A coating on the tongue without other symptoms is not a sign of anything serious. It is a build-up of bacteria and debris due to a dry mouth, mild dehydration, or as a side effect of not eating or talking. Drinking more water and removing the coating with a tongue scraper is usually sufficient to solve the problem
Occasionally a coated or discoloured tongue really is an indication of a more serious problem. Some oral infection are characterised by a white tongue. Infections like scarlet fever can produce red spots on the tongue. Smoking can produce a coated tongue and dry mouth amid all its other side effects.
Individuals should consult a dentist if their tongue changes colour or a coating appears for no apparent reason; if their tongue feels sore in any way; if a coating on the tongue persists for any length of time, perhaps two weeks.
Discoloured patches that develop on the tongue or cheek may be a reaction to irritation in the mouth, possibly from recent dental work, possibly from teeth that aggravate the surrounding tissue and need dental work. In rare cases they are linked to cancer.
Discolouration and coatings are usually not harmful in themselves, but may indicate other health problems in the individual.