THC Levels Do Not Predict Driving Impairment

The February 5, 2021 issue of Traffic Injury Prevention published results from an Australian study assessing the relationship between THC levels and driving performance.  

Volunteers consumed vaporized cannabis samples of varying potencies containing THC, CBD and a placebo.  Blood and saliva samples were collected 30 minutes after inhalation and 3.5 hours later.  Researchers evaluated driving performance on a simulator.

Nearly half of the participants failed to show driving impairment after 30 minutes despite showing THC levels above the per se limits, e.g., 5 ng/ml.  Several participants did show impairment at the 3.5 hour mark, but their THC levels were below 5 ng/ml.  

The authors reported: “The blood and oral fluid per se limits examined often failed to discriminate between impaired and unimpaired drivers.” “Moreover, blood and oral fluid THC concentrations were poorly correlated with driving impairment . . . .  It is almost impossible to infer how much cannabis was consumed, or when it was consumed, based solely on a given concentration of THC in any biological matrix.”

They reached the following conclusions:

“Due to erratic and route-dependent differences in THC pharmacokinetics as well as significant inter- and intra-individual variability, blood and oral fluid THC concentrations, unlike BAC [blood alcohol concentrations] for alcohol, provide little information as to the amount of cannabis consumed or the extent to which an individual may be intoxicated. Collectively, these results suggest that the per se limits examined here do not reliably represent thresholds for impaired driving.”


“There appears to be a poor and inconsistent relationship between magnitude of impairment and THC concentrations in biological samples, meaning that per se limits cannot reliably discriminate between impaired [and] unimpaired drivers.”

This is consistent with prior research on the topic that demonstrates that per se limits for cannabinoids are not evidence-based.

Speaking of something else that is not evidence-based . . . .  Iowa does not use a per se limit in OWI-drug cases.  Instead, operating with “any amount” of a schedule I or II controlled substance is prohibited.  This is typically established with a urine test result showing the presence of the inactive metabolite, 11-Nor-9-carboxy-Δ9-tetrahydrocannabinol, or carboxy THC (THC-COOH). 

Not surprising, there is no evidence that the smallest detectable amount of inactive carboxy-THC can discriminate between impaired and unimpaired drivers either.  

You hear “follow the science” a lot these days.  Perhaps it is time for the backdoor prohibition of THC to get a second look in Iowa.