A Cannabis FST Trial Found Both Group Differences and Frequent Placebo Classifications

A randomized cannabis field-test trial found a clear difference between THC and placebo groups, alongside frequent impairment classifications in the placebo group. Both findings matter: group discrimination does not make every positive classification specific to cannabis.

The initial comparison showed a treatment difference

Marcotte and colleagues analyzed 184 cannabis users assigned to active THC or placebo smoking conditions. About 70 minutes after smoking, officers classified 81% of the THC group and 49.2% of the placebo group as field-test impaired.

The difference was statistically significant, approximately 31.8 percentage points. It would therefore misrepresent the trial to say the field tests detected no THC-related signal. Selected test components also showed associations with simulator performance.

The placebo classifications constrain attribution

At the same time, nearly half of the placebo group received an impairment classification. A classification alone therefore did not reliably identify active THC administration in every participant under these study conditions.

Officers did not observe the participants’ simulator driving. The study’s selected field assessments should not be treated as identical to an entire roadside investigation, and its results should not be transferred unchanged to every examination setting.

Separate the questions before quoting accuracy

One question is whether groups differ after random assignment. Another is whether a particular classification identifies THC exposure. A third is whether it corresponds to independently measured driving impairment. The reference outcome changes the interpretation.

For a case review, examine the actual components and the basis of the final opinion. This article concerns the initial classification comparison; improvement during repeated testing raises the separate issue of practice effects. The trial supports qualified interpretation of both positive and null findings.

Related: How the reference specimen changes a cannabis accuracy claim.

Research cited

Marcotte TD; Umlauf A; Grelotti DJ; et al.; JAMA Psychiatry; 2023. Evaluation of Field Sobriety Tests for Identifying Drivers Under the Influence of Cannabis: A Randomized Clinical Trial.