Tongue diseases that cause a white coating include oral lichen planus, mouth cancer, tongue cancer and oral thrush, according to Mayo Clinic. Other conditions that also coat the tongue in white include syphilis, HIV/AIDS, leukoplakia, geographic tongue and prolonged use of medications such as antibiotics.
The coating on the tongue appears white because of dead cells, bacteria and debris that collect between the papillae on its surface, explains Mayo Clinic. Most times, when not set off by a disease, the white coating is harmless and goes away with gentle brushing, scraping and drinking lots of water. However, a painful white tongue that lasts longer than a few weeks needs medical attention.