Polydrug Use: The New Normal in Toxicological Casework

Published 25 June 2026 · By Dr Marcus Oyelaran, Senior Scientist, Forensic and Toxicology Standards

Ask any forensic toxicologist how often they close a case on a single substance and the answer is increasingly: rarely. Post-mortem, driving-under-the-influence and drug-facilitated crime casework now routinely returns two, three or more analytes, often from different pharmacological classes and sometimes including substances that did not exist as reference materials when the method was validated. Polydrug findings are no longer an interesting complication. They are the baseline condition of the work.

What is driving the shift

Several trends have compounded one another over the past decade:

• Synthetic opioid adulteration. Nitazenes and fentanyl analogues appear in samples where the user believed they were taking heroin, oxycodone or a benzodiazepine, producing combinations that were never intentional.
• Designer benzodiazepines. Substances such as bromazolam and flualprazolam circulate alongside prescribed medicines, and immunoassay screens designed for classical benzodiazepines frequently miss them.
• Stimulant and depressant co-use. Cocaine or methamphetamine combined with opioids remains one of the most common fatal patterns.
• Rapid NPS turnover. New psychoactive substances enter and leave the market faster than scheduling or method validation cycles can follow.
• Cannabinoid diversification. Semi-synthetic cannabinoids and a widening range of synthetic receptor agonists complicate what was once a straightforward finding.

The analytical consequences

A polydrug reality changes method strategy in three concrete ways. First, targeted panels alone are insufficient; laboratories increasingly pair a broad LC-QTOF or Orbitrap screen against a curated high-resolution library with targeted LC-MS/MS confirmation and quantitation. Second, detection windows and sample volumes become limiting, particularly in post-mortem work where blood may be scarce and degraded. Third, and most demanding, the laboratory must be able to add a newly encountered substance to its scope quickly — which is fundamentally a reference standards problem, not an instrumentation one.

Standards: the rate-limiting step

Identifying an unknown by accurate mass and fragmentation gives a candidate. Confirming it to evidential standard requires a certified reference material analysed under the same conditions. In practice, three things determine how fast a laboratory can respond to a new substance:

1. Availability. Whether a characterised neat material or solution exists at all, and whether it can be shipped under the relevant controlled drug licensing.
2. Internal standards. Whether a deuterated or 13C-labelled analogue is available for quantitation, since matrix suppression in post-mortem blood is severe and uncorrected results are not defensible.
3. Documentation. Whether the certificate supports court use — identity confirmation, purity determination method, assigned value with uncertainty, and traceability.

Laboratories that maintain a standing relationship with their supplier, monitor early warning networks such as national drug alert systems, and pre-emptively acquire materials for substances detected regionally are consistently weeks or months ahead of those that order reactively.

Interpretation is harder than detection

Reporting a list of concentrations no longer answers the question a coroner or a court is asking. Combined central nervous system depression from an opioid, a benzodiazepine and alcohol is not the arithmetic sum of three separate risks. Post-mortem redistribution alters opioid and antidepressant concentrations in ways that depend on sampling site. Tolerance in chronic users means a concentration that would be fatal in a naive individual may be routine. Metabolite ratios can distinguish recent administration from historic use, but only where the metabolite standard is available and validated.

Good interpretive reporting states the findings, the limitations of the matrix, the analytical uncertainty, and the combined pharmacological picture in language a non-scientist can follow — without overstating what a concentration can prove.

Practical steps for laboratories

• Review scope against regional intelligence quarterly rather than annually.
• Maintain deuterated internal standards for every quantified analyte, not only the historically common ones.
• Keep high-resolution spectral libraries current and version-controlled.
• Use proficiency testing schemes that include multi-analyte and NPS-containing samples.
• Document the reference standard lot used for every evidential confirmation.

Why it matters

Behind each case is a family waiting for an explanation, a court deciding on liability, or a public health team trying to understand what is circulating in a community. Accurate polydrug toxicology feeds directly into drug alerts, harm reduction messaging and prosecution decisions. The laboratories doing this work well are the ones treating reference standard readiness as an operational priority, not a procurement afterthought.