A long-standing belief among law enforcement that marijuana turns your tongue green is used in DUI cases across the country.

Mythology abounds in the marijuana world. Prohibition forced cannabis communities underground, which resulted in the formation of a marijuana counterculture in the 60s and 70s. As a result, marijuana tips and tricks — how black pepper sedates paranoia, joint rolling techniques, etc. — were often passed around by word of mouth. There was little existing literature outside High Times and Terence McKenna to educate yourself.

Law enforcement crafted its own mythology regarding marijuana users. Among the popular tropes were weed is a gateway drug and legalizing cannabis would inhibit police from doing their jobs. This is simply not true, as one study funded by the Department of Justice found. And while the internet has empowered many to dispel rumors and fake news on both sides, some still persist in a subtle yet pernicious manner.

Take, for example, the myth that smoking marijuana will create a viscous green film atop your tongue. Smoking marijuana can dry out your mouth, creating the dreaded “cottonmouth” many daily users describe. For anyone with cannabis experience, the idea that marijuana will create more color-tinted saliva in your mouth is a funny one.

RELATED: Is Cotton Mouth From Marijuana Use Actually Dangerous?

Yet for some Americans, the green tongue myth is no laughing matter. Law enforcement across the country reference the green coating as probable cause in DUI cases. In fact, many police officers are trained to look for a green tongue in marijuana-related cases. According to The York Daily Record, 28 out of a possible 1,300 DUI cases in the York County Court of Common Pleas last year detail a “green film,” “green tint,” and “green coating.”

“The science behind marijuana consumption turning your tongue green is about as sound as the science behind the earth being flat or that lying makes your nose grow,” Erik Altieri, executive director of NORML, told the York Daily Record in an email.

To identify possible persons under the influence, police undergo a Drug Evaluation and Classification Program. “Possible green coating on the tongue” is listed as a potential characteristic of marijuana users, in the Feb. 2018 Instructor Guide manual. The course, as well as law enforcement advocates, reference two specific peer-reviewed studies as the basis for the “green tongue” theory.

In the first, a 1998 study published in the Journal of the American Optometric Association, two of the five authors worked in law enforcement. The paper, without citation, states those who recently smoked marijuana “might have a greenish coating” on the back of their tongue. Karl Citek, an optometry professor at Pacific University, told the Record they were just reporting on what police were taught in the Drug Evaluation and Classification Program. In other words, the police’s reference was their own program.

RELATED: Can I Get High From Second-Hand Marijuana Smoke?

The second, a 2017 study published in the Journal of Forensic Sciences, was an analysis of persons suspected of driving under the influence via blood samples. In their reports, police mentioned a “coating on the tongue” in 185 cases. Toxicology reports stated that 96.2% of these drivers had THC in their system. In a scientific sense, this is a classic case of confusing causation with correlation. These researchers did not discover smoking marijuana caused a coating of the tongue. Instead, they connected reports of what officers saw in the field against their lab results.

By the way, the first page of the second study notes that, “Authors all work for The Orange County Crime Laboratory testifying on driving under the influence cases, specifically in regard to marijuana, which represents a possible conflict of interest.”

All of this appears far from conclusive. One expert told The York Daily Record the initial myth began because of a 1986 handbook called “Identifying the Marihuana User.” Included in the book is picture provided by the author, physician Forest Tennant, which shows the “Green coated tongue of marijuana-hashish smoker.”  While that’s possibly where it all started, hopefully new research is conducted outside possible police influence. In the world of law enforcement, facts should lead the way, not mythology.