Federal Study Reports that THC Level Testing is Unreliable in Assessing Marijuana Intoxication

July 8, 2021 Criminal Defense, Drug Charges, Drunk Driving/DUI, News & Announcements

Marijuana DUI Florida

Marijuana has been a hot topic in the press and political spheres for years now due to the rapidly changing laws surrounding the drug. Many states are repealing or amending their marijuana laws in light of studies and social consciousness that promote the plant’s benefits[1], like reducing pain for those suffering from arthritis, fibromyalgia, endometriosis, and migraines, managing anxiety, epilepsy, post-traumatic stress disorder, and depression, and sleep management.  These benefits have become common knowledge, as well as the limited number of negative side-effects[2] compared to other drugs that treat similar ailments. Despite this, some states, including Florida, have not completely legalized the use of marijuana. To learn more about marijuana charges, consequences, and defenses, please visit our blog post here.

What is a Per Se Marijuana DUI?

In several states, blood tests are used to exhibit minimal tetrahydrocannabinol (THC) concentration prior to the filing of a per se marijuana DUI charge.[3] After a driver is arrested, a blood test is conducted to see if THC metabolites are present. If sufficient metabolites are found, the driver can be charged with per se marijuana DUI. This approach mimics the blood alcohol level (BAC) tests in alcohol DUI cases, where above 0.08 is illegal.

What Has the Study Revealed About Per Se Marijuana DUI Laws?

The results of the study[4] conducted by scientists from RTI International and funded by the National Institute of Justice call into question the per se marijuana DUI laws that are in effect in several states in the country. The study concluded that an individual’s THC levels after partaking in marijuana do not accurately gauge impairment. The purpose of the study was to find out whether the biofluid levels of THC circulating in an individual’s body correlate accurately with performance in sobriety and cognitive exams typically used to determine whether a driver is impaired. Twenty participants were assessed in this study. These participants ate or vaporized marijuana and researchers found that, while individuals who consumed or partook in doses of 5 milligrams of THC or more were impaired to “some degree,” the THC levels present in the biofluids of the participants were not reliable or accurate indicators of marijuana impairment or intoxication. During the test, the researchers collected biofluid such a blood, urine, and saliva from participants for examination. Interestingly, the researchers found no participant’s motor functions were impaired after using marijuana. These toxicology tests also revealed that the  cannabis components, THC, cannabidiol, and cannabinol, present in the biofluid of the participants did not correlate with any cognitive or motor impairment measured for ingested or vaporized marijuana intake.

The study does not conclude that THC does not impair an individual. However, researchers found that the concentration of THC in a person’s biofluid is not a reliable indicator of such impairment. During the study, some participants that had significantly impaired motor and cognitive function even when their biofluid showed low THC levels. Researchers also found that the sobriety tests commonly associated with alcohol impairment stops were ineffective in determining whether an individual was impaired or intoxicated by marijuana.

This study casts a dubious shadow over the per se marijuana DUI laws because “there exists no science demonstrating that these arbitrary limits are reliable predictors of either recent cannabis exposure or impairment.”[5]

Other Related Findings

This is not the first report to conclude that levels of marijuana, or THC, in a person’s body is a reliable indicator of impairment. A 2019 congressional report regarding marijuana-impaired driving also found that evidence is currently inconclusive to support the claim that marijuana impairs driving.[6] In the report, the Congressional Research Service wrote that “although laboratory studies have shown that marijuana consumption can affect a person’s response times and motor performance, studies of the impact of marijuana consumption on a driver’s risk of being involved in a crash have produced conflicting results, with some studies finding little or no increased risk of a crash from marijuana usage.”[7] As far as THC levels and accuracy go, the report details that “THC’s unique absorption profile and prolonged detection window in blood makes it so that—unlike alcohol—the detection of THC in blood is not necessarily indicative of either recency of use or behavioral impairment.”[8] The report authors stated that “as of yet [there is] no scientifically demonstrated correlation between levels of THC and degrees of impairment of driver performance, and epidemiological studies disagree as to whether marijuana use by a driver results in increased crash risk.”[9]

Tallahassee Marijuana Defense Lawyer

Marijuana charges in Florida can carry serious penalties. As stated above, not all measurements of intoxication or impairment are accurate. That is why it is incredibly important to hire an educated and zealous Tallahassee criminal defense attorney that will ensure the rights of you or a loved one are protected. Don Pumphrey and the legal team at Pumphrey Law Firm have a wealth of experience helping clients navigate marijuana charges, and will ensure you receive the best, and most strategic, defense possible. Call (850) 681-7777 or send an online message today to discuss your options during an open and free consultation with an attorney in our team.

This article was written by Gabi D’Esposito

gabi d'esposito pumphrey law









[1] Kristeen Cherney, What Are the Benefits of Marijuana?, Healthline (Jan. 6, 2020), available at: https://www.healthline.com/health/medical-marijuana/benefits-of-marijuana.

[2] Id.

[3] Marijuana DUI Laws, Marijuana and the Law, available at: https://marijuanaandthelaw.com/resources/marijuana-dui-laws/.

[4] National Institute of Justice, “Field Sobriety Tests and THC Levels Unreliable Indicators of Marijuana Intoxication,” (April 5, 2021), nij.ojp.gov:


[5] Testing THC Levels Is “Not Reliable” Method of Gauging Marijuana Impairment, Federal Study Finds, Marijuana and the Law (June 15, 2021), available at: https://marijuanaandthelaw.com/testing-thc-levels-is-not-reliable-method-of-gauging-marijuana-impairment/.

[6] Congressional Report Questions Whether Marijuana Use Impairs Driving, Marijuana and the Law (June 26 , 2019), available at: https://marijuanaandthelaw.com/congressional-report-questions-whether-marijuana-use-impairs-driving/.

[7] Marijuana Use and Highway Safety, Congressional Research Service (May 14, 2019), available at: https://crsreports.congress.gov/product/pdf/R/R45719.

[8] Supra note 6.

[9] Supra note 7.

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