The International Association of Chiefs of Police (IACP) continues to list a “possible green coating on the tongue” as an indicator of cannabis use in its drug recognition course.

Alleged green tongues have been cited in arrests and trials across the US, but the practice was recently scrutinised by the York Daily Record in the US state of Pennsylvania.

Last year, at least 28 cases brought before the York County Court of Common Pleas mentioned a “green coating”, “green film” or “green tint”.

Attorney Scott Harper, who dealt with one of the cases, called it a “kind of junk science”.

He added: “I don’t even think it’s real. They can take pictures of these things. I’m still waiting to see a green tongue someday.”

Another local attorney, Joe Gothie, said: “This is just a pretty easy one to pick on because it just seems so facially ridiculous. Yet it has been accepted, unquestioningly, for years.

“And what does that say about our ability, through the legal system, to seriously evaluate the quality of scientific evidence?”