White tongue occurs as a result of overgrowth and swelling of the finger-like projections (papillae) on the surface of your tongue. The appearance of a white coating is caused by the settling of debris, bacteria and dead cells between the enlarged and sometimes inflamed papillae.

Causes of papilla hypertrophy or inflammation include, for example:

poor oral hygiene
dry mouth
dehydration
Smoking or other oral tobacco use
Alcohol use (excessive)
mouth breathing
Low roughage diet (eating mostly soft or pureed foods)
Mechanical irritation from sharp tooth edges or dental instruments
Fire

“Mechanical irritation from sharp tooth edges or dental instruments may be a preliminary symptom.”

Examples of conditions associated with white patches or other discolorations on your tongue include the use of certain medications, such as long-term use of antibiotics, which can cause a fungal infection in the mouth.