Many pro-marijuana legalization groups, such as the National Organization for the Reform of Marijuana Laws (NORML), have sensationalized a green coating on the tongue as a sign of cannabis impairment. They have published outrageous photos of radioactive looking tongues mocking this sign of impairment, such as this photo in a news article here shared on NORML's social media feed.
NORML is remarkably cavalier about the green tongue observation as not a "valid" sign of impairment, and they are quick to quip this observation has "never been proven by science." What do NORML and other pro-marijuana groups define as "science"? What does their "science" consider when evaluating cannabis impairment? More often than not, these groups claim cannabis causes very little to no impairment. However, there are published studies, such as A Two-Year Study of THC in Drivers by Figueroa et al., that showed a coating on the tongue was present in 96.2% of marijuana-impaired driving arrests.
Recently I attended the Illinois Impaired Driving Summit, and former Adam's (IL) County District Attorney Jonathan Barnard presented about court testimony in impaired driving cases. He spoke about cross-examination when asked about explanations by defense attorneys for signs and symptoms of drug and alcohol impairment. Mr. Barnard made a great analogy about explaining signs and symptoms not "proven by science." Jaw or back pain, pain in the left arm or shoulder, shortness of breath, nausea, cold sweats, and chest discomfort individually are not "validated" signs and symptoms. Collectively, however, any first-year medical student - or anyone with an inkling of medical knowledge - will tell you someone with those signs and symptoms probably is having a heart attack, and they need urgent medical attention. Alcohol and drug impairment signs of impairment should be considered collectively, and combined with other evidence; the only reasonable explanation is impairment.
I have never seen the bright shade of green on someone's tongue depicted by NORML after someone smoked marijuana flower. Not once! If I saw this shade of green on someone's tongue combined with other signs of cannabis impairment, I would suspect this person ingested something other than smoked cannabis, such as an edible. If I didn't suspect impairment, I would consider the always referenced alternative explanation – the Jolly Rancher!
An investigator should never rely on just one sign of use because there could be other explanations for that particular sign of impairment. However, combining more than one observation of impairment (odor of burnt marijuana, bloodshot and glossy eyes, the list goes on), a prudent investigator considers everything in the totality before deciding on whether or not someone is impaired.
In reality, with smoked marijuana, the back or sides of the tongue will usually have a light green coating, and it will have a "striped" appearance down the tongue. The inside of the person’s mouth will also more often than not have a dry mouth and tongue along with the aforementioned green coating.
A Jolly Rancher may have caused a radioactive green looking tongue, or it could have been caused by marijuana edibles if you have other signs of cannabis impairment with the bright green tongue. When a man falls to the ground, sweating, and holding his chest, don't be the one in disbelief of the obvious heart attack signs yelling, "that's not scientifically validated!" The bottom line is never rely on just one indicator - look at everything in the totality of the circumstances.