USA: Testing People for Marijuana Impairment Based on THC Levels is ‘Not Reliable,’ Federally Funded Study Finds

The amount of THC in a person’s system after consuming marijuana is not an accurate predictor of impairment, a federally funded study has determined.

The research, backed by a grant from the National Institute of Justice (NIJ), involved 20 people who either ate or vaporized cannabis with varying levels of THC. They were then tested with basic field sobriety and cognitive tests.

While the groups that received doses higher than 5mg of THC were all “negatively impacted” and experienced observable psychomotor impairment, the RTI International researchers found that “THC levels in biofluids were not reliable indicators of marijuana intoxication for their study participants.”


“Researchers investigated how marijuana affects skills required for safe driving and found that biofluid levels of THC did not correlate with field sobriety test performance or marijuana intoxication, regardless of how the cannabis was ingested.”


Read more at Marijuana Moment

Testing People For Marijuana Impairment Based On THC Levels Is ‘Not Reliable,’ Federally Funded Study Finds



Field Sobriety Tests and THC Levels Unreliable Indicators of Marijuana Intoxication

Researchers investigated how marijuana affects skills required for safe driving and found that biofluid levels of THC did not correlate with field sobriety test performance or marijuana intoxication, regardless of how the cannabis was ingested.

April 5, 2021

Laws regarding driving under the influence of marijuana vary from state to state, with a growing trend toward “per se” laws that use a level of delta-9-tetrahydrocannabinol (THC, one of the psychoactive substances in marijuana) in the blood, urine, or oral fluid as a determinant of intoxication.[1] However, there is little evidence correlating a specific THC level with impaired driving, making marijuana per se laws controversial and difficult to prosecute.[2]

In an effort to better understand marijuana intoxication and, ultimately, improve marijuana intoxication legislation, National Institute of Justice-supported researchers from RTI International studied how specific cannabis doses and administration methods (eaten or vaped) affect THC levels in the body and how that correlates with performance on impairment tests. Results from their clinical dosing sessions showed that THC levels in study participants’ biofluids varied depending on cannabis dose and administration method and that timing of maximum impairment for each dose – and performance on impairment tests – also varied by dose and administration method. Therefore, the RTI team concluded that, although THC has been proven to affect areas of the brain that control movement, balance, coordination, memory, and judgment,[3] – skills required for safe driving – THC levels in biofluids were not reliable indicators of marijuana intoxication for their study participants.

THC Dosing Study Design and Results

Through six double-blind clinical dosing sessions, the RTI researchers evaluated how oral and vaporized administration of known doses of THC affected behavior and performance, as well as forensic toxicology testing of blood, urine, and oral fluid samples in 20 study participants.

Each of the participants completed all six dosing sessions. They ate cannabis brownies with 0, 10 mg, and 25 mg of THC and inhaled vapor containing 0, 5 mg, and 20 mg of THC. The dosing sessions were spaced at least one week apart.

Cognitive and Psychomotor Effects of THC

Study participants’ cognitive and psychomotor performances were assessed using common impairment tests – none of which are currently part of a legal protocol for determining marijuana intoxication – before and after THC dosing, including:

  • Paced serial addition test.
  • Digit symbol substitution test.
  • Divided attention test.
  • Tasks 1-4 from the DRUID iOS smartphone app.
  • Standardized field sobriety tests to detect alcohol impairment, including standing on one leg, walk and turn, modified Romberg balance, and eye tracking for nystagmus and pupillary response.

After THC dosing, study participants reported feeling heightened drug effects with increased cannabis doses. These subjective effects peaked on average three to five hours after oral administration and zero to one hour after vaped administration.

Study participants’ cognitive and psychomotor functioning were negatively impacted after all oral and vaped doses of cannabis except for the lowest vaped dose, which contained 5 mg THC.

For vaped THC doses over 5 mg, peak cognitive and psychomotor effects were observed zero to two hours after administration and returned to baseline after four hours.

For oral THC doses, cognitive and psychomotor effects were observed one hour after administration and peak effects were seen about five hours after administration. Participants’ cognitive and psychomotor functioning returned to baseline eight hours after oral administration.

The researchers reported that the one leg stand, walk and turn, and modified Romberg balance tests were not sensitive to cannabis intoxication for any of the study participants.

Testing Biofluids for THC

Samples of blood, urine, and oral fluid were collected from study participants before cannabis dosing and then nearly every hour for eight hours after dosing. The researchers sent all biofluid samples to commercial forensic toxicology laboratories to be analyzed for THC as well as nonpsychoactive cannabis components cannabidiol and cannabinol.

Results from the toxicology tests showed that the levels of all three targeted cannabis components (THC, cannabidiol, and cannabinol) in blood, urine, and oral fluid did not correlate with cognitive or psychomotor impairment measures for oral or vaporized cannabis administration.

Conclusions and Implications for Law Enforcement

RTI concluded that, for their dosing study, THC levels in biofluid were not reliable indicators of marijuana intoxication. Many of their study participants had significantly decreased cognitive and psychomotor functioning even when their blood, urine, and oral fluid contained low levels of THC. The researchers also observed that standardized field sobriety tests commonly used to detect driving under the influence of drugs or alcohol were not effective in detecting marijuana intoxication.

The RTI researchers hope their work will inform policy for cannabis impairment and driving under the influence of drugs and help establish scientifically-based thresholds for marijuana intoxication.

About this Article

The research described in this article was funded by NIJ grant 2016-DN-BX-0193, awarded to RTI International. This article is based on the grantee final report “Differences in Cannabis Impairment and its Measurement Due to Route of Administration” (March 2020) by Megan Grabenauer.

[note 1] National Highway Traffic Safety Administration, Governors Highway Safety Association, & the Volpe National Transportation Systems Center, “Impact of the Legalization and Decriminalization of Marijuana on the DWI System: Highlights from the Expert Panel Meeting,” Washington, DC: National Highway Traffic Safety Administration, June 2017, Report No. DOT HS 812 430,

[note 2] Mark Asbridge, Jill A. Hayden, and Jennifer L. Cartwright, “Acute Cannabis Consumption and Motor Vehicle Collision Risk: Systematic Review of Observational Studies and Meta-Analysis,” The British Medical Journal, 344 (2012): e536,

Rebecca L. Hartman and Marilyn A. Huestis, “Cannabis Effects on Driving Skills,” Clinical Chemistry, 59 no. 3 (2013):478-492,

[note 3] Immaculada Fierro, Juan Carlos Gonzáles-Luque, and F. Javier Álvarez, “The Relationship Between Observed Signs of Impairment and THC Concentration in Oral Fluid,” Drug and Alcohol Dependence, 144 (2014): 231-238,

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