If you use smokeless tobacco for a long time and hold it in the same place in your mouth, the surface of your lip or cheek, called the mucosa, where the tobacco sits might thicken and turn white. A layer of white keratin is reacting to the irritating effect of the tobacco. The reaction is similar to the development of calluses on a worker’s hand. White patches in the mouth related to smokeless tobacco are nearly always benign.
Physicians and dentists refer to these spots as “leukoplakia” – “leuko” meaning white and “plakia” meaning plaque or patch. Regrettably, many health professionals don’t know that smokeless use is far safer than smoking and that a wealth of research shows that smokeless tobacco-related callouses hardly ever turn into anything more serious.
As an oral pathologist, I have decades of experience examining these patches and I have researched this issue extensively. A 1990 study (abstract here) is typical; it showed that white patches were common in baseball players who chewed and dipped. Upon biopsy, all were benign. The link between white patches and cancer is virtually zero for dippers and chewers.
On the other hand, white patches in smokers are matters of concern. Smoke permeates the lining of the mouth, throat, airway and lungs, delivering thousands of toxic agents. As I discussed in a 1995 journal article, studies show that white patches in smokers are much more likely to be associated with mouth cancer (abstract here).