Police Tests for Marijuana Impairment ‘Pseudoscience’

Police officers across the United States are using roadside tests to detect marijuana-impaired driving that are “not much better than a coin toss,” according to a scathing new analysis published in the Journal of Studies on Alcohol and Drugs.

The editorial argues that methods employed by thousands of specially trained officers rely on pseudoscience rather than validated scientific techniques, creating serious legal and public safety concerns.

With marijuana now legal in many states, the need for reliable impairment detection has become urgent. Unlike alcohol, which can be measured objectively with breathalyzers, marijuana presents unique challenges that have led law enforcement to rely on subjective evaluation methods that lack scientific foundation.

“The existing evidence suggests they’re ‘not much better than a coin toss,'” said William J. McNichol, an adjunct professor at Rutgers University Camden School of Law and author of the perspective piece.

The Drug Recognition Expert Problem

At the center of the controversy are Drug Recognition Experts (DREs)—more than 8,000 police officers nationwide who follow a standardized protocol claimed to detect drug impairment and identify specific substances. The process involves multiple steps including coordination tests, blood pressure checks, pulse measurements, and examining pupil size and eye movements.

However, McNichol argues this approach represents “police science”—techniques developed by officers for police work rather than evidence-based methods. The protocol includes questionable indicators such as squeezing drivers’ limbs to assess “muscle tone” and diagnosing psychiatric conditions like paranoia on the roadside.

A critical detail not emphasized in the press release: A 1998 controlled laboratory study found the DRE protocol produced false negative or false positive results 45.5% of the time. Even more troubling, when two groups of DREs evaluated the same subjects, they agreed on cannabis impairment only 69% of the time.

Scientific Challenges

The fundamental problem stems from attempting to apply alcohol-detection methods to marijuana without accounting for crucial differences between these substances:

• Alcohol is water-soluble; cannabinoids are fat-soluble and metabolized differently
• THC blood levels don’t correlate with impairment levels
• Heavy users develop tolerance, complicating standardized testing
• Multiple cannabinoids affect impairment, not just THC
• Some cannabinoids persist long after impairment ends

Legal and Social Consequences

The stakes extend far beyond academic debate. DUI convictions carry severe penalties including license suspension, job loss, and lasting criminal records. McNichol warns that relying on “untrustworthy evidence undermines the credibility” of impairment laws and “contains within itself the seeds of its own destruction.”

Several courts have begun excluding DRE testimony, with Maryland, Michigan, and Rhode Island rejecting it entirely. New Jersey allows DREs to testify only that behavior was “consistent with” marijuana use, while Minnesota forbids officers from calling themselves “experts.”

The problem has expanded beyond roadside testing. Workplace Impairment Recognition Experts (WIREs) now apply similar methods in employment settings, despite the same scientific limitations.

A Path Forward

Experts emphasize the urgent need for scientifically validated alternatives. “Developing more robust tools to identify cannabis-impaired drivers in an unbiased fashion is essential to keeping our roadways safe,” write Thomas D. Marcotte and Robert L. Fitzgerald from UC San Diego’s Center for Medicinal Cannabis Research.

McNichol advocates for new psychomotor tests and biochemical markers that undergo rigorous validation, with known error rates transparent to courts and juries. Crucially, he argues that leadership in developing these standards must shift from law enforcement to the scientific community.

Funding for such research already exists through marijuana tax revenue, McNichol notes. “The money is there,” he said, “if only it can be allocated properly.” Rather than supporting current DRE programs, these funds could finance the robust scientific research needed to develop truly reliable impairment detection methods.

As marijuana legalization continues expanding, the pressure mounts to replace pseudoscientific police techniques with evidence-based approaches that protect both public safety and individual rights.