Forensic toxicologist
Expert class library · Forensic science · Either side
A forensic toxicologist or laboratory chemist who tests blood and urine for alcohol and drugs and explains what the results mean: concentrations, how drugs affect a person, and whether the findings are consistent with impairment. Courts readily admit the laboratory results and testimony about the general effects of drugs, but the interpretive step is the battleground. New Hampshire's Supreme Court reversed a conviction because a toxicologist's method for linking drugs in a driver's blood to observed signs of impairment was not shown to be reliable.
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What this expert is retained to answer
- Does the testing distinguish between related compounds, such as the isomers of methamphetamine or active THC and its inactive metabolites?
- What is the laboratory's measurement uncertainty, and how does it affect the result?
- Is the expert reporting a test result, or interpreting whether the person was impaired?
- For drugs other than alcohol, is there any accepted relationship between blood concentration and impairment?
- Does the toxicologist have training in the specific drug effects being described?
Methods
- Gas chromatography and mass spectrometry testing of blood and urine
- Blood alcohol analysis and retrograde extrapolation
- Interpretation of drug concentrations and metabolites
- Comparison of observed signs and symptoms with known drug effects
- Laboratory quality control, chain of custody and measurement uncertainty
How the testimony is attacked
- Impairment opinions without a reliable method. Linking drugs in the blood to impairment is contested. In State v. Keller the New Hampshire Supreme Court held a human performance toxicologist's method of matching a driver's signs and symptoms to the drugs found was not shown to be reliable, citing a record lacking evidence of peer review and the absence of any certification for that interpretive work, and reversed.
- Lab results without a hearing. Defendants seek reliability hearings on standard lab testing. In State v. Anderson the South Dakota Supreme Court upheld denying a Daubert hearing on the state laboratory chemists' drug testing, which used accepted methods.
- Qualifications for drug effects. Opponents challenge whether a lab toxicologist may describe how a drug affects people. In Macdonald v. State the Texas court upheld letting an experienced forensic toxicologist describe the general effects of a high dose of methamphetamine.
- Defense experts outside their field. Defense experts may be confined to their expertise. In Anderson the court upheld limiting a defense expert on methamphetamine isomers who was not an expert in forensic testing.
- Retrograde extrapolation. Estimates of blood alcohol at the time of driving are attacked when they rest on assumptions about when the person last drank, ate or reached peak absorption.
What the public record shows
Courts have filed at least 34 opinions since 2015 that address this class of testimony under Daubert or Rule 702, as of October 4, 2026. Three that show how courts handle this class of testimony:
| Outcome | Case | Court | Year | Why |
|---|---|---|---|---|
| Excluded | State v. Keller2024 N.H. 42 | N.H. | 2024 | The New Hampshire Supreme Court held the trial court erred in admitting the State's human performance forensic toxicology expert's opinion that a driver showed signs of impairment consistent with the drugs in his system, because her interpretive method was not shown to be reliable, and reversed the aggravated driving while intoxicated conviction. |
| Admitted | State v. Anderson2025 S.D. 45 | S.D. | 2025 | Affirming a conviction for driving under the influence of drugs, the South Dakota Supreme Court upheld admitting state laboratory chemists' blood and urine test results without a Daubert hearing and upheld limiting a defense expert on methamphetamine isomers to her area of expertise. |
| Admitted | Macdonald v. StateNo. 13-22-00123-CR | Tex. App. | 2023 | Affirming a felony DWI conviction, the court held the trial court did not abuse its discretion in finding a forensic toxicologist with eighteen years of experience qualified to testify about the general effects of a high dose of methamphetamine. |
Each case links to the free opinion text on CourtListener.
Under amended Rule 702
Since December 1, 2023, Rule 702 says expressly that the party offering an expert must show the court it is more likely than not that the testimony meets each requirement: that it rests on sufficient facts or data, uses reliable methods, and reflects a reliable application of those methods to the case. Questions about the basis of an opinion are no longer automatically matters of weight for the jury. For how the circuits have applied the amendment, see the Rule 702 tracker, which follows each court of appeals; for what that means for preparing or attacking this class of expert, see the guide on amended Rule 702.
Related classes
- Alcohol toxicology and retrograde extrapolation expert
- Forensic pathologist (cause and manner of death)
- Toxicologist (dose-response)
Guides
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Last reviewed October 4, 2026. How this page is built: rulings are found by searching court opinions on CourtListener, and each one is read in the opinion before it is summarised here; outcomes are labelled by what the court did with the expert's testimony. No individual expert is named. This page summarises public decisions for orientation and is not legal advice; read the opinion before relying on any ruling.