By Shaun D. Carstairs, MD, DFACMT, FACEP
Board-certified in Emergency Medicine, Medical Toxicology, and Addiction Medicine
Last medically reviewed: September 7, 2026
A fentanyl result is one piece of evidence. The legal-medical question usually requires much more than the number itself.
Cases involving fentanyl frequently raise questions about exposure, intoxication, overdose, tolerance, co-ingestants, emergency treatment, and causation. A laboratory result can be important, but it should be interpreted in the context of the specimen tested, timing, clinical course, other substances, medications, treatment, and the outcome at issue.
1. What does the test actually measure?
Start with the specimen and the laboratory method. Blood, urine, and other specimens answer different questions. A result may establish that fentanyl or a related analyte was detected, but the significance of that finding depends on how and when the specimen was collected and what the laboratory method can reliably establish.
Attorneys should obtain the complete toxicology report rather than relying only on a summary in a medical record, police report, or death certificate.
2. Can the concentration establish impairment or overdose by itself?
Usually, the more useful question is not whether a concentration is “high” or “low” in isolation, but whether the entire medical record supports fentanyl as a material contributor to the clinical condition or outcome. Interpretation can be affected by tolerance, timing, chronic versus intermittent exposure, co-exposures, medical illness, and treatment before specimen collection.
A concentration should therefore be interpreted alongside the history, vital signs, neurologic and respiratory findings, response to naloxone when given, laboratory data, imaging, and the overall clinical course.
3. What other substances were present?
Fentanyl cases often involve more than one substance. Alcohol, benzodiazepines, other opioids, stimulants, sedating medications, and prescribed drugs may alter the clinical picture. The presence of multiple substances does not automatically establish that each one caused the outcome, but it may materially affect a causation analysis.
4. What was the patient’s opioid tolerance?
Tolerance can substantially change the relationship between an opioid exposure and its clinical effects. Records that may help address tolerance include prescription history, prior treatment for opioid use disorder, previous overdose encounters, toxicology results, substance-use history, and documentation from emergency, inpatient, addiction-treatment, or pain-management settings.
5. Was naloxone given, and what happened afterward?
Naloxone use is often important but should not be interpreted simplistically. The dose, timing, route, observed response, recurrence of symptoms, airway management, and other concurrent treatments may all matter. A documented improvement after naloxone can support opioid effect, but the complete clinical context remains important.
6. Are there competing explanations for the outcome?
A sound causation review should consider reasonable alternatives. Depending on the case, these may include trauma, infection, cardiovascular disease, pulmonary disease, metabolic abnormalities, aspiration, other drugs, delayed treatment, or unrelated medical conditions. The goal is not merely to identify fentanyl in the record, but to determine how strongly the available evidence supports or weakens a causal relationship.
7. What records are most useful for review?
- Complete emergency medical services and emergency department records
- Medication administration records, including naloxone
- Complete toxicology and laboratory reports
- Prescription and medication history when relevant
- Prior records bearing on opioid tolerance or substance use disorder
- Hospital, ICU, or specialty consultation records
- Autopsy and postmortem toxicology records when applicable
- Relevant deposition testimony or witness accounts concerning timing and exposure
When medical toxicology and addiction medicine overlap
Some fentanyl cases require both a toxicology analysis of the exposure and an addiction-medicine analysis of opioid use disorder, tolerance, withdrawal, or treatment. Others also require evaluation of the emergency care provided. Shaun D. Carstairs, MD, DFACMT, FACEP is board-certified in Medical Toxicology, Addiction Medicine, and Emergency Medicine and can assess matters that cross those clinical boundaries when the requested assignment fits his expertise.
Related services: Medical Toxicology Expert Witnesses, Addiction Medicine Expert Witness, and Emergency Medicine Expert Witnesses.
Selected References
- Palmer RB. Fentanyl in postmortem forensic toxicology. Clin Toxicol (Phila). 2010;48(8):771-784. doi:10.3109/15563650.2010.525514.
- Gill JR, Lin PT, Nelson L. Reliability of postmortem fentanyl concentrations in determining the cause of death. J Med Toxicol. 2013;9(1):34-41. doi:10.1007/s13181-012-0253-z.
- Stolbach A, Connors N, Nelson L, Kulig K. ACMT Position Statement: Interpretation of Urine Opiate and Opioid Tests. J Med Toxicol. 2022;18(2):176-179. doi:10.1007/s13181-021-00864-1.
- Concheiro M, Chesser R, Pardi J, Cooper G. Postmortem Toxicology of New Synthetic Opioids. Front Pharmacol. 2018;9:1210. doi:10.3389/fphar.2018.01210.
- Reiter A, Mueller A, Otto B, et al. Fast increase of postmortem fentanyl blood concentrations after transdermal application: A call to careful interpretation. Forensic Sci Int. 2019;302:109896. doi:10.1016/j.forsciint.2019.109896.
- Labay LM, Kacinko SL, Casey BK, Brower JO. Tolerant or Intolerant? Learning From Studying Drug Concentrations in the Living and the Dead. Am J Forensic Med Pathol. 2024;45(4):292-296. doi:10.1097/PAF.0000000000000962.
These references address general principles relevant to fentanyl testing and interpretation. A case-specific review may require additional literature based on the specimen, clinical circumstances, and questions presented.
Discuss a fentanyl-related case
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This resource is intended for general educational purposes for attorneys and does not constitute patient-specific medical advice or a case opinion.