A white tongue is the result of an overgrowth and swelling of the fingerlike projections (papillae) on the surface of your tongue. The appearance of a white coating is caused by debris, bacteria and dead cells getting lodged between the enlarged and sometimes inflamed papillae.

Causes of papillae hypertrophy or inflammation include, for example:
<ol class="bullet">
 	<li>Poor oral hygiene</li>
 	<li>Dry mouth</li>
 	<li>Dehydration</li>
 	<li>Smoking or other oral tobacco use</li>
 	<li>Alcohol use (excessive)</li>
 	<li>Mouth breathing</li>
 	<li>Low roughage diet (eating mostly soft or mashed foods)</li>
 	<li>Mechanical irritation from sharp tooth edges or dental appliances</li>
 	<li>Fever</li>
</ol>
<blockquote>“Mechanical irritation from sharp tooth edges or dental appliances might be a prior symptom.”</blockquote>
Examples of conditions associated with white patches or other discolorations of your tongue include the use of certain medications, such as prolonged use of antibiotics that may bring on an oral yeast infection.

Source: www.mayoclinic.org