Safe driving: Cannabis intoxication not reliably measured by field tests, THC levels

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United States researchers investigated how marijuana affects safe driving skills and found that biofluid levels of THC did not correlate with field sobriety test performance or intoxication, regardless of how the marijuana was ingested. The National Institute of Justice study hopes to help establish scientifically-based thresholds for cannabis intoxication.

Laws regarding driving under the influence of marijuana vary from state to state in America, 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.

However, there is little evidence correlating a specific THC level with impaired driving, making marijuana per se laws controversial and difficult to prosecute, says material from the National Institute of Justice (NIJ) published on 5 April 2021.

In an effort to better understand marijuana intoxication and, ultimately, improve marijuana intoxication legislation, NIJ-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, – skills required for safe driving – THC levels in biofluids were not reliable indicators of cannabis intoxication for their study participants.

THC dosing study

Through six double-blind clinical dosing sessions, the RTI researchers evaluated how oral and vaporised administration of known doses of THC affected behaviour 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 analysed for THC as well as non-psychoactive 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 vaporised 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 standardised 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.


Research report

Differences in Cannabis Impairment and its Measurement Due to Route of Administration

Megan Grabenauer

Affiliation: RTI International

Submitted to the US Department of Justice Office of Justice Programs at the National Institute of Justice in Washington DC on 31 March 2021.




Current laws regarding cannabis impairment are either difficult to prosecute or are controversial.

Currently, most law enforcement use a combination of biological and behavioural assessments administered by drug recognition experts (DREs) and blood THC levels, with cutoffs ranging from 1 to 5 ng/mL, to judge cases of suspected Driving Under the Influence of Drugs (DUID) involving cannabis.

However, the behavioural assessments have not been explicitly developed to be sensitive for detecting acute intoxication from cannabis, and there are significant limitations to the use of blood THC levels as a proxy for acute intoxication.

Also, though smoking remains the most common route of cannabis administration, cannabis is increasingly available in a wide array of “edibles” intended for oral ingestion and there has been a substantial increase in the use of vaporizers to inhale cannabis products.

The purpose of this project was to better define the pharmacokinetics and associated pharmacodynamics of cannabis administered via vaporization and oral consumption in order to evaluate methods of determining whether or not an individual under the influence of cannabis is impaired.

Read the full report via the link to the report below.


National Institute of Justice article – Field Sobriety Tests and THC Levels Unreliable Indicators of Marijuana Intoxication (Open access)

Report via the National Criminal Justice Reference Service – Differences in Cannabis Impairment and its Measurement Due to Route of Administration (Open access)




We should not allow a cannabis free-for-all – Yale ethicist

Parliament should reconsider ‘woefully lacking’ Cannabis for Private Purposes Bill

United Nations – and United States – reclassify cannabis as less risky narcotic

Study shows cannabis use leads to less alcohol. Are we just swapping vices?



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