LC-MS/MS Reveals Fentanyl Tolerance in Users

Fentanyl continues to be one of the top drugs linked to overdose deaths. However, there is a complicating factor in that some people who use the drug regularly build up a tolerance, and their blood can show high fentanyl levels even though they are alive and not overdosing. This makes it tricky for toxicologists to interpret test results, especially since there is little existing data connecting specific fentanyl blood levels to how someone is actually doing physically or mentally.

To help fill that gap, a joint study by the University of California Davis and Toronto's Centre of Forensic Sciences looked at 39 recreational drug users who came to the emergency room for reasons other than an overdose. The researchers gathered information on each person's drug use history, their symptoms and condition at the hospital, and what showed up in their toxicology tests. To measure fentanyl and 36 other drugs and their breakdown products in blood samples, they used a multidrug-targeted liquid-chromatography-tandem mass spectrometry (LC-MS/MS) method that is routinely employed for forensic casework. A paper based on this work was published in the journal Legal Medicine.¹

Why is Fentanyl Such a Critical Public Health Concern?

Deaths linked to synthetic opioids in the US, mostly driven by illegally made fentanyl, have climbed over the past ten years, with 73,838 overdose deaths reported in 2022 alone. Fentanyl remained the number one drug involved in overdose deaths in the US in both 2022 and 2023.^2,3 Deaths have been linked to blood fentanyl levels as low as 3.0 ng/mL, but it is often unclear whether the fentanyl was there on purpose or if it accidentally contaminated another drug the person was taking.^4-6In a large study looking at 153,234 impaired-driving cases across 25 states between 2010 and 2020, researchers found that average blood fentanyl levels tripled over that decade, climbing from 3.2 to 9.6 ng/mL. This suggests that drug users may be building up higher tolerance over time, using bigger doses, or both.⁷

This has inspired the researchers involved in this study to examine blood fentanyl levels, physical symptoms, and self-reported drug use among living patients with substance use disorders who came to the emergency room for reasons unrelated to an overdose. By studying this specific group of people, the team hopes to provide more context for understanding fentanyl levels that might otherwise be dismissed as "incidental" findings in toxicology testing.¹

How Did Blood Fentanyl Levels in Participants Compare to Overdose/Impaired-Driving Cases, and Did Those Levels Match Visible Signs of Toxicity?

While the researchers initially looked at 39 recreational drug users, after eliminating patients who had been given fentanyl by hospital staff, or whose fentanyl levels were too low to accurately measure, 24 patients remained. On average, their blood fentanyl levels came in at 20.1 ng/mL (with a midpoint value of 9.6 ng/mL). Interestingly, many of these levels overlapped with fentanyl concentrations previously reported in impaired-driving cases and even in fatal overdose cases, yet these two dozen patients showed no visible signs of drug toxicity. Using more than one drug at a time was also common in this group, with methamphetamine turning up most often alongside fentanyl.¹

“Our findings,” write the authors of the paper,¹ “demonstrate that blood fentanyl concentrations are unreliable predictors for effects, particularly in the drug-using population.”

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References

• Yu, K.; Medeiros, E. A.; Dillon, D. G. et al. Blood Fentanyl Concentrations and Clinical Presentation Among Patients with Substance Use Disorders in the Emergency Department. Leg Med (Tokyo) 2026, 86, 102993. DOI: 10.1016/j.legalmed.2026.102993
• Drug Overdose Deaths: Facts and Figures. National Institute on Drug abuse website 2024. https://nida.nih.gov/research-topics/trends-statistics/overdose-death-rates
• Overdose Deaths Decrease in 2023, First Time Since 2018. Centers for Disease Control and Prevention website. https://www.cdc.gov/nchs/pressroom/nchs_press_releases/2024/20240515.htm#print(accessed 2024-11-13)
• B. Pardo, B.; J. Taylor, J.; J.P. Caulkins, J. P. et al. The Future of Fentanyl and Other Synthetic Opioids. RAND Corporation website 2019. https://www.rand.org/pubs/research_reports/RR3117.html
• Martin, T. L.; Woodall, K. L.; McLellan, B. A. Fentanyl-related Deaths in Ontario, Canada: Toxicological Findings and Circumstances of Death in 112 cases (2002-2004). J Anal Toxicol. 2006, 30 (8), 603-610. DOI: 10.1093/jat/30.8.603
• Woodall, K. L.; Martin, T. L.; McLellan, B. A. Oral Abuse of Fentanyl Patches (Duragesic): Seven Case Reports. J Forensic Sci.2008, 53 (1), 222-225. DOI: 10.1111/j.1556-4029.2007.00597.x
• Chan-Hosokawa, A.; Bierly, J. J. 11-Year Study of Fentanyl in Driving Under the Influence of Drugs Casework. J Anal Toxicol. 2022, 46 (3), 337-341. DOI: 10.1093/jat/bkab049