What Is Forensic Toxicology and How Is It Used in Court?

Forensic toxicology is the branch of science that detects and measures drugs, alcohol, poisons, and other chemicals in blood, urine, tissue, and other biological samples so that courts, medical examiners, and regulators can decide cases on evidence rather than guesswork. A toxicologist’s report can determine whether a death was accidental or criminal, whether a driver was impaired, or whether a safety-sensitive employee violated federal rules. The findings only carry weight when the chemistry, the sample handling, and the laboratory methods behind them can survive a legal challenge, which makes the discipline as much about procedure as it is about science.

What a Forensic Toxicologist Does

A forensic toxicologist takes custody of a biological sample, runs it through laboratory analysis, interprets what the results mean, and then explains those results to people without scientific training. Every stage of that work is built around the possibility that a lawyer will later question it.

Chain of custody is the first requirement. From the moment a vial of blood or a urine specimen is collected, every person who touches it must be documented with dates, times, and signatures. A gap in that documentation gives a defense attorney an opening to argue the sample was tampered with or confused with someone else’s, and judges take those arguments seriously.

The second requirement is testimony. Toxicologists regularly appear as expert witnesses, and their job on the stand is to translate molecular data into plain language. That translation is harder than it sounds. An expert may need to explain why a drug concentration measured after death does not necessarily reflect what the person experienced while alive, or why a screening test flagged a substance that confirmatory testing later ruled out.

Credentials matter on cross-examination. The American Board of Forensic Toxicology, established in 1975, runs a certification program designed to identify professionals with verified qualifications and competence in the field.¹ Board certification is one of the clearest benchmarks a toxicologist can point to when opposing counsel starts probing qualifications.

Where Forensic Toxicology Shows Up in Legal Cases

Death Investigations

Post-mortem toxicology examines fluids and tissues from a deceased person to determine whether chemicals contributed to or caused the death. Medical examiners rely on these findings to classify a death as accidental, suicidal, homicidal, or natural. The report can be the single piece of evidence that distinguishes a fatal heroin overdose from a homicide staged to look like one. Timing matters: public toxicology laboratories average around 55 days to complete a case, while private labs average closer to four days.² Families and prosecutors often wait on those results before anything else moves.

Impaired Driving

Human performance toxicology evaluates how a substance affected someone’s ability to function. Impaired driving is the most common application. Every state sets the legal blood alcohol concentration limit at 0.08% for noncommercial adult drivers, a threshold effectively required nationwide by federal law tying highway funding to its adoption.³ Alcohol is only part of the picture now. Drug-impaired driving cases are growing, particularly as more jurisdictions legalize marijuana, and detecting drug impairment is far more complicated than measuring blood alcohol. Law enforcement officers trained as Drug Recognition Experts conduct a structured 12-step evaluation to form an opinion about what category of drug may be impairing a driver, and a laboratory toxicology analysis then supports or contradicts that opinion.⁴

Workplace and Transportation Testing

Federal regulations require drug testing for safety-sensitive employees in the transportation sector. Under 49 CFR Part 40, employers must test for five drug classes: marijuana metabolites, cocaine metabolites, amphetamines, phencyclidine, and opioids. Testing can occur before hiring, randomly, after an accident, when a supervisor has reasonable suspicion, or as part of a return-to-duty process. Every collection is a split specimen, meaning the sample is divided so the employee can request independent testing of the second portion if the first comes back positive.⁵

A Medical Review Officer reviews all non-negative results before they reach the employer and checks whether a legitimate medical explanation exists, such as a valid prescription. An employee who tests positive cannot return to safety-sensitive duties until they complete an evaluation by a Substance Abuse Professional, finish any recommended treatment, and pass a return-to-duty test followed by at least six unannounced follow-up tests in the next twelve months.⁵ A refusal to test carries the same consequences as a positive result.

What Toxicology Labs Look For

The range of substances a lab can detect is wide, but most cases cluster around a few categories. Controlled substances scheduled under the Controlled Substances Act are a primary focus,⁶ including heroin, cocaine, methamphetamine, fentanyl and its analogs, and synthetic cannabinoids. Prescription medications, particularly opioids and benzodiazepines, appear frequently in overdose investigations and drug-facilitated crime cases. Over-the-counter drugs like antihistamines and common pain relievers matter when they contribute to impairment.

