Expert Testimony in DUI Cases
When you're charged with driving under the influence (DUI) in California, you might assume the most critical witnesses in your case are police officers or eyewitnesses. But in many DUI cases—especially those involving alleged drug impairment or elevated blood alcohol levels—the most influential testimony can come from a toxicologist.
As a California DUI defense attorney, I frequently deal with cases where toxicologists are central to the prosecution's attempt to prove impairment. Understanding their role can be crucial for mounting an effective defense.
What Is a Toxicologist?
A toxicologist is a scientist who studies the effects of chemicals—like drugs and alcohol—on the human body. In DUI cases, toxicologists are often called by the prosecution to interpret blood, breath, or urine test results and offer opinions about how those substances may have impacted a driver’s ability to operate a vehicle safely.
These professionals typically work for crime labs, government agencies, or as expert witnesses hired specifically for litigation.
How Toxicologists Are Used in DUI Prosecutions
In California DUI trials, toxicologists may:
Explain Blood Alcohol Concentration (BAC): They interpret lab results and explain how BAC levels correlate to levels of impairment.
Evaluate the Timing of Consumption: Toxicologists often use mathematical modeling to estimate whether a driver was above or below the legal limit at the time of driving. Click here to read more about estimating BAC at an earlier point in time.
Analyze Drug Impairment: In drug DUIs, toxicologists may attempt to link detected substances (e.g., THC, prescription medications, or narcotics) to impaired driving behavior.
Discuss Tolerance and Metabolism: A key part of testimony may include how individuals metabolize substances differently, based on factors like body weight, gender, health, and prior usage.
Challenges for the Defense
From a defense standpoint, toxicologist testimony can be powerful—but it’s not infallible. Here’s why:
Scientific Assumptions: Toxicological models often rely on averages, not individual data. If the prosecution’s toxicologist bases their opinion on flawed assumptions (like the exact time of alcohol consumption or a uniform metabolic rate), we may be able to challenge their conclusions.
Timing of Testing: Blood or breath tests are typically performed after the alleged driving. This opens the door for "rising BAC" defenses—where your BAC was under the legal limit while driving but increased afterward.
Drug Detection ≠ Impairment: Just because a drug is present in the bloodstream doesn’t mean the driver was impaired. Many substances (like marijuana) can linger in the system for days or weeks. Toxicologists may be forced to admit that drug presence doesn't prove functional impairment at the time of driving.
Cross-Examination Opportunities: A skilled defense attorney can often highlight inconsistencies, overgeneralizations, or limitations in the toxicologist’s testimony—especially if they’re relying on theoretical models instead of concrete evidence.
How We Use Our Own Experts
To level the playing field, our firm often works with independent toxicologists who can:
Review the prosecution’s lab results and methodology
Offer alternative explanations for test results
Help the jury understand the true limitations of chemical testing
These experts can be crucial in raising reasonable doubt about whether you were actually impaired or legally over the limit at the time of driving.
Final Thoughts
Toxicologists play a critical role in many DUI prosecutions in California. Their testimony can be persuasive—but it’s not beyond challenge. As an experienced DUI defense attorney, I know how to scrutinize this kind of scientific evidence, cross-examine toxicology experts, and bring in qualified experts of our own when necessary.
If you’re facing DUI charges and toxicology evidence is involved, don’t leave your defense to chance. Contact my office today for a confidential consultation.
