ADHD. Cocaine use accounts for a significant fraction of illicit drug use, with an estimated 14.9 percent of people aged twelve and over reporting cocaine use in their lifetime, with an additional 3.5 percent using crack (smoked cocaine). SAMHSA, 2019. Methamphetamine use accounts for another significant fraction of illicit drug use in the U.S., with an estimated 5.4 percent of people aged twelve and over reporting using methamphetamine in their lifetime as of 2018.

Screening for cocaine uses an ELISA assay designed to detect a specific metabolite of cocaine metabolism, benzoylecgonine (BZE), which may be present in the urine for up to 5 days.  Moeller et al., 2017. Thus, false positives for cocaine are almost non-existent. Id. Although urine testing has improved for methamphetamine, false positives include commonly used over-the-counter drugs, such as pseudoephedrine (Sudafed), metformin, labetalol, phentermine, and buproprion (Wellbutrin). Id.  In methamphetamine cases, it is especially important to understand whether the individual has a noted history of methamphetamine use, or a more definitive assay such as a GC/MS or LC/MS of blood must be used to confirm the drug is present.

Users of cocaine or methamphetamine can exhibit several, pronounced psychomotor impairments important for driving, including deficits in executive function, also known as goal-oriented decision making. Other deficits include increased impulsive behavior, lack of object recognition, adjusted behavior to external circumstances, loss of emotional control, inconsistency in delayed gratification tasks, lack of interest in others’ needs, and a preference for magical or irrational explanations to solve problems. Czermainski et al., 2019; Ballard et al., 2015. High impulsiveness is an additional hallmark of stimulant use.  These specific deficits of stimulants can have a profound effect on driving ability.

Multiple Drugs

Recent survey data estimates that a majority of drivers (63.2 percent) who test positive for drugs are under the influence of multiple drugs at the time of a motor vehicle accident. SAMHSA, 2019. Using multiple drugs creates novel euphoric experiences for users that are more powerful than single drugs alone. Nestler, 2004. Since each specific drug or drug family produces its own impairments, the impairments from multiple drugs would be worse than for a single drug. Sometimes, these effects can be synergistic, such that the combined impairment is greater than what would occur if only one or the other was present alone.

One example is the combined use of alcohol and marijuana. The combination of these two drugs produces stronger euphoria and stronger impairment than either drug alone. Garriott, 2015; Downey et al., 2012. For example, low doses of alcohol (0.05 g alcohol/dL blood), when combined with marijuana, increased the effective BAC impairment to an equivalent of 0.14 g alcohol/dL blood. A separate driving simulator study found significant impairments in “straddling the solid line,” “straddling the barrier line,” and “insufficient stopping clear space,” when THC was consumed with alcohol. Downey et al., 2013. Interestingly, people who consumed both alcohol and marijuana had higher concentrations of THC in the blood, suggesting people increase THC consumption while drinking alcohol. Id.

Another example is cocaine and alcohol. The combined use of cocaine and alcohol results in several physiological effects that would increase impairment in a driver. Garriott, 2015; Althobaiti and Sari, 2016. One effect is that the combination of these two drugs can result in the production of a unique metabolite, cocaethylene, that acts similarly to cocaine in its pharmacological properties, but stays in the system for much longer than cocaine. Althobaiti and Sari, 2016. Second, concurrent alcohol reduces the metabolic rate of cocaine clearance and increases the concentration of cocaine in the plasma by as much as 15 percent, resulting in a higher concentration of the drug available to cause impairing effects on an individual. Id. There is also a reported heightened sense of pleasure and euphoria when users combine these substances as compared to individual use of these substances, thus, increasing the chances of additional use of both drugs together. Id.


Driving is a complicated skill that requires experience and full cognitive abilities for the consistent successful completion of at least four different tasks at the same time. Other activities while driving include proper goal-oriented behavior, understanding and remembering multiple moving visual cues, and deciding which information is important and which information is not. By taking drugs, an individual increases the chance of failure of any or all of the steps necessary to operate a motor vehicle safely and properly. Toxicology experts can provide the expertise for evaluation of cases involving motor vehicle accidents and drugs. Each type of drug consumed has different properties, and an understanding of the various changes in an individual’s ability to function is critical to understanding culpability for motor vehicle accidents.

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