Cannabis Expert Witnesses In Impaired Driving Cases


Cannabis Expert Witnesses

As both medical and recreational marijuana become legal in an increasing number of jurisdictions, the issue of driving while impaired by the use of marijuana becomes more prevalent.  Marijuana laws vary by state for both the legality of use of the drug, as well as the legality of driving while having an amount of the drug present within one’s system.  The government naturally may turn to experts to assist in meeting their burden of proof by offering expert testimony to establish impairment or evidence of a per se level of Tetrahydrocannabinol (THC) in the blood of an alleged driver.

How Alcohol Impairment and Marijuana Impairment Differ

Impairment based on marijuana usage has challenges not present in cases based on blood alcohol concentration.  Alcohol contains a single intoxicant (ethanol, or ethyl alcohol) which is not stored in the body upon consumption.  Instead, alcohol is excreted from the body linearly, starting at the time of consumption.  Thus, measuring the intoxicant in the blood is relatively easy.  One can extrapolate the measured quantity in the blood to a standardized degree of impairment.  One can also perform a retrograde analysis to calculate a driver’s blood alcohol concentration hours before the sample was collected.

Marijuana use, on the other hand, involves an intoxicating constituent, THC, a portion of which is absorbed immediately by the fat cells of the body.  The fat cells then release this THC slowly over the course of time.  While generally not intoxicating in the small dose released, the presence of marijuana can be detected in the blood anywhere from three days to one month after consumption.

Expert Testimony in Marijuana Cases

Expert testimony may come in two forms: Drug Recognition Experts (DRE), who offer their opinion as to impairment of an alleged driver, and toxicologists, who provide the finder of fact with data about the amount of THC or metabolites found in a person’s blood, taken after the stop of the car.   Often times, offering both the observations of the DRE and the data of the toxicologist is necessary.

A properly trained DRE engages in a 12 step protocol of observation and interaction with an alleged driver, seeking signs of impairment based on drug use.  They observe pulse, eye movement, muscle tone, and the ability to perform certain standardized tests that may indicate impairment, as well as other indications to assess a driver’s potential impairment.  Part of the DRE protocol is to form an opinion not only about impairment, but also what category or categories of drugs may have contributed to the impairment.  Of note, the 12th and final step of the DRE protocol is to have a toxicological examination that “provides additional scientific admissible evidence to support the DRE’s opinion.”

When is Someone “Impaired” by Marijuana Use – Per Se State Laws

According to the Governors Highway Safety Association (GHSA):

  • 9 states maintain a zero-tolerance law for both THC or a metabolite
  • 3 states maintain a zero-tolerance law for THC but no similar bar on metabolites
  • 5 states have per se limits for the presence of THC, which range from 1 ng to 5 ng
  • 1 state (Colorado) has a “reasonable inference” law regarding THC

In these states, determining impairment requires toxicology to determine first whether THC or (depending on the state) a metabolite is present.  Next, where there is a per se limit, whether the limit has been established based on the proper mathematical analysis.  Of course, just as with measurements of blood alcohol, toxicology measurements of THC should include an estimate of measurement uncertainty.

When is Someone “Impaired” by Marijuana Use – Observation State Laws

While some states rely on a mathematical calculation to determine impairment for driving under the influence of marijuana, most states do not.  Most states simply include impairment due to the use of a drug in their DUI/DWI statutes, by stating it is illegal to either:

  1. Drive with a blood alcohol concentration of 0.08 or more
  2. Drive while impaired by the consumption of alcohol
  3. Drive while impaired due to the use of any drug, either over the counter, prescription, or illegal
  4. Drive while impaired by the use of any drug and alcohol in combination

In these states, the question of marijuana impairment cannot be answered by a simple toxicological report.  Instead, there must be testimony from someone who observed facts and circumstances to support the inference of impairment.  This may include the arresting officer as well as a DRE.  Additional support for impairment (but not proof of impairment standing alone) could include the toxicology report.  This, of course, necessitates the testimony of the testing toxicologist.

A Caution to Toxicologists

Whether a toxicologist testifies in a per se jurisdiction or an observation state jurisdiction, toxicologists should approach the issue of impairment with caution.

There are many variables that can impact marijuana impairment.  THC blood concentrations alone are rarely a sufficient basis to establish impairment in one of the 32 states that does not directly tie marijuana impairment to a specific number value.  (However, at least one study published by the National Institute of Health has established THC concentrations of greater than 30 ng/ml result in significant impairment at a rate of 100 % for participants tested.)

There are a host of factors that can impact marijuana impairment that are specific to the individual, including:

  1. Absorption rate
  2. Distribution rate
  3. Metabolism rate
  4. Excretion rate
  5. Quantity of past marijuana use
  6. THC tolerance
  7. Time of last use
  8. Time since the user last ate
  9. The fat content of the last meal the user consumed
  10. The smoking technique employed

A variety of studies, along with the National District Attorney’s Association (NDAA) caution against correlating drug concentrations in the blood alone with driving impairment.

A Caution to Attorneys

Every case is different.  Different toxicologists may have different opinions about what a given level of THC in the blood signifies.  In every case, attorneys should meet with and discuss the toxicologist’s findings well before trial.  Unless in a per se jurisdiction, where the mere presence of a substance is enough, an attorney should explore the issues of both impairment and measurement uncertainty with their witness to discover the limits of their testimony, and the possible significance or lack thereof of the toxicologist’s findings.

About The Author

Christine Funk, J.D., is a dual-qualified criminal defense attorney and forensic science consultant who has trained lawyers, judges, and law enforcement across three continents in various forensic science disciplines.

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