Multiple Drugs in One Hair Sample Do Not Establish Same-Day Co-Use

Finding THC and cocaine in the same length of hair does not establish that the drugs were taken together on a particular day. A retrospective hair study examined concurrent exposure over months, rather than simultaneous intoxication at a driving event.

The denominator was already-positive cases

The investigators reviewed an Italian forensic laboratory’s records from 2010 through 2020. The population involved people undergoing testing associated with driver-license assessment. Routine specimens were approximately four centimetres of scalp hair, analyzed for selected cannabinoids, cocaine and opiates by GC-MS.

Among 1,432 positive cases, 174 contained more than one substance, a proportion of 12.15%. Cocaine and THC were the most frequent combination in the multiple-substance group. The proportion describes positive cases in this referral dataset; it is not the prevalence of polydrug use among all drivers or all people tested.

A shared segment is a broad observation interval

A hair specimen combines material accumulated over an extended interval. Two detected substances can therefore reflect separate episodes within that interval. Their presence together does not show a shared dose time, establish which was used first or demonstrate overlapping pharmacological effects.

The study reported demographic associations and changes in the composition of positive cases over the years. These are useful descriptive findings. They do not by themselves establish that age, sex or use of one drug caused use of another, and the selected laboratory population constrains generalization.

What is missing from an acute-event opinion

The analysis did not measure participants’ driving performance during the episodes represented by their hair. It therefore cannot quantify impairment from the particular drug combination or attribute a collision to simultaneous exposure.

An event-specific assessment needs evidence addressing timing, specimen interpretation and observed function. Hair can contribute a longer-term exposure history, subject to analyte, contamination and sampling limitations. It should not be converted into a same-day blood result or a retrospective intoxication measurement.

Related: the uses and limitations of cannabis hair testing.

Primary research

Hair Analysis to Evaluate Polydrug Use. DOI: 10.3390/healthcare9080972.