Banner for The Criminal Justice Systems: A Guide for Law Enforcement Officers and Expert Witnesses in Impaired Driving Cases

Toxicologists, Crash Reconstructionists,
Optometrists, and Other Medical Personnel


The prosecutor will ask you what you did with the sample in this case:

  • What was the condition of the  blood/urine sample (explain the condition it was in when received—sealed, clearly labeled, no signs of tampering, etc.)?
  • Did you follow established lab protocols—rules—when conducting the tests?
  • Please provide a brief, non-scientific description of the lab instruments used.
  • What were the results—cocaine, methamphetamine, etc. present?
  • If urine, explain how long prior to the taking of the sample the drug was in the blood and thus affecting the person’s brain.
  • Explain the effects of the drug on a person (sleepy, hyper, hard to concentrate, slows reaction times, etc.).
  • Explain the potential for wide variations in a drug’s effect—new versus experienced user, amount taken (over what period of time), other drugs being used at the same time (“synergistic effect”
    1 + 1 > 2), “going up” or “coming down” off the drug.
  • Discuss the physical and mental abilities needed to safely drive a car.
  • Discuss how the effects of the drug interfere with driving a car (e.g., delayed reaction time means the driver is slower to notice (even in split seconds) the light turning from green to red; and in that split second of not braking, the car has traveled further down the road.
  • Are you able to give an opinion that on this date, at this time, this defendant was impaired by this drug to a degree that made him unable to drive safely?

The Need to Use Ordinary Language

Part of your job in testifying is to take a potentially complex or unfamiliar term and convert it into ordinary language. While you won’t be testifying to a group of kindergarten children, it is helpful to think back to your late grade school/early high school days and the vocabulary you used at that time. Common toxicology terms defined in laymen’s terms include:

Route of Administration - how the drug gets into the person’s body

  • Oral - taken by mouth; swallowed
  • Inhalation - breathed in through nose and/or mouth
  • Intravenous - injected with a needle, similar to getting a shot at the doctor’s office
  • Smoking - as with a cigarette, cigar, or pipe but perhaps in another form, such as a small glass tube open at both ends to suck smoke through one’s mouth
  • Intranasal – drug placed directly into the nose where it is absorbed by the skin inside the nose and breathed in deeply
  • Dermal - through the skin, as with a patch used to quit smoking



getting the drug into the body (following “route of administration”)



the movement of the drug throughout the body



the body’s way of breaking down a complex substance into smaller pieces in an effort to get rid of it



getting the drug or its smaller pieces out of the body through urine, feces, sweat, saliva, or breath



The polysyllabic words used in toxicology have the potential to become mind-numbing to the non-toxicologist audience (pharmacokinetics, pharmacodynamics, epidemiological studies, methylenedioxymethamphetamine, tetrahydrocannabinol, etc.) This is not to suggest you can never use these words. However, when you use them, immediately explain their meanings in common, everyday language—and then try to limit repeating the words and substitute as much as possible common language to convey your message. Refrain from using several of these terms within a short statement:

Studies conducted by some of our most renowned forensic toxicologists demonstrate the pharmacokinetics of methylenedioxymethamphetamine to be typically excreted 65% unchanged, with the principal metabolite being methylenedioxyamphetamine. Our epidemiological studies further show that the driving population and others are adversely affected by ingestion of MDMA, with symptoms including ataxia and diaphoresis, accompanied by psychomotor and memory dysfunction.

Plain Language
Scientific studies done by experts show that MDMA, a relatively common street drug—you may have heard of it as “ecstasy”—seems to make people feel more alert, even hyper. In reality, someone on MDMA will frequently be uncoordinated, restless, sweat heavily, have trouble controlling their movements, and be unable to think clearly. Obviously, a person in this condition is a risk to himself and others when driving.

Having thus painted a “word picture” of what a person on MDMA looks like, the toxicologist can now compare the picture of this uncoordinated individual to what a person should look and act like when driving.

The Science of Toxicology

“Alcohol is a drug, but not all drugs are alcohol.”

When testifying about the effects of alcohol and/or other drugs on a person’s ability to drive, it will be necessary to go into some detail about the physical and mental effects of the substance(s) involved. You may be asked about retrograde extrapolation, zero order kinetics, first order kinetics, pharmacodynamics, and a host of other complex (to the lay person) topics. It is safe to assume the average juror has no knowledge whatsoever of these concepts, and therefore it is imperative to limit scientific jargon as much as possible.


When discussing a scientific principle, immediately follow it up with a simple analogy. Example:

Scientific principle: Alcohol is eliminated at a fixed or linear rate (“zero order kinetics”), thus enabling a toxicologist, in many circumstances, to perform retrograde extrapolation (use a math equation to determine someone’s BAC a short period of time in the past, measured in hours, not days). However, the elimination rate of other drugs, depending upon their specific half-life (the period of time it takes the body to get rid of one-half of the substance) cannot be plotted on a straight line, but rather, is plotted on a curved line over time (“first order kinetics”). Drug pharmacodynamics—the processes of absorption, distribution, metabolism, and elimination—do not occur in set chronological fashion, one after the other, but rather, in combination with each other. Drug effects are also related to the timeline of drug use, will change over time, and may be further complicated by the presence of some active metabolites, which themselves will cause certain effects.

Simple analogy:5 A baseball dropped by a 10-year-old sitting in a tree house, high above the ground, will fall straight down (alcohol zero order elimination). With practice, the child might even be able to reasonably predict how long the ball takes to hit the ground (alcohol retrograde). If that same 10-year-old then drops a maple leaf attached to an acorn, it should hit the ground at about the same time as the baseball (other drugs with zero order elimination). However, what would happen if he dropped only the leaf, without the acorn? It will drop much more slowly—as it is tossed and turned in the breeze—than the baseball or the leaf with the acorn. The leaf’s size also changes during descent as pieces break off in the wind (changing drug half-life); this also causes its rate of descent to slow. Eventually the leaf gets to the ground, but not in a straight line nor in a predictable time frame (drug first order elimination).


5 This analogy first appeared in Drug Toxicology for Prosecutors: Targeting Hardcore Impaired Drivers, Dr. Sarah Kerrigan, Ph.D. Published by the American Prosecutors Research Institute, October 2004 and available for downloading at