Alcohol is the single most commonly detected substance in legal samples. Ethanol in beverages and methanol in industrial products are both relevant for different reasons: ethanol drives impaired-driving cases, while methanol points toward poisoning or contaminated product exposure. Labs also screen for environmental toxins and poisons such as carbon monoxide, cyanide, and heavy metals like arsenic or lead. Their presence can indicate foul play, workplace negligence, or accidental exposure, and interpretation depends heavily on context and concentration.

How a Sample Becomes a Result

Screening

Laboratory analysis starts with a screening phase designed to identify broad classes of substances quickly and cheaply. Most labs use immunoassay techniques, including the Enzyme-Linked Immunosorbent Assay, which uses antibodies to detect specific chemical structures in a sample. Screening is efficient but blunt. It flags the likely presence of a substance without definitively identifying it, and false positives happen. A positive screening result is never treated as a final answer. It triggers a more rigorous confirmation step.

Confirmation

Confirmation testing uses instruments like gas chromatography-mass spectrometry or liquid chromatography-mass spectrometry. These machines separate a sample into its individual chemical components and identify each one by its unique molecular signature. The combination of separation and identification provides the specificity that screening lacks. Confirmation also quantifies the exact concentration of a substance in the sample, which matters enormously for legal interpretation. The difference between a therapeutic dose and a lethal one is often just a matter of concentration.

Novel Psychoactive Substances

One of the hardest challenges facing forensic labs is the constant emergence of novel psychoactive substances. The synthetic opioid market is especially volatile, with new compounds sometimes appearing and disappearing within three to six months. Developing, validating, and implementing a test for a new substance typically takes six to nine months, so labs are often a step behind. Because many novel synthetic opioids are structurally distinct from fentanyl or heroin, they can slip through standard testing undetected if the lab does not know to look for them. Some labs now use non-targeted workflows that flag unexpected compounds and maintain databases tracking nearly 1,000 drugs simultaneously.⁷

Accreditation

Accreditation gives a laboratory’s results the structural credibility they need in court. The primary international standard for forensic testing laboratories is ISO/IEC 17025, which requires demonstrations of competence, impartiality, and consistent operation. The ANSI National Accreditation Board is the exclusive provider of accreditation to the American Board of Forensic Toxicology checklist in conjunction with ISO/IEC 17025.⁸ ANSI/ASB Standard 054 adds minimum requirements for quality control programs across subdisciplines, including post-mortem toxicology, human performance cases, non-regulated employment testing, and court-ordered testing for probation or drug courts.⁹ A lab that cannot show compliance gives defense attorneys a credible basis to challenge every result it produces.

Interpreting the Numbers

Raw numbers from a lab instrument mean little without context. A toxicologist compares detected concentrations against established therapeutic, toxic, and lethal ranges. A therapeutic level means the drug was present at a concentration consistent with medical use. A toxic level means the amount was high enough to cause harm or impairment. A lethal concentration is one typically associated with death in the general population. These ranges come from published scientific literature and population studies, but individual variation is significant. A chronic opioid user might tolerate a concentration that would kill someone with no tolerance.

Post-mortem cases add another layer. After death, drugs redistribute from organs into the blood, a phenomenon called post-mortem redistribution. This can artificially inflate blood concentrations and make a substance appear more dangerous than it was when the person was alive. Experienced toxicologists address this by collecting samples from multiple sites. Peripheral blood drawn from the femoral vein is preferred over cardiac blood because it sits farther from the organs driving redistribution.¹⁰ Vitreous humor from the eye provides another useful comparison point because it is relatively isolated from the rest of the body. When peripheral blood and vitreous humor tell different stories, the toxicologist has to weigh which is more reliable for the particular substance, and that judgment is often where the expertise lies.

How Courts Decide Whether the Evidence Comes In

Federal courts and many state courts evaluate expert scientific testimony under the framework established in Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993). Before Daubert, courts asked only whether a technique was “generally accepted” in the scientific community under the older Frye standard. Daubert broadened the inquiry. Trial judges now act as gatekeepers, weighing whether the theory or technique has been tested, whether it has undergone peer review, its known error rate, whether standards control its operation, and whether it has widespread acceptance in the relevant field. No single factor is decisive, and a judge can exclude testimony that fails this reliability analysis under Federal Rule of Evidence 702.¹¹

Rule 702 was amended in December 2023 to tighten the standard for admitting expert testimony. The amendment clarified two things that had drifted in practice. The proponent of expert testimony must now demonstrate by a preponderance of the evidence that the testimony meets the rule’s reliability requirements, correcting what had become a more permissive standard in many courts. The amendment also emphasized that an expert’s opinion must stay within the bounds of what a reliable application of the methodology actually supports. The committee notes specifically flagged forensic experts, warning that they should avoid assertions of absolute or 100 percent certainty when the underlying methodology is subjective and potentially subject to error.¹²

How Toxicology Evidence Gets Challenged

Toxicology evidence is powerful but not unassailable. Defense attorneys have several avenues to challenge it, and the vulnerabilities tend to cluster in a handful of places.

Collection and Storage Errors

The quality of any toxicology result depends first on the quality of the sample. Drawing blood through a “blind stick” rather than direct visualization of the vein increases the probability of false results. Using an antiseptic swab containing alcohol before drawing blood for ethanol analysis can contaminate the sample and produce a falsely elevated reading.¹⁰ If metal analysis is at issue, using metal instruments introduces contamination.

Storage matters as much as collection. Blood samples require a fluoride salt preservative, and published guidelines call this mandatory for peripheral blood. Without it, concentrations of opioids, ethanol, cocaine, cyanide, and GHB can shift after collection. Short-term storage requires refrigeration at 4°C, and long-term storage demands freezing at −20°C or colder.¹⁰ Volatile substances like ethanol or inhalants require glass containers with specialized lids because plastic allows greater diffusion and loss. A sample stored in the wrong container at the wrong temperature for too long can produce results that no longer reflect reality.

Post-Mortem Artifacts

In death investigations, mixing peripheral blood with cardiac blood, or collecting liver tissue from the wrong lobe, can produce misleadingly high concentrations. The deep right lobe of the liver is preferred because the left lobe sits close to the stomach, where drug residue from the last dose can diffuse into surrounding tissue.¹⁰ When vitreous humor shows ethanol but blood does not, the discrepancy may indicate ethanol was produced after death by bacterial activity rather than consumed before death. A reviewing expert who fails to account for these artifacts opens the door to effective cross-examination.

Independent Retesting

Defendants can request that a third-party laboratory independently retest the biological sample. Courts generally must approve the request and may allocate funds for the testing, particularly for indigent defendants.¹³ In federal DOT-regulated cases, the split specimen requirement exists specifically for this purpose. If an employee disputes a positive result, the second portion of the original collection can be sent to a different certified lab.⁵

Methodology and Overstatement

The 2023 amendment to Rule 702 gave courts sharper tools to scrutinize expert testimony. It specifically warns that forensic experts should avoid assertions of absolute certainty when the underlying methodology is subjective and potentially subject to error.¹² A toxicologist who testifies that a drug “definitely caused” a death when the data only supports “likely contributed to” has stepped beyond what the methodology can reliably support. Many forensic cases are won or lost here, not on the lab results themselves, but on whether the expert stayed within the bounds of what the science actually proves.

1
American Board of Forensic Toxicology. American Board of Forensic Toxicology
2
National Forensic Laboratory Information System. 2021 Toxicology Laboratory Survey Report
3
Alcohol Policy Information System. Adult Operators of Noncommercial Motor Vehicles
4
International Association of Chiefs of Police. 12 Step Process
5
eCFR. Procedures for Transportation Workplace Drug and Alcohol Testing Programs
6
Office of the Law Revision Counsel. 21 USC 811 – Authority and Criteria for Classification of Substances
7
National Institute of Justice. Challenges in Identifying Novel Psychoactive Substances and a Stronger Path Forward
8
ANSI National Accreditation Board. ISO/IEC 17025 Forensic Testing Laboratory Accreditation
9
American Academy of Forensic Sciences. Standard for a Quality Control Program in Forensic Toxicology Laboratories
10
PubMed Central. Guidelines for Collection of Biological Samples for Clinical and Forensic Toxicological Analysis
11
Legal Information Institute. Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993)
12
Legal Information Institute. Rule 702 – Testimony by Expert Witnesses
13
National Institute of Justice. Law 101 – Legal Guide for the Forensic Expert – Independent Testing