Reports of green-coloured tongues are not limited to the 21st century. "The Pathogenetic Cyclopaedia" published in 1859 lists "green tongue" among other symptoms associated with the mouth. Modern medicine lists known and typically benign factors involving green tongue symptoms.
A green tongue often denotes a benign condition know as "black hairy tongue", where the tongue may appear green, black, white or yellow, along with a furry appearance. While the major cause is not known, the condition is temporary and usually subsides without any prescribed treatment.
The process leading to black hairy tongue involves a build-up of the tongue's papillae, as new papillae cells grow while the old cells do not shed. This build-up traps food and bacteria on the tongue, thereby producing an unsightly appearance.
While the cause is unknown, listed risk factors contributing to black hairy tongue include poor oral hygiene, smoking cigarettes, taking certain antibiotics, chemotherapy and radiation treatments applied to the head and neck, and poor diabetes management.