An Inactive THC Metabolite’s Correlation Did Not Make It an Impairing Drug

A statistical association between carboxy-THC and performance does not make that metabolite an impairing drug. A cannabis meta-regression illustrates how an exposure marker can correlate with an outcome without establishing the mechanism causing it.

The analysis combined driving-related outcomes

McCartney and colleagues analyzed 822 driving-related outcomes reported in 28 publications. Those figures count outcomes and publications, not 822 independent participants or 822 trials. The analysis considered THC, metabolites, oral fluid and subjective intoxication.

Many associations were statistically significant but weak or very weak. The reported relationship for blood carboxy-THC after inhalation was stronger than some parent-THC relationships, with a correlation of approximately −0.43, while the ingestion relationship was not significant.

Association is not pharmacological activity

Carboxy-THC is an inactive metabolite used as evidence of exposure. Its association with performance can reflect relationships within the exposure and sampling process without making it the agent producing impairment.

A stronger correlation for an inactive marker therefore does not justify assigning it an intoxicating mechanism. Statistical significance, association strength and causal interpretation remain separate properties of a result.

The subgroup and individual questions remain

The analysis did not find significant biomarker relationships in regular users, but that evidence was less robust. A null subgroup result does not establish immunity to cannabis effects. Conversely, a pooled association does not determine an individual’s driving ability.

For a case review, identify route, specimen, analyte and the measured performance outcome. The meta-regression supports caution about individual prediction from concentrations while preserving the significant associations it actually found. Neither the significant nor null findings justify an absolute claim about all users.

Related: Why the blood fraction matters for THC interpretation.

Research cited

McCartney D; Arkell TR; Irwin C; et al.; Neuroscience and Biobehavioral Reviews; 2022. Are blood and oral fluid delta-9-tetrahydrocannabinol and metabolite concentrations related to impairment? A meta-regression analysis